What’s New In Python 3.4
************************

Author:
   R. David Murray <rdmurray@bitdance.com> (Editor)

This article explains the new features in Python 3.4, compared to 3.3.
Python 3.4 was released on March 16, 2014.  For full details, see the
changelog.

See also: **PEP 429** – Python 3.4 Release Schedule


Summary – Release Highlights
============================

New syntax features:

* No new syntax features were added in Python 3.4.

Other new features:

* pip should always be available (**PEP 453**).

* Newly created file descriptors are non-inheritable (**PEP 446**).

* command line option for isolated mode (bpo-16499).

* improvements in the handling of codecs that are not text encodings
  (multiple issues).

* A ModuleSpec Type for the Import System (**PEP 451**).  (Affects
  importer authors.)

* The "marshal" format has been made more compact and efficient
  (bpo-16475).

New library modules:

* "asyncio": New provisional API for asynchronous IO (**PEP 3156**).

* "ensurepip": Bootstrapping the pip installer (**PEP 453**).

* "enum": Support for enumeration types (**PEP 435**).

* "pathlib": Object-oriented filesystem paths (**PEP 428**).

* "selectors": High-level and efficient I/O multiplexing, built upon
  the "select" module primitives (part of **PEP 3156**).

* "statistics": A basic numerically stable statistics library (**PEP
  450**).

* "tracemalloc": Trace Python memory allocations (**PEP 454**).

Significantly improved library modules:

* Single-dispatch generic functions in "functools" (**PEP 443**).

* New "pickle" protocol 4 (**PEP 3154**).

* "multiprocessing" now has an option to avoid using os.fork on Unix
  (bpo-8713).

* "email" has a new submodule, "contentmanager", and a new "Message"
  subclass ("EmailMessage") that simplify MIME handling (bpo-18891).

* The "inspect" and "pydoc" modules are now capable of correct
  introspection of a much wider variety of callable objects, which
  improves the output of the Python "help()" system.

* The "ipaddress" module API has been declared stable

Security improvements:

* Secure and interchangeable hash algorithm (**PEP 456**).

* Make newly created file descriptors non-inheritable (**PEP 446**) to
  avoid leaking file descriptors to child processes.

* New command line option for isolated mode, (bpo-16499).

* "multiprocessing" now has an option to avoid using os.fork on Unix.
  *spawn* and *forkserver* are more secure because they avoid sharing
  data with child processes.

* "multiprocessing" child processes on Windows no longer inherit all
  of the parent’s inheritable handles, only the necessary ones.

* A new "hashlib.pbkdf2_hmac()" function provides the PKCS#5 password-
  based key derivation function 2.

* TLSv1.1 and TLSv1.2 support for "ssl".

* Retrieving certificates from the Windows system cert store support
  for "ssl".

* Server-side SNI (Server Name Indication) support for "ssl".

* The "ssl.SSLContext" class has a lot of improvements.

* All modules in the standard library that support SSL now support
  server certificate verification, including hostname matching
  ("ssl.match_hostname()") and CRLs (Certificate Revocation lists, see
  "ssl.SSLContext.load_verify_locations()").

CPython implementation improvements:

* Safe object finalization (**PEP 442**).

* Leveraging **PEP 442**, in most cases module globals are no longer
  set to None during finalization (bpo-18214).

* Configurable memory allocators (**PEP 445**).

* Argument Clinic (**PEP 436**).

Please read on for a comprehensive list of user-facing changes,
including many other smaller improvements, CPython optimizations,
deprecations, and potential porting issues.


New Features
============


PEP 453: Explicit Bootstrapping of PIP in Python Installations
--------------------------------------------------------------


Bootstrapping pip By Default
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The new "ensurepip" module (defined in **PEP 453**) provides a
standard cross-platform mechanism to bootstrap the pip installer into
Python installations and virtual environments. The version of "pip"
included with Python 3.4.0 is "pip" 1.5.4, and future 3.4.x
maintenance releases will update the bundled version to the latest
version of "pip" that is available at the time of creating the release
candidate.

By default, the commands "pipX" and "pipX.Y" will be installed on all
platforms (where X.Y stands for the version of the Python
installation), along with the "pip" Python package and its
dependencies. On Windows and in virtual environments on all platforms,
the unversioned "pip" command will also be installed. On other
platforms, the system wide unversioned "pip" command typically refers
to the separately installed Python 2 version.

The "pyvenv" command line utility and the "venv" module make use of
the "ensurepip" module to make "pip" readily available in virtual
environments. When using the command line utility, "pip" is installed
by default, while when using the "venv" module API installation of
"pip" must be requested explicitly.

For CPython source builds on POSIX systems, the "make install" and
"make altinstall" commands bootstrap "pip" by default.  This behaviour
can be controlled through configure options, and overridden through
Makefile options.

On Windows and Mac OS X, the CPython installers now default to
installing "pip" along with CPython itself (users may opt out of
installing it during the installation process). Window users will need
to opt in to the automatic "PATH" modifications to have "pip"
available from the command line by default, otherwise it can still be
accessed through the Python launcher for Windows as "py -m pip".

As **discussed in the PEP** platform packagers may choose not to
install these commands by default, as long as, when invoked, they
provide clear and simple directions on how to install them on that
platform (usually using the system package manager).

Note:

  To avoid conflicts between parallel Python 2 and Python 3
  installations, only the versioned "pip3" and "pip3.4" commands are
  bootstrapped by default when "ensurepip" is invoked directly - the "
  --default-pip" option is needed to also request the unversioned
  "pip" command. "pyvenv" and the Windows installer ensure that the
  unqualified "pip" command is made available in those environments,
  and "pip" can always be invoked via the "-m" switch rather than
  directly to avoid ambiguity on systems with multiple Python
  installations.


Documentation Changes
~~~~~~~~~~~~~~~~~~~~~

As part of this change, the Installing Python Modules and Distributing
Python Modules sections of the documentation have been completely
redesigned as short getting started and FAQ documents. Most packaging
documentation has now been moved out to the Python Packaging Authority
maintained Python Packaging User Guide and the documentation of the
individual projects.

However, as this migration is currently still incomplete, the legacy
versions of those guides remaining available as Building C and C++
Extensions with setuptools and Building C and C++ Extensions with
setuptools.

See also:

  **PEP 453** – Explicit bootstrapping of pip in Python installations
     PEP written by Donald Stufft and Nick Coghlan, implemented by
     Donald Stufft, Nick Coghlan, Martin von Löwis and Ned Deily.


PEP 446: Newly Created File Descriptors Are Non-Inheritable
-----------------------------------------------------------

**PEP 446** makes newly created file descriptors non-inheritable.  In
general, this is the behavior an application will want: when launching
a new process, having currently open files also open in the new
process can lead to all sorts of hard to find bugs, and potentially to
security issues.

However, there are occasions when inheritance is desired.  To support
these cases, the following new functions and methods are available:

* "os.get_inheritable()", "os.set_inheritable()"

* "os.get_handle_inheritable()", "os.set_handle_inheritable()"

* "socket.socket.get_inheritable()", "socket.socket.set_inheritable()"

See also:

  **PEP 446** – Make newly created file descriptors non-inheritable
     PEP written and implemented by Victor Stinner.


Improvements to Codec Handling
------------------------------

Since it was first introduced, the "codecs" module has always been
intended to operate as a type-neutral dynamic encoding and decoding
system. However, its close coupling with the Python text model,
especially the type restricted convenience methods on the builtin
"str", "bytes" and "bytearray" types, has historically obscured that
fact.

As a key step in clarifying the situation, the "codecs.encode()" and
"codecs.decode()" convenience functions are now properly documented in
Python 2.7, 3.3 and 3.4. These functions have existed in the "codecs"
module (and have been covered by the regression test suite) since
Python 2.4, but were previously only discoverable through runtime
introspection.

Unlike the convenience methods on "str", "bytes" and "bytearray", the
"codecs" convenience functions support arbitrary codecs in both Python
2 and Python 3, rather than being limited to Unicode text encodings
(in Python 3) or "basestring" <-> "basestring" conversions (in Python
2).

In Python 3.4, the interpreter is able to identify the known non-text
encodings provided in the standard library and direct users towards
these general purpose convenience functions when appropriate:

   >>> b"abcdef".decode("hex")
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   LookupError: 'hex' is not a text encoding; use codecs.decode() to handle arbitrary codecs

   >>> "hello".encode("rot13")
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   LookupError: 'rot13' is not a text encoding; use codecs.encode() to handle arbitrary codecs

   >>> open("foo.txt", encoding="hex")
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   LookupError: 'hex' is not a text encoding; use codecs.open() to handle arbitrary codecs

In a related change, whenever it is feasible without breaking
backwards compatibility, exceptions raised during encoding and
decoding operations are wrapped in a chained exception of the same
type that mentions the name of the codec responsible for producing the
error:

   >>> import codecs

   >>> codecs.decode(b"abcdefgh", "hex")
   Traceback (most recent call last):
     File "/usr/lib/python3.4/encodings/hex_codec.py", line 20, in hex_decode
       return (binascii.a2b_hex(input), len(input))
   binascii.Error: Non-hexadecimal digit found

   The above exception was the direct cause of the following exception:

   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   binascii.Error: decoding with 'hex' codec failed (Error: Non-hexadecimal digit found)

   >>> codecs.encode("hello", "bz2")
   Traceback (most recent call last):
     File "/usr/lib/python3.4/encodings/bz2_codec.py", line 17, in bz2_encode
       return (bz2.compress(input), len(input))
     File "/usr/lib/python3.4/bz2.py", line 498, in compress
       return comp.compress(data) + comp.flush()
   TypeError: 'str' does not support the buffer interface

   The above exception was the direct cause of the following exception:

   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   TypeError: encoding with 'bz2' codec failed (TypeError: 'str' does not support the buffer interface)

Finally, as the examples above show, these improvements have permitted
the restoration of the convenience aliases for the non-Unicode codecs
that were themselves restored in Python 3.2. This means that encoding
binary data to and from its hexadecimal representation (for example)
can now be written as:

   >>> from codecs import encode, decode
   >>> encode(b"hello", "hex")
   b'68656c6c6f'
   >>> decode(b"68656c6c6f", "hex")
   b'hello'

The binary and text transforms provided in the standard library are
detailed in Binary Transforms and Text Transforms.

(Contributed by Nick Coghlan in bpo-7475, bpo-17827, bpo-17828 and
bpo-19619.)


PEP 451: A ModuleSpec Type for the Import System
------------------------------------------------

**PEP 451** provides an encapsulation of the information about a
module that the import machinery will use to load it (that is, a
module specification).  This helps simplify both the import
implementation and several import-related APIs. The change is also a
stepping stone for several future import-related improvements.

The public-facing changes from the PEP are entirely backward-
compatible. Furthermore, they should be transparent to everyone but
importer authors.  Key finder and loader methods have been deprecated,
but they will continue working. New importers should use the new
methods described in the PEP.  Existing importers should be updated to
implement the new methods.  See the Deprecated section for a list of
methods that should be replaced and their replacements.


Other Language Changes
----------------------

Some smaller changes made to the core Python language are:

* Unicode database updated to UCD version 6.3.

* "min()" and "max()" now accept a *default* keyword-only argument
  that can be used to specify the value they return if the iterable
  they are evaluating has no elements.  (Contributed by Julian Berman
  in bpo-18111.)

* Module objects are now weakly referenceable.

* Module "__file__" attributes (and related values) should now always
  contain absolute paths by default, with the sole exception of
  "__main__.__file__" when a script has been executed directly using a
  relative path.  (Contributed by Brett Cannon in bpo-18416.)

* All the UTF-* codecs (except UTF-7) now reject surrogates during
  both encoding and decoding unless the "surrogatepass" error handler
  is used, with the exception of the UTF-16 decoder (which accepts
  valid surrogate pairs) and the UTF-16 encoder (which produces them
  while encoding non-BMP characters). (Contributed by Victor Stinner,
  Kang-Hao (Kenny) Lu and Serhiy Storchaka in bpo-12892.)

* New German EBCDIC codec "cp273".  (Contributed by Michael Bierenfeld
  and Andrew Kuchling in bpo-1097797.)

* New Ukrainian codec "cp1125".  (Contributed by Serhiy Storchaka in
  bpo-19668.)

* "bytes".join() and "bytearray".join() now accept arbitrary buffer
  objects as arguments.  (Contributed by Antoine Pitrou in bpo-15958.)

* The "int" constructor now accepts any object that has an "__index__"
  method for its *base* argument.  (Contributed by Mark Dickinson in
  bpo-16772.)

* Frame objects now have a "clear()" method that clears all references
  to local variables from the frame.  (Contributed by Antoine Pitrou
  in bpo-17934.)

* "memoryview" is now registered as a "Sequence", and supports the
  "reversed()" builtin.  (Contributed by Nick Coghlan and Claudiu Popa
  in bpo-18690 and bpo-19078.)

* Signatures reported by "help()" have been modified and improved in
  several cases as a result of the introduction of Argument Clinic and
  other changes to the "inspect" and "pydoc" modules.

* "__length_hint__()" is now part of the formal language specification
  (see **PEP 424**).  (Contributed by Armin Ronacher in bpo-16148.)


New Modules
===========


asyncio
-------

The new "asyncio" module (defined in **PEP 3156**) provides a standard
pluggable event loop model for Python, providing solid asynchronous IO
support in the standard library, and making it easier for other event
loop implementations to interoperate with the standard library and
each other.

For Python 3.4, this module is considered a *provisional API*.

See also:

  **PEP 3156** – Asynchronous IO Support Rebooted: the “asyncio”
  Module
     PEP written and implementation led by Guido van Rossum.


ensurepip
---------

The new "ensurepip" module is the primary infrastructure for the **PEP
453** implementation.  In the normal course of events end users will
not need to interact with this module, but it can be used to manually
bootstrap "pip" if the automated bootstrapping into an installation or
virtual environment was declined.

"ensurepip" includes a bundled copy of "pip", up-to-date as of the
first release candidate of the release of CPython with which it ships
(this applies to both maintenance releases and feature releases).
"ensurepip" does not access the internet.  If the installation has
internet access, after "ensurepip" is run the bundled "pip" can be
used to upgrade "pip" to a more recent release than the bundled one.
(Note that such an upgraded version of "pip" is considered to be a
separately installed package and will not be removed if Python is
uninstalled.)

The module is named *ensure*pip because if called when "pip" is
already installed, it does nothing.  It also has an "--upgrade" option
that will cause it to install the bundled copy of "pip" if the
existing installed version of "pip" is older than the bundled copy.


enum
----

The new "enum" module (defined in **PEP 435**) provides a standard
implementation of enumeration types, allowing other modules (such as
"socket") to provide more informative error messages and better
debugging support by replacing opaque integer constants with backwards
compatible enumeration values.

See also:

  **PEP 435** – Adding an Enum type to the Python standard library
     PEP written by Barry Warsaw, Eli Bendersky and Ethan Furman,
     implemented by Ethan Furman.


pathlib
-------

The new "pathlib" module offers classes representing filesystem paths
with semantics appropriate for different operating systems.  Path
classes are divided between *pure paths*, which provide purely
computational operations without I/O, and *concrete paths*, which
inherit from pure paths but also provide I/O operations.

For Python 3.4, this module is considered a *provisional API*.

See also:

  **PEP 428** – The pathlib module – object-oriented filesystem paths
     PEP written and implemented by Antoine Pitrou.


selectors
---------

The new "selectors" module (created as part of implementing **PEP
3156**) allows high-level and efficient I/O multiplexing, built upon
the "select" module primitives.


statistics
----------

The new "statistics" module (defined in **PEP 450**) offers some core
statistics functionality directly in the standard library. This module
supports calculation of the mean, median, mode, variance and standard
deviation of a data series.

See also:

  **PEP 450** – Adding A Statistics Module To The Standard Library
     PEP written and implemented by Steven D’Aprano


tracemalloc
-----------

The new "tracemalloc" module (defined in **PEP 454**) is a debug tool
to trace memory blocks allocated by Python. It provides the following
information:

* Trace where an object was allocated

* Statistics on allocated memory blocks per filename and per line
  number: total size, number and average size of allocated memory
  blocks

* Compute the differences between two snapshots to detect memory leaks

See also:

  **PEP 454** – Add a new tracemalloc module to trace Python memory
  allocations
     PEP written and implemented by Victor Stinner


Improved Modules
================


abc
---

New function "abc.get_cache_token()" can be used to know when to
invalidate caches that are affected by changes in the object graph.
(Contributed by Łukasz Langa in bpo-16832.)

New class "ABC" has "ABCMeta" as its meta class. Using "ABC" as a base
class has essentially the same effect as specifying
"metaclass=abc.ABCMeta", but is simpler to type and easier to read.
(Contributed by Bruno Dupuis in bpo-16049.)


aifc
----

The "getparams()" method now returns a namedtuple rather than a plain
tuple.  (Contributed by Claudiu Popa in bpo-17818.)

"aifc.open()" now supports the context management protocol: when used
in a "with" block, the "close()" method of the returned object will be
called automatically at the end of the block.  (Contributed by Serhiy
Storchacha in bpo-16486.)

The "writeframesraw()" and "writeframes()" methods now accept any
*bytes-like object*.  (Contributed by Serhiy Storchaka in bpo-8311.)


argparse
--------

The "FileType" class now accepts *encoding* and *errors* arguments,
which are passed through to "open()".  (Contributed by Lucas Maystre
in bpo-11175.)


audioop
-------

"audioop" now supports 24-bit samples.  (Contributed by Serhiy
Storchaka in bpo-12866.)

New "byteswap()" function converts big-endian samples to little-endian
and vice versa.  (Contributed by Serhiy Storchaka in bpo-19641.)

All "audioop" functions now accept any *bytes-like object*.  Strings
are not accepted: they didn’t work before, now they raise an error
right away. (Contributed by Serhiy Storchaka in bpo-16685.)


base64
------

The encoding and decoding functions in "base64" now accept any *bytes-
like object* in cases where it previously required a "bytes" or
"bytearray" instance.  (Contributed by Nick Coghlan in bpo-17839.)

New functions "a85encode()", "a85decode()", "b85encode()", and
"b85decode()" provide the ability to encode and decode binary data
from and to "Ascii85" and the git/mercurial "Base85" formats,
respectively.  The "a85" functions have options that can be used to
make them compatible with the variants of the "Ascii85" encoding,
including the Adobe variant.  (Contributed by Martin Morrison, the
Mercurial project, Serhiy Storchaka, and Antoine Pitrou in bpo-17618.)


collections
-----------

The "ChainMap.new_child()" method now accepts an *m* argument
specifying the child map to add to the chain.  This allows an existing
mapping and/or a custom mapping type to be used for the child.
(Contributed by Vinay Sajip in bpo-16613.)


colorsys
--------

The number of digits in the coefficients for the RGB — YIQ conversions
have been expanded so that they match the FCC NTSC versions.  The
change in results should be less than 1% and may better match results
found elsewhere. (Contributed by Brian Landers and Serhiy Storchaka in
bpo-14323.)


contextlib
----------

The new "contextlib.suppress" context manager helps to clarify the
intent of code that deliberately suppresses exceptions from a single
statement.  (Contributed by Raymond Hettinger in bpo-15806 and Zero
Piraeus in bpo-19266.)

The new "contextlib.redirect_stdout()" context manager makes it easier
for utility scripts to handle inflexible APIs that write their output
to "sys.stdout" and don’t provide any options to redirect it.  Using
the context manager, the "sys.stdout" output can be redirected to any
other stream or, in conjunction with "io.StringIO", to a string. The
latter can be especially useful, for example, to capture output from a
function that was written to implement a command line interface. It is
recommended only for utility scripts because it affects the global
state of "sys.stdout".  (Contributed by Raymond Hettinger in
bpo-15805.)

The "contextlib" documentation has also been updated to include a
discussion of the differences between single use, reusable and
reentrant context managers.


dbm
---

"dbm.open()" objects now support the context management protocol.
When used in a "with" statement, the "close" method of the database
object will be called automatically at the end of the block.
(Contributed by Claudiu Popa and Nick Coghlan in bpo-19282.)


dis
---

Functions "show_code()", "dis()", "distb()", and "disassemble()" now
accept a keyword-only *file* argument that controls where they write
their output.

The "dis" module is now built around an "Instruction" class that
provides object oriented access to the details of each individual
bytecode operation.

A new method, "get_instructions()", provides an iterator that emits
the Instruction stream for a given piece of Python code.  Thus it is
now possible to write a program that inspects and manipulates a
bytecode object in ways different from those provided by the "dis"
module itself.  For example:

   >>> import dis
   >>> for instr in dis.get_instructions(lambda x: x + 1):
   ...     print(instr.opname)
   LOAD_FAST
   LOAD_CONST
   BINARY_ADD
   RETURN_VALUE

The various display tools in the "dis" module have been rewritten to
use these new components.

In addition, a new application-friendly class "Bytecode" provides an
object-oriented API for inspecting bytecode in both in human-readable
form and for iterating over instructions.  The "Bytecode" constructor
takes the same arguments that "get_instruction()" does (plus an
optional *current_offset*), and the resulting object can be iterated
to produce "Instruction" objects.  But it also has a "dis" method,
equivalent to calling "dis" on the constructor argument, but returned
as a multi-line string:

   >>> bytecode = dis.Bytecode(lambda x: x + 1, current_offset=3)
   >>> for instr in bytecode:
   ...     print('{} ({})'.format(instr.opname, instr.opcode))
   LOAD_FAST (124)
   LOAD_CONST (100)
   BINARY_ADD (23)
   RETURN_VALUE (83)
   >>> bytecode.dis().splitlines()
   ['  1           0 LOAD_FAST                0 (x)',
    '      -->     3 LOAD_CONST               1 (1)',
    '              6 BINARY_ADD',
    '              7 RETURN_VALUE']

"Bytecode" also has a class method, "from_traceback()", that provides
the ability to manipulate a traceback (that is,
"print(Bytecode.from_traceback(tb).dis())" is equivalent to
"distb(tb)").

(Contributed by Nick Coghlan, Ryan Kelly and Thomas Kluyver in
bpo-11816 and Claudiu Popa in bpo-17916.)

New function "stack_effect()" computes the effect on the Python stack
of a given opcode and argument, information that is not otherwise
available. (Contributed by Larry Hastings in bpo-19722.)


doctest
-------

A new option flag, "FAIL_FAST", halts test running as soon as the
first failure is detected.  (Contributed by R. David Murray and Daniel
Urban in bpo-16522.)

The "doctest" command line interface now uses "argparse", and has two
new options, "-o" and "-f".  "-o" allows doctest options to be
specified on the command line, and "-f" is a shorthand for "-o
FAIL_FAST" (to parallel the similar option supported by the "unittest"
CLI).  (Contributed by R. David Murray in bpo-11390.)

"doctest" will now find doctests in extension module "__doc__"
strings. (Contributed by Zachary Ware in bpo-3158.)


email
-----

"as_string()" now accepts a *policy* argument to override the default
policy of the message when generating a string representation of it.
This means that "as_string" can now be used in more circumstances,
instead of having to create and use a "generator" in order to pass
formatting parameters to its "flatten" method.  (Contributed by R.
David Murray in bpo-18600.)

New method "as_bytes()" added to produce a bytes representation of the
message in a fashion similar to how "as_string" produces a string
representation.  It does not accept the *maxheaderlen* argument, but
does accept the *unixfrom* and *policy* arguments. The "Message"
"__bytes__()" method calls it, meaning that "bytes(mymsg)" will now
produce the intuitive result:  a bytes object containing the fully
formatted message.  (Contributed by R. David Murray in bpo-18600.)

The "Message.set_param()" message now accepts a *replace* keyword
argument. When specified, the associated header will be updated
without changing its location in the list of headers.  For backward
compatibility, the default is "False".  (Contributed by R. David
Murray in bpo-18891.)

A pair of new subclasses of "Message" have been added ("EmailMessage"
and "MIMEPart"), along with a new sub-module, "contentmanager" and a
new "policy" attribute "content_manager".  All documentation is
currently in the new module, which is being added as part of email’s
new *provisional API*.  These classes provide a number of new methods
that make extracting content from and inserting content into email
messages much easier.  For details, see the "contentmanager"
documentation and the email: Examples.  These API additions complete
the bulk of the work that was planned as part of the email6 project.
The currently provisional API is scheduled to become final in Python
3.5 (possibly with a few minor additions in the area of error
handling).  (Contributed by R. David Murray in bpo-18891.)


filecmp
-------

A new "clear_cache()" function provides the ability to clear the
"filecmp" comparison cache, which uses "os.stat()" information to
determine if the file has changed since the last compare.  This can be
used, for example, if the file might have been changed and re-checked
in less time than the resolution of a particular filesystem’s file
modification time field. (Contributed by Mark Levitt in bpo-18149.)

New module attribute "DEFAULT_IGNORES" provides the list of
directories that are used as the default value for the *ignore*
parameter of the "dircmp()" function.  (Contributed by Eli Bendersky
in bpo-15442.)


functools
---------

The new "partialmethod()" descriptor brings partial argument
application to descriptors, just as "partial()" provides for normal
callables. The new descriptor also makes it easier to get arbitrary
callables (including "partial()" instances) to behave like normal
instance methods when included in a class definition. (Contributed by
Alon Horev and Nick Coghlan in bpo-4331.)

The new "singledispatch()" decorator brings support for single-
dispatch generic functions to the Python standard library. Where
object oriented programming focuses on grouping multiple operations on
a common set of data into a class, a generic function focuses on
grouping multiple implementations of an operation that allows it to
work with *different* kinds of data.

See also:

  **PEP 443** – Single-dispatch generic functions
     PEP written and implemented by Łukasz Langa.

"total_ordering()" now supports a return value of "NotImplemented"
from the underlying comparison function.  (Contributed by Katie Miller
in bpo-10042.)

A pure-python version of the "partial()" function is now in the
stdlib; in CPython it is overridden by the C accelerated version, but
it is available for other implementations to use.  (Contributed by
Brian Thorne in bpo-12428.)


gc
--

New function "get_stats()" returns a list of three per-generation
dictionaries containing the collections statistics since interpreter
startup. (Contributed by Antoine Pitrou in bpo-16351.)


glob
----

A new function "escape()" provides a way to escape special characters
in a filename so that they do not become part of the globbing
expansion but are instead matched literally.  (Contributed by Serhiy
Storchaka in bpo-8402.)


hashlib
-------

A new "hashlib.pbkdf2_hmac()" function provides the PKCS#5 password-
based key derivation function 2.  (Contributed by Christian Heimes in
bpo-18582.)

The "name" attribute of "hashlib" hash objects is now a formally
supported interface.  It has always existed in CPython’s "hashlib"
(although it did not return lower case names for all supported
hashes), but it was not a public interface and so some other Python
implementations have not previously supported it.  (Contributed by
Jason R. Coombs in bpo-18532.)


hmac
----

"hmac" now accepts "bytearray" as well as "bytes" for the *key*
argument to the "new()" function, and the *msg* parameter to both the
"new()" function and the "update()" method now accepts any type
supported by the "hashlib" module.  (Contributed by Jonas Borgström in
bpo-18240.)

The *digestmod* argument to the "hmac.new()" function may now be any
hash digest name recognized by "hashlib".  In addition, the current
behavior in which the value of *digestmod* defaults to "MD5" is
deprecated: in a future version of Python there will be no default
value.  (Contributed by Christian Heimes in bpo-17276.)

With the addition of "block_size" and "name" attributes (and the
formal documentation of the "digest_size" attribute), the "hmac"
module now conforms fully to the **PEP 247** API. (Contributed by
Christian Heimes in bpo-18775.)


html
----

New function "unescape()" function converts HTML5 character references
to the corresponding Unicode characters.  (Contributed by Ezio Melotti
in bpo-2927.)

"HTMLParser" accepts a new keyword argument *convert_charrefs* that,
when "True", automatically converts all character references.  For
backward-compatibility, its value defaults to "False", but it will
change to "True" in a future version of Python, so you are invited to
set it explicitly and update your code to use this new feature.
(Contributed by Ezio Melotti in bpo-13633.)

The *strict* argument of "HTMLParser" is now deprecated. (Contributed
by Ezio Melotti in bpo-15114.)


http
----

"send_error()" now accepts an optional additional *explain* parameter
which can be used to provide an extended error description, overriding
the hardcoded default if there is one. This extended error description
will be formatted using the "error_message_format" attribute and sent
as the body of the error response.  (Contributed by Karl Cow in
bpo-12921.)

The "http.server" command line interface now has a "-b/--bind" option
that causes the server to listen on a specific address. (Contributed
by Malte Swart in bpo-17764.)


idlelib and IDLE
----------------

Since idlelib implements the IDLE shell and editor and is not intended
for import by other programs, it gets improvements with every release.
See "Lib/idlelib/NEWS.txt" for a cumulative list of changes since
3.3.0, as well as changes made in future 3.4.x releases. This file is
also available from the IDLE Help ‣ About IDLE dialog.


importlib
---------

The "InspectLoader" ABC defines a new method, "source_to_code()" that
accepts source data and a path and returns a code object.  The default
implementation is equivalent to "compile(data, path, 'exec',
dont_inherit=True)". (Contributed by Eric Snow and Brett Cannon in
bpo-15627.)

"InspectLoader" also now has a default implementation for the
"get_code()" method.  However, it will normally be desirable to
override the default implementation for performance reasons.
(Contributed by Brett Cannon in bpo-18072.)

The "reload()" function has been moved from "imp" to "importlib" as
part of the "imp" module deprecation.  (Contributed by Berker Peksag
in bpo-18193.)

"importlib.util" now has a "MAGIC_NUMBER" attribute providing access
to the bytecode version number.  This replaces the "get_magic()"
function in the deprecated "imp" module. (Contributed by Brett Cannon
in bpo-18192.)

New "importlib.util" functions "cache_from_source()" and
"source_from_cache()" replace the same-named functions in the
deprecated "imp" module.  (Contributed by Brett Cannon in bpo-18194.)

The "importlib" bootstrap "NamespaceLoader" now conforms to the
"InspectLoader" ABC, which means that "runpy" and "python -m" can now
be used with namespace packages.  (Contributed by Brett Cannon in
bpo-18058.)

"importlib.util" has a new function "decode_source()" that decodes
source from bytes using universal newline processing.  This is useful
for implementing "InspectLoader.get_source()" methods.

"importlib.machinery.ExtensionFileLoader" now has a "get_filename()"
method.  This was inadvertently omitted in the original
implementation.  (Contributed by Eric Snow in bpo-19152.)


inspect
-------

The "inspect" module now offers a basic command line interface to
quickly display source code and other information for modules, classes
and functions.  (Contributed by Claudiu Popa and Nick Coghlan in
bpo-18626.)

"unwrap()" makes it easy to unravel wrapper function chains created by
"functools.wraps()" (and any other API that sets the "__wrapped__"
attribute on a wrapper function).  (Contributed by Daniel Urban, Aaron
Iles and Nick Coghlan in bpo-13266.)

As part of the implementation of the new "enum" module, the "inspect"
module now has substantially better support for custom "__dir__"
methods and dynamic class attributes provided through metaclasses.
(Contributed by Ethan Furman in bpo-18929 and bpo-19030.)

"getfullargspec()" and "getargspec()" now use the "signature()" API.
This allows them to support a much broader range of callables,
including those with "__signature__" attributes, those with metadata
provided by argument clinic, "functools.partial()" objects and more.
Note that, unlike "signature()", these functions still ignore
"__wrapped__" attributes, and report the already bound first argument
for bound methods, so it is still necessary to update your code to use
"signature()" directly if those features are desired. (Contributed by
Yury Selivanov in bpo-17481.)

"signature()" now supports duck types of CPython functions, which adds
support for functions compiled with Cython.  (Contributed by Stefan
Behnel and Yury Selivanov in bpo-17159.)


ipaddress
---------

"ipaddress" was added to the standard library in Python 3.3 as a
*provisional API*. With the release of Python 3.4, this qualification
has been removed: "ipaddress" is now considered a stable API, covered
by the normal standard library requirements to maintain backwards
compatibility.

A new "is_global" property is "True" if an address is globally
routeable.  (Contributed by Peter Moody in bpo-17400.)


logging
-------

The "TimedRotatingFileHandler" has a new *atTime* parameter that can
be used to specify the time of day when rollover should happen.
(Contributed by Ronald Oussoren in bpo-9556.)

"SocketHandler" and "DatagramHandler" now support Unix domain sockets
(by setting *port* to "None").  (Contributed by Vinay Sajip in commit
ce46195b56a9.)

"fileConfig()" now accepts a "configparser.RawConfigParser" subclass
instance for the *fname* parameter.  This facilitates using a
configuration file when logging configuration is just a part of the
overall application configuration, or where the application modifies
the configuration before passing it to "fileConfig()".  (Contributed
by Vinay Sajip in bpo-16110.)

Logging configuration data received from a socket via the
"logging.config.listen()" function can now be validated before being
processed by supplying a verification function as the argument to the
new *verify* keyword argument.  (Contributed by Vinay Sajip in
bpo-15452.)


marshal
-------

The default "marshal" version has been bumped to 3.  The code
implementing the new version restores the Python2 behavior of
recording only one copy of interned strings and preserving the
interning on deserialization, and extends this “one copy” ability to
any object type (including handling recursive references).  This
reduces both the size of ".pyc" files and the amount of memory a
module occupies in memory when it is loaded from a ".pyc" (or ".pyo")
file.  (Contributed by Kristján Valur Jónsson in bpo-16475, with
additional speedups by Antoine Pitrou in bpo-19219.)


mmap
----

mmap objects are now weakly referenceable. (Contributed by Valerie
Lambert in bpo-4885.)


multiprocessing
---------------

On Unix two new start methods, "spawn" and "forkserver", have been
added for starting processes using "multiprocessing".  These make the
mixing of processes with threads more robust, and the "spawn" method
matches the semantics that multiprocessing has always used on Windows.
New function "get_all_start_methods()" reports all start methods
available on the platform, "get_start_method()" reports the current
start method, and "set_start_method()" sets the start method.
(Contributed by Richard Oudkerk in bpo-8713.)

"multiprocessing" also now has the concept of a "context", which
determines how child processes are created.  New function
"get_context()" returns a context that uses a specified start method.
It has the same API as the "multiprocessing" module itself, so you can
use it to create "Pool"s and other objects that will operate within
that context.  This allows a framework and an application or different
parts of the same application to use multiprocessing without
interfering with each other.  (Contributed by Richard Oudkerk in
bpo-18999.)

Except when using the old *fork* start method, child processes no
longer inherit unneeded handles/file descriptors from their parents
(part of bpo-8713).

"multiprocessing" now relies on "runpy" (which implements the "-m"
switch) to initialise "__main__" appropriately in child processes when
using the "spawn" or "forkserver" start methods. This resolves some
edge cases where combining multiprocessing, the "-m" command line
switch, and explicit relative imports could cause obscure failures in
child processes.  (Contributed by Nick Coghlan in bpo-19946.)


operator
--------

New function "length_hint()" provides an implementation of the
specification for how the "__length_hint__()" special method should be
used, as part of the **PEP 424** formal specification of this language
feature.  (Contributed by Armin Ronacher in bpo-16148.)

There is now a pure-python version of the "operator" module available
for reference and for use by alternate implementations of Python.
(Contributed by Zachary Ware in bpo-16694.)


os
--

There are new functions to get and set the inheritable flag of a file
descriptor ("os.get_inheritable()", "os.set_inheritable()") or a
Windows handle ("os.get_handle_inheritable()",
"os.set_handle_inheritable()").

New function "cpu_count()" reports the number of CPUs available on the
platform on which Python is running (or "None" if the count can’t be
determined).  The "multiprocessing.cpu_count()" function is now
implemented in terms of this function).  (Contributed by Trent Nelson,
Yogesh Chaudhari, Victor Stinner, and Charles-François Natali in
bpo-17914.)

"os.path.samestat()" is now available on the Windows platform (and the
"os.path.samefile()" implementation is now shared between Unix and
Windows).  (Contributed by Brian Curtin in bpo-11939.)

"os.path.ismount()" now recognizes volumes mounted below a drive root
on Windows.  (Contributed by Tim Golden in bpo-9035.)

"os.open()" supports two new flags on platforms that provide them,
"O_PATH" (un-opened file descriptor), and "O_TMPFILE" (unnamed
temporary file; as of 3.4.0 release available only on Linux systems
with a kernel version of 3.11 or newer that have uapi headers).
(Contributed by Christian Heimes in bpo-18673 and Benjamin Peterson,
respectively.)


pdb
---

"pdb" has been enhanced to handle generators, "yield", and "yield
from" in a more useful fashion.  This is especially helpful when
debugging "asyncio" based programs.  (Contributed by Andrew Svetlov
and Xavier de Gaye in bpo-16596.)

The "print" command has been removed from "pdb", restoring access to
the Python "print()" function from the pdb command line.  Python2’s
"pdb" did not have a "print" command; instead, entering "print"
executed the "print" statement.  In Python3 "print" was mistakenly
made an alias for the pdb "p" command.  "p", however, prints the
"repr" of its argument, not the "str" like the Python2 "print" command
did.  Worse, the Python3 "pdb print" command shadowed the Python3
"print" function, making it inaccessible at the "pdb" prompt.
(Contributed by Connor Osborn in bpo-18764.)


pickle
------

"pickle" now supports (but does not use by default) a new pickle
protocol, protocol 4.  This new protocol addresses a number of issues
that were present in previous protocols, such as the serialization of
nested classes, very large strings and containers, and classes whose
"__new__()" method takes keyword-only arguments.  It also provides
some efficiency improvements.

See also:

  **PEP 3154** – Pickle protocol 4
     PEP written by Antoine Pitrou and implemented by Alexandre
     Vassalotti.


plistlib
--------

"plistlib" now has an API that is similar to the standard pattern for
stdlib serialization protocols, with new "load()", "dump()",
"loads()", and "dumps()" functions.  (The older API is now
deprecated.)  In addition to the already supported XML plist format
("FMT_XML"), it also now supports the binary plist format
("FMT_BINARY").  (Contributed by Ronald Oussoren and others in
bpo-14455.)


poplib
------

Two new methods have been added to "poplib": "capa()", which returns
the list of capabilities advertised by the POP server, and "stls()",
which switches a clear-text POP3 session into an encrypted POP3
session if the POP server supports it.  (Contributed by Lorenzo
Catucci in bpo-4473.)


pprint
------

The "pprint" module’s "PrettyPrinter" class and its "pformat()", and
"pprint()" functions have a new option, *compact*, that controls how
the output is formatted.  Currently setting *compact* to "True" means
that sequences will be printed with as many sequence elements as will
fit within *width* on each (indented) line. (Contributed by Serhiy
Storchaka in bpo-19132.)

Long strings are now wrapped using Python’s normal line continuation
syntax.  (Contributed by Antoine Pitrou in bpo-17150.)


pty
---

"pty.spawn()" now returns the status value from "os.waitpid()" on the
child process, instead of "None".  (Contributed by Gregory P. Smith.)


pydoc
-----

The "pydoc" module is now based directly on the "inspect.signature()"
introspection API, allowing it to provide signature information for a
wider variety of callable objects. This change also means that
"__wrapped__" attributes are now taken into account when displaying
help information. (Contributed by Larry Hastings in bpo-19674.)

The "pydoc" module no longer displays the "self" parameter for already
bound methods. Instead, it aims to always display the exact current
signature of the supplied callable.  (Contributed by Larry Hastings in
bpo-20710.)

In addition to the changes that have been made to "pydoc" directly,
its handling of custom "__dir__" methods and various descriptor
behaviours has also been improved substantially by the underlying
changes in the "inspect" module.

As the "help()" builtin is based on "pydoc", the above changes also
affect the behaviour of "help()".


re
--

New "fullmatch()" function and "regex.fullmatch()" method anchor the
pattern at both ends of the string to match.  This provides a way to
be explicit about the goal of the match, which avoids a class of
subtle bugs where "$" characters get lost during code changes or the
addition of alternatives to an existing regular expression.
(Contributed by Matthew Barnett in bpo-16203.)

The repr of regex objects now includes the pattern and the flags; the
repr of match objects now includes the start, end, and the part of the
string that matched.  (Contributed by Hugo Lopes Tavares and Serhiy
Storchaka in bpo-13592 and bpo-17087.)


resource
--------

New "prlimit()" function, available on Linux platforms with a kernel
version of 2.6.36 or later and glibc of 2.13 or later, provides the
ability to query or set the resource limits for processes other than
the one making the call.  (Contributed by Christian Heimes in
bpo-16595.)

On Linux kernel version 2.6.36 or later, there are also some new Linux
specific constants: "RLIMIT_MSGQUEUE", "RLIMIT_NICE", "RLIMIT_RTPRIO",
"RLIMIT_RTTIME", and "RLIMIT_SIGPENDING". (Contributed by Christian
Heimes in bpo-19324.)

On FreeBSD version 9 and later, there some new FreeBSD specific
constants: "RLIMIT_SBSIZE", "RLIMIT_SWAP", and "RLIMIT_NPTS".
(Contributed by Claudiu Popa in bpo-19343.)


select
------

"epoll" objects now support the context management protocol. When used
in a "with" statement, the "close()" method will be called
automatically at the end of the block.  (Contributed by Serhiy
Storchaka in bpo-16488.)

"devpoll" objects now have "fileno()" and "close()" methods, as well
as a new attribute "closed".  (Contributed by Victor Stinner in
bpo-18794.)


shelve
------

"Shelf" instances may now be used in "with" statements, and will be
automatically closed at the end of the "with" block. (Contributed by
Filip Gruszczyński in bpo-13896.)


shutil
------

"copyfile()" now raises a specific "Error" subclass, "SameFileError",
when the source and destination are the same file, which allows an
application to take appropriate action on this specific error.
(Contributed by Atsuo Ishimoto and Hynek Schlawack in bpo-1492704.)


smtpd
-----

The "SMTPServer" and "SMTPChannel" classes now accept a *map* keyword
argument which, if specified, is passed in to "asynchat.async_chat" as
its *map* argument.  This allows an application to avoid affecting the
global socket map.  (Contributed by Vinay Sajip in bpo-11959.)


smtplib
-------

"SMTPException" is now a subclass of "OSError", which allows both
socket level errors and SMTP protocol level errors to be caught in one
try/except statement by code that only cares whether or not an error
occurred. (Contributed by Ned Jackson Lovely in bpo-2118.)


socket
------

The socket module now supports the "CAN_BCM" protocol on platforms
that support it.  (Contributed by Brian Thorne in bpo-15359.)

Socket objects have new methods to get or set their inheritable flag,
"get_inheritable()" and "set_inheritable()".

The "socket.AF_*" and "socket.SOCK_*" constants are now enumeration
values using the new "enum" module.  This allows meaningful names to
be printed during debugging, instead of integer “magic numbers”.

The "AF_LINK" constant is now available on BSD and OSX.

"inet_pton()" and "inet_ntop()" are now supported on Windows.
(Contributed by Atsuo Ishimoto in bpo-7171.)


sqlite3
-------

A new boolean parameter to the "connect()" function, *uri*, can be
used to indicate that the *database* parameter is a "uri" (see the
SQLite URI documentation).  (Contributed by poq in bpo-13773.)


ssl
---

"PROTOCOL_TLSv1_1" and "PROTOCOL_TLSv1_2" (TLSv1.1 and TLSv1.2
support) have been added; support for these protocols is only
available if Python is linked with OpenSSL 1.0.1 or later.
(Contributed by Michele Orrù and Antoine Pitrou in bpo-16692.)

New function "create_default_context()" provides a standard way to
obtain an "SSLContext" whose settings are intended to be a reasonable
balance between compatibility and security.  These settings are more
stringent than the defaults provided by the "SSLContext" constructor,
and may be adjusted in the future, without prior deprecation, if best-
practice security requirements change.  The new recommended best
practice for using stdlib libraries that support SSL is to use
"create_default_context()" to obtain an "SSLContext" object, modify it
if needed, and then pass it as the *context* argument of the
appropriate stdlib API.  (Contributed by Christian Heimes in
bpo-19689.)

"SSLContext" method "load_verify_locations()" accepts a new optional
argument *cadata*, which can be used to provide PEM or DER encoded
certificates directly via strings or bytes, respectively. (Contributed
by Christian Heimes in bpo-18138.)

New function "get_default_verify_paths()" returns a named tuple of the
paths and environment variables that the "set_default_verify_paths()"
method uses to set OpenSSL’s default "cafile" and "capath".  This can
be an aid in debugging default verification issues.  (Contributed by
Christian Heimes in bpo-18143.)

"SSLContext" has a new method, "cert_store_stats()", that reports the
number of loaded "X.509" certs, "X.509 CA" certs, and certificate
revocation lists ("crl"s), as well as a "get_ca_certs()" method that
returns a list of the loaded "CA" certificates.  (Contributed by
Christian Heimes in bpo-18147.)

If OpenSSL 0.9.8 or later is available, "SSLContext" has a new
attribute "verify_flags" that can be used to control the certificate
verification process by setting it to some combination of the new
constants "VERIFY_DEFAULT", "VERIFY_CRL_CHECK_LEAF",
"VERIFY_CRL_CHECK_CHAIN", or "VERIFY_X509_STRICT". OpenSSL does not do
any CRL verification by default.  (Contributed by Christien Heimes in
bpo-8813.)

New "SSLContext" method "load_default_certs()" loads a set of default
“certificate authority” (CA) certificates from default locations,
which vary according to the platform.  It can be used to load both TLS
web server authentication certificates ("purpose=""SERVER_AUTH") for a
client to use to verify a server, and certificates for a server to use
in verifying client certificates ("purpose=""CLIENT_AUTH").
(Contributed by Christian Heimes in bpo-19292.)

Two new windows-only functions, "enum_certificates()" and
"enum_crls()" provide the ability to retrieve certificates,
certificate information, and CRLs from the Windows cert store.
(Contributed by Christian Heimes in bpo-17134.)

Support for server-side SNI (Server Name Indication) using the new
"ssl.SSLContext.set_servername_callback()" method. (Contributed by
Daniel Black in bpo-8109.)

The dictionary returned by "SSLSocket.getpeercert()" contains
additional "X509v3" extension items: "crlDistributionPoints",
"calIssuers", and "OCSP" URIs.  (Contributed by Christian Heimes in
bpo-18379.)


stat
----

The "stat" module is now backed by a C implementation in "_stat". A C
implementation is required as most of the values aren’t standardized
and are platform-dependent.  (Contributed by Christian Heimes in
bpo-11016.)

The module supports new "ST_MODE" flags, "S_IFDOOR", "S_IFPORT", and
"S_IFWHT".  (Contributed by Christian Hiemes in bpo-11016.)


struct
------

New function "iter_unpack" and a new "struct.Struct.iter_unpack()"
method on compiled formats provide streamed unpacking of a buffer
containing repeated instances of a given format of data. (Contributed
by Antoine Pitrou in bpo-17804.)


subprocess
----------

"check_output()" now accepts an *input* argument that can be used to
provide the contents of "stdin" for the command that is run.
(Contributed by Zack Weinberg in bpo-16624.)

"getstatus()" and "getstatusoutput()" now work on Windows.  This
change was actually inadvertently made in 3.3.4. (Contributed by Tim
Golden in bpo-10197.)


sunau
-----

The "getparams()" method now returns a namedtuple rather than a plain
tuple.  (Contributed by Claudiu Popa in bpo-18901.)

"sunau.open()" now supports the context management protocol: when used
in a "with" block, the "close" method of the returned object will be
called automatically at the end of the block.  (Contributed by Serhiy
Storchaka in bpo-18878.)

"AU_write.setsampwidth()" now supports 24 bit samples, thus adding
support for writing 24 sample using the module.  (Contributed by
Serhiy Storchaka in bpo-19261.)

The "writeframesraw()" and "writeframes()" methods now accept any
*bytes-like object*.  (Contributed by Serhiy Storchaka in bpo-8311.)


sys
---

New function "sys.getallocatedblocks()" returns the current number of
blocks allocated by the interpreter.  (In CPython with the default "--
with-pymalloc" setting, this is allocations made through the
"PyObject_Malloc()" API.)  This can be useful for tracking memory
leaks, especially if automated via a test suite.  (Contributed by
Antoine Pitrou in bpo-13390.)

When the Python interpreter starts in interactive mode, it checks for
an "__interactivehook__" attribute on the "sys" module.  If the
attribute exists, its value is called with no arguments just before
interactive mode is started.  The check is made after the
"PYTHONSTARTUP" file is read, so it can be set there.  The "site"
module sets it to a function that enables tab completion and history
saving (in "~/.python-history") if the platform supports "readline".
If you do not want this (new) behavior, you can override it in
"PYTHONSTARTUP", "sitecustomize", or "usercustomize" by deleting this
attribute from "sys" (or setting it to some other callable).
(Contributed by Éric Araujo and Antoine Pitrou in bpo-5845.)


tarfile
-------

The "tarfile" module now supports a simple Command-Line Interface when
called as a script directly or via "-m".  This can be used to create
and extract tarfile archives.  (Contributed by Berker Peksag in
bpo-13477.)


textwrap
--------

The "TextWrapper" class has two new attributes/constructor arguments:
"max_lines", which limits the number of lines in the output, and
"placeholder", which is a string that will appear at the end of the
output if it has been truncated because of *max_lines*.  Building on
these capabilities, a new convenience function "shorten()" collapses
all of the whitespace in the input to single spaces and produces a
single line of a given *width* that ends with the *placeholder* (by
default, "[...]").  (Contributed by Antoine Pitrou and Serhiy
Storchaka in bpo-18585 and bpo-18725.)


threading
---------

The "Thread" object representing the main thread can be obtained from
the new "main_thread()" function.  In normal conditions this will be
the thread from which the Python interpreter was started.
(Contributed by Andrew Svetlov in bpo-18882.)


traceback
---------

A new "traceback.clear_frames()" function takes a traceback object and
clears the local variables in all of the frames it references,
reducing the amount of memory consumed.  (Contributed by Andrew
Kuchling in bpo-1565525.)


types
-----

A new "DynamicClassAttribute()" descriptor provides a way to define an
attribute that acts normally when looked up through an instance
object, but which is routed to the *class* "__getattr__" when looked
up through the class.  This allows one to have properties active on a
class, and have virtual attributes on the class with the same name
(see "Enum" for an example). (Contributed by Ethan Furman in
bpo-19030.)


urllib
------

"urllib.request" now supports "data:" URLs via the "DataHandler"
class.  (Contributed by Mathias Panzenböck in bpo-16423.)

The http method that will be used by a "Request" class can now be
specified by setting a "method" class attribute on the subclass.
(Contributed by Jason R Coombs in bpo-18978.)

"Request" objects are now reusable: if the "full_url" or "data"
attributes are modified, all relevant internal properties are updated.
This means, for example, that it is now possible to use the same
"Request" object in more than one "OpenerDirector.open()" call with
different *data* arguments, or to modify a "Request"‘s "url" rather
than recomputing it from scratch.  There is also a new
"remove_header()" method that can be used to remove headers from a
"Request".  (Contributed by Alexey Kachayev in bpo-16464, Daniel
Wozniak in bpo-17485, and Damien Brecht and Senthil Kumaran in
bpo-17272.)

"HTTPError" objects now have a "headers" attribute that provides
access to the HTTP response headers associated with the error.
(Contributed by Berker Peksag in bpo-15701.)


unittest
--------

The "TestCase" class has a new method, "subTest()", that produces a
context manager whose "with" block becomes a “sub-test”.  This context
manager allows a test method to dynamically generate subtests  by,
say, calling the "subTest" context manager inside a loop.  A single
test method can thereby produce an indefinite number of separately
identified and separately counted tests, all of which will run even if
one or more of them fail.  For example:

   class NumbersTest(unittest.TestCase):
       def test_even(self):
           for i in range(6):
               with self.subTest(i=i):
                   self.assertEqual(i % 2, 0)

will result in six subtests, each identified in the unittest verbose
output with a label consisting of the variable name "i" and a
particular value for that variable ("i=0", "i=1", etc).  See
Distinguishing test iterations using subtests for the full version of
this example.  (Contributed by Antoine Pitrou in bpo-16997.)

"unittest.main()" now accepts an iterable of test names for
*defaultTest*, where previously it only accepted a single test name as
a string.  (Contributed by Jyrki Pulliainen in bpo-15132.)

If "SkipTest" is raised during test discovery (that is, at the module
level in the test file), it is now reported as a skip instead of an
error.  (Contributed by Zach Ware in bpo-16935.)

"discover()" now sorts the discovered files to provide consistent test
ordering.  (Contributed by Martin Melin and Jeff Ramnani in
bpo-16709.)

"TestSuite" now drops references to tests as soon as the test has been
run, if the test is successful.  On Python interpreters that do
garbage collection, this allows the tests to be garbage collected if
nothing else is holding a reference to the test.  It is possible to
override this behavior by creating a "TestSuite" subclass that defines
a custom "_removeTestAtIndex" method.  (Contributed by Tom Wardill,
Matt McClure, and Andrew Svetlov in bpo-11798.)

A new test assertion context-manager, "assertLogs()", will ensure that
a given block of code emits a log message using the "logging" module.
By default the message can come from any logger and have a priority of
"INFO" or higher, but both the logger name and an alternative minimum
logging level may be specified.  The object returned by the context
manager can be queried for the "LogRecord"s and/or formatted messages
that were logged.  (Contributed by Antoine Pitrou in bpo-18937.)

Test discovery now works with namespace packages (Contributed by
Claudiu Popa in bpo-17457.)

"unittest.mock" objects now inspect their specification signatures
when matching calls, which means an argument can now be matched by
either position or name, instead of only by position.  (Contributed by
Antoine Pitrou in bpo-17015.)

"mock_open()" objects now have "readline" and "readlines" methods.
(Contributed by Toshio Kuratomi in bpo-17467.)


venv
----

"venv" now includes activation scripts for the "csh" and "fish"
shells.  (Contributed by Andrew Svetlov in bpo-15417.)

"EnvBuilder" and the "create()" convenience function take a new
keyword argument *with_pip*, which defaults to "False", that controls
whether or not "EnvBuilder" ensures that "pip" is installed in the
virtual environment.  (Contributed by Nick Coghlan in bpo-19552 as
part of the **PEP 453** implementation.)


wave
----

The "getparams()" method now returns a namedtuple rather than a plain
tuple.  (Contributed by Claudiu Popa in bpo-17487.)

"wave.open()" now supports the context management protocol.
(Contributed by Claudiu Popa in bpo-17616.)

"wave" can now write output to unseekable files.  (Contributed by
David Jones, Guilherme Polo, and Serhiy Storchaka in bpo-5202.)

The "writeframesraw()" and "writeframes()" methods now accept any
*bytes-like object*.  (Contributed by Serhiy Storchaka in bpo-8311.)


weakref
-------

New "WeakMethod" class simulates weak references to bound methods.
(Contributed by Antoine Pitrou in bpo-14631.)

New "finalize" class makes it possible to register a callback to be
invoked when an object is garbage collected, without needing to
carefully manage the lifecycle of the weak reference itself.
(Contributed by Richard Oudkerk in bpo-15528.)

The callback, if any, associated with a "ref" is now exposed via the
"__callback__" attribute.  (Contributed by Mark Dickinson in
bpo-17643.)


xml.etree
---------

A new parser, "XMLPullParser", allows a non-blocking applications to
parse XML documents.  An example can be seen at Pull API for non-
blocking parsing.  (Contributed by Antoine Pitrou in bpo-17741.)

The "xml.etree.ElementTree" "tostring()" and "tostringlist()"
functions, and the "ElementTree" "write()" method, now have a
*short_empty_elements* keyword-only parameter providing control over
whether elements with no content are written in abbreviated ("<tag
/>") or expanded ("<tag></tag>") form.  (Contributed by Ariel Poliak
and Serhiy Storchaka in bpo-14377.)


zipfile
-------

The "writepy()" method of the "PyZipFile" class has a new *filterfunc*
option that can be used to control which directories and files are
added to the archive.  For example, this could be used to exclude test
files from the archive. (Contributed by Christian Tismer in
bpo-19274.)

The *allowZip64* parameter to "ZipFile" and "PyZipfile" is now "True"
by default.  (Contributed by William Mallard in bpo-17201.)


CPython Implementation Changes
==============================


PEP 445: Customization of CPython Memory Allocators
---------------------------------------------------

**PEP 445** adds new C level interfaces to customize memory allocation
in the CPython interpreter.

See also:

  **PEP 445** – Add new APIs to customize Python memory allocators
     PEP written and implemented by Victor Stinner.


PEP 442: Safe Object Finalization
---------------------------------

**PEP 442** removes the current limitations and quirks of object
finalization in CPython. With it, objects with "__del__()" methods, as
well as generators with "finally" clauses, can be finalized when they
are part of a reference cycle.

As part of this change, module globals are no longer forcibly set to
"None" during interpreter shutdown in most cases, instead relying on
the normal operation of the cyclic garbage collector.  This avoids a
whole class of interpreter-shutdown-time errors, usually involving
"__del__" methods, that have plagued Python since the cyclic GC was
first introduced.

See also:

  **PEP 442** – Safe object finalization
     PEP written and implemented by Antoine Pitrou.


PEP 456: Secure and Interchangeable Hash Algorithm
--------------------------------------------------

**PEP 456** follows up on earlier security fix work done on Python’s
hash algorithm to address certain DOS attacks to which public facing
APIs backed by dictionary lookups may be subject.  (See bpo-14621 for
the start of the current round of improvements.)  The PEP unifies
CPython’s hash code to make it easier for a packager to substitute a
different hash algorithm, and switches Python’s default implementation
to a SipHash implementation on platforms that have a 64 bit data type.
Any performance differences in comparison with the older FNV algorithm
are trivial.

The PEP adds additional fields to the "sys.hash_info" named tuple to
describe the hash algorithm in use by the currently executing binary.
Otherwise, the PEP does not alter any existing CPython APIs.


PEP 436: Argument Clinic
------------------------

“Argument Clinic” (**PEP 436**) is now part of the CPython build
process and can be used to simplify the process of defining and
maintaining accurate signatures for builtins and standard library
extension modules implemented in C.

Some standard library extension modules have been converted to use
Argument Clinic in Python 3.4, and "pydoc" and "inspect" have been
updated accordingly.

It is expected that signature metadata for programmatic introspection
will be added to additional callables implemented in C as part of
Python 3.4 maintenance releases.

Note:

  The Argument Clinic PEP is not fully up to date with the state of
  the implementation. This has been deemed acceptable by the release
  manager and core development team in this case, as Argument Clinic
  will not be made available as a public API for third party use in
  Python 3.4.

See also:

  **PEP 436** – The Argument Clinic DSL
     PEP written and implemented by Larry Hastings.


Other Build and C API Changes
-----------------------------

* The new "PyType_GetSlot()" function has been added to the stable
  ABI, allowing retrieval of function pointers from named type slots
  when using the limited API.  (Contributed by Martin von Löwis in
  bpo-17162.)

* The new "Py_SetStandardStreamEncoding()" pre-initialization API
  allows applications embedding the CPython interpreter to reliably
  force a particular encoding and error handler for the standard
  streams. (Contributed by Bastien Montagne and Nick Coghlan in
  bpo-16129.)

* Most Python C APIs that don’t mutate string arguments are now
  correctly marked as accepting "const char *" rather than "char *".
  (Contributed by Serhiy Storchaka in bpo-1772673.)

* A new shell version of "python-config" can be used even when a
  python interpreter is not available (for example, in cross
  compilation scenarios).

* "PyUnicode_FromFormat()" now supports width and precision
  specifications for "%s", "%A", "%U", "%V", "%S", and "%R".
  (Contributed by Ysj Ray and Victor Stinner in bpo-7330.)

* New function "PyStructSequence_InitType2()" supplements the existing
  "PyStructSequence_InitType()" function.  The difference is that it
  returns "0" on success and "-1" on failure.

* The CPython source can now be compiled using the address sanity
  checking features of recent versions of GCC and clang:  the false
  alarms in the small object allocator have been silenced.
  (Contributed by Dhiru Kholia in bpo-18596.)

* The Windows build now uses Address Space Layout Randomization and
  Data Execution Prevention.  (Contributed by Christian Heimes in
  bpo-16632.)

* New function "PyObject_LengthHint()" is the C API equivalent of
  "operator.length_hint()".  (Contributed by Armin Ronacher in
  bpo-16148.)


Other Improvements
------------------

* The python command has a new option, "-I", which causes it to run in
  “isolated mode”, which means that "sys.path" contains neither the
  script’s directory nor the user’s "site-packages" directory, and all
  "PYTHON*" environment variables are ignored (it implies both "-s"
  and "-E").  Other restrictions may also be applied in the future,
  with the goal being to isolate the execution of a script from the
  user’s environment.  This is appropriate, for example, when Python
  is used to run a system script.  On most POSIX systems it can and
  should be used in the "#!" line of system scripts.  (Contributed by
  Christian Heimes in bpo-16499.)

* Tab-completion is now enabled by default in the interactive
  interpreter on systems that support "readline".  History is also
  enabled by default, and is written to (and read from) the file
  "~/.python-history". (Contributed by Antoine Pitrou and Éric Araujo
  in bpo-5845.)

* Invoking the Python interpreter with "--version" now outputs the
  version to standard output instead of standard error (bpo-18338).
  Similar changes were made to "argparse" (bpo-18920) and other
  modules that have script-like invocation capabilities (bpo-18922).

* The CPython Windows installer now adds ".py" to the "PATHEXT"
  variable when extensions are registered, allowing users to run a
  python script at the windows command prompt by just typing its name
  without the ".py" extension.  (Contributed by Paul Moore in
  bpo-18569.)

* A new "make" target coverage-report will build python, run the test
  suite, and generate an HTML coverage report for the C codebase using
  "gcov" and lcov.

* The "-R" option to the python regression test suite now also checks
  for memory allocation leaks, using "sys.getallocatedblocks()".
  (Contributed by Antoine Pitrou in bpo-13390.)

* "python -m" now works with namespace packages.

* The "stat" module is now implemented in C, which means it gets the
  values for its constants from the C header files, instead of having
  the values hard-coded in the python module as was previously the
  case.

* Loading multiple python modules from a single OS module (".so",
  ".dll") now works correctly (previously it silently returned the
  first python module in the file).  (Contributed by Václav Šmilauer
  in bpo-16421.)

* A new opcode, "LOAD_CLASSDEREF", has been added to fix a bug in the
  loading of free variables in class bodies that could be triggered by
  certain uses of __prepare__.  (Contributed by Benjamin Peterson in
  bpo-17853.)

* A number of MemoryError-related crashes were identified and fixed by
  Victor Stinner using his **PEP 445**-based "pyfailmalloc" tool
  (bpo-18408, bpo-18520).

* The "pyvenv" command now accepts a "--copies" option to use copies
  rather than symlinks even on systems where symlinks are the default.
  (Contributed by Vinay Sajip in bpo-18807.)

* The "pyvenv" command also accepts a "--without-pip" option to
  suppress the otherwise-automatic bootstrapping of pip into the
  virtual environment.  (Contributed by Nick Coghlan in bpo-19552 as
  part of the **PEP 453** implementation.)

* The encoding name is now optional in the value set for the
  "PYTHONIOENCODING" environment variable.  This makes it possible to
  set just the error handler, without changing the default encoding.
  (Contributed by Serhiy Storchaka in bpo-18818.)

* The "bz2", "lzma", and "gzip" module "open" functions now support
  "x" (exclusive creation) mode.  (Contributed by Tim Heaney and
  Vajrasky Kok in bpo-19201, bpo-19222, and bpo-19223.)


Significant Optimizations
-------------------------

* The UTF-32 decoder is now 3x to 4x faster.  (Contributed by Serhiy
  Storchaka in bpo-14625.)

* The cost of hash collisions for sets is now reduced.  Each hash
  table probe now checks a series of consecutive, adjacent key/hash
  pairs before continuing to make random probes through the hash
  table.  This exploits cache locality to make collision resolution
  less expensive. The collision resolution scheme can be described as
  a hybrid of linear probing and open addressing.  The number of
  additional linear probes defaults to nine.  This can be changed at
  compile-time by defining LINEAR_PROBES to be any value.  Set
  LINEAR_PROBES=0 to turn-off linear probing entirely.  (Contributed
  by Raymond Hettinger in bpo-18771.)

* The interpreter starts about 30% faster. A couple of measures lead
  to the speedup. The interpreter loads fewer modules on startup, e.g.
  the "re", "collections" and "locale" modules and their dependencies
  are no longer imported by default. The marshal module has been
  improved to load compiled Python code faster.  (Contributed by
  Antoine Pitrou, Christian Heimes and Victor Stinner in bpo-19219,
  bpo-19218, bpo-19209, bpo-19205 and bpo-9548.)

* "bz2.BZ2File" is now as fast or faster than the Python2 version for
  most cases.  "lzma.LZMAFile" has also been optimized.  (Contributed
  by Serhiy Storchaka and Nadeem Vawda in bpo-16034.)

* "random.getrandbits()" is 20%-40% faster for small integers (the
  most common use case).  (Contributed by Serhiy Storchaka in
  bpo-16674.)

* By taking advantage of the new storage format for strings, pickling
  of strings is now significantly faster.  (Contributed by Victor
  Stinner and Antoine Pitrou in bpo-15596.)

* A performance issue in "io.FileIO.readall()" has been solved.  This
  particularly affects Windows, and significantly speeds up the case
  of piping significant amounts of data through "subprocess".
  (Contributed by Richard Oudkerk in bpo-15758.)

* "html.escape()" is now 10x faster.  (Contributed by Matt Bryant in
  bpo-18020.)

* On Windows, the native "VirtualAlloc" is now used instead of the CRT
  "malloc" in "obmalloc".  Artificial benchmarks show about a 3%
  memory savings.

* "os.urandom()" now uses a lazily opened persistent file descriptor
  so as to avoid using many file descriptors when run in parallel from
  multiple threads.  (Contributed by Antoine Pitrou in bpo-18756.)


Deprecated
==========

This section covers various APIs and other features that have been
deprecated in Python 3.4, and will be removed in Python 3.5 or later.
In most (but not all) cases, using the deprecated APIs will produce a
"DeprecationWarning" when the interpreter is run with deprecation
warnings enabled (for example, by using "-Wd").


Deprecations in the Python API
------------------------------

* As mentioned in PEP 451: A ModuleSpec Type for the Import System, a
  number of "importlib" methods and functions are deprecated:
  "importlib.find_loader()" is replaced by
  "importlib.util.find_spec()";
  "importlib.machinery.PathFinder.find_module()" is replaced by
  "importlib.machinery.PathFinder.find_spec()";
  "importlib.abc.MetaPathFinder.find_module()" is replaced by
  "importlib.abc.MetaPathFinder.find_spec()";
  "importlib.abc.PathEntryFinder.find_loader()" and "find_module()"
  are replaced by "importlib.abc.PathEntryFinder.find_spec()"; all of
  the "*xxx*Loader" ABC "load_module" methods
  ("importlib.abc.Loader.load_module()",
  "importlib.abc.InspectLoader.load_module()",
  "importlib.abc.FileLoader.load_module()",
  "importlib.abc.SourceLoader.load_module()") should no longer be
  implemented, instead loaders should implement an "exec_module"
  method ("importlib.abc.Loader.exec_module()",
  "importlib.abc.InspectLoader.exec_module()"
  "importlib.abc.SourceLoader.exec_module()") and let the import
  system take care of the rest; and
  "importlib.abc.Loader.module_repr()",
  "importlib.util.module_for_loader()", "importlib.util.set_loader()",
  and "importlib.util.set_package()" are no longer needed because
  their functions are now handled automatically by the import system.

* The "imp" module is pending deprecation. To keep compatibility with
  Python 2/3 code bases, the module’s removal is currently not
  scheduled.

* The "formatter" module is pending deprecation and is slated for
  removal in Python 3.6.

* "MD5" as the default *digestmod* for the "hmac.new()" function is
  deprecated.  Python 3.6 will require an explicit digest name or
  constructor as *digestmod* argument.

* The internal "Netrc" class in the "ftplib" module has been
  documented as deprecated in its docstring for quite some time.  It
  now emits a "DeprecationWarning" and will be removed completely in
  Python 3.5.

* The undocumented *endtime* argument to "subprocess.Popen.wait()"
  should not have been exposed and is hopefully not in use; it is
  deprecated and will mostly likely be removed in Python 3.5.

* The *strict* argument of "HTMLParser" is deprecated.

* The "plistlib" "readPlist()", "writePlist()",
  "readPlistFromBytes()", and "writePlistToBytes()" functions are
  deprecated in favor of the corresponding new functions "load()",
  "dump()", "loads()", and "dumps()".  "Data()" is deprecated in favor
  of just using the "bytes" constructor.

* The "sysconfig" key "SO" is deprecated, it has been replaced by
  "EXT_SUFFIX".

* The "U" mode accepted by various "open" functions is deprecated. In
  Python3 it does not do anything useful, and should be replaced by
  appropriate uses of "io.TextIOWrapper" (if needed) and its *newline*
  argument.

* The *parser* argument of "xml.etree.ElementTree.iterparse()" has
  been deprecated, as has the *html* argument of "XMLParser()".  To
  prepare for the removal of the latter, all arguments to "XMLParser"
  should be passed by keyword.


Deprecated Features
-------------------

* Running IDLE — Python editor and shell with the "-n" flag (no
  subprocess) is deprecated. However, the feature will not be removed
  until bpo-18823 is resolved.

* The site module adding a “site-python” directory to sys.path, if it
  exists, is deprecated (bpo-19375).


Removed
=======


Operating Systems No Longer Supported
-------------------------------------

Support for the following operating systems has been removed from the
source and build tools:

* OS/2 (bpo-16135).

* Windows 2000 (changeset e52df05b496a).

* Windows systems where "COMSPEC" points to "command.com" (bpo-14470).

* VMS (bpo-16136).


API and Feature Removals
------------------------

The following obsolete and previously deprecated APIs and features
have been removed:

* The unmaintained "Misc/TextMate" and "Misc/vim" directories have
  been removed (see the devguide for suggestions on what to use
  instead).

* The "SO" makefile macro is removed (it was replaced by the
  "SHLIB_SUFFIX" and "EXT_SUFFIX" macros) (bpo-16754).

* The "PyThreadState.tick_counter" field has been removed; its value
  has been meaningless since Python 3.2, when the “new GIL” was
  introduced (bpo-19199).

* "PyLoader" and "PyPycLoader" have been removed from "importlib".
  (Contributed by Taras Lyapun in bpo-15641.)

* The *strict* argument to "HTTPConnection" and "HTTPSConnection" has
  been removed.  HTTP 0.9-style “Simple Responses” are no longer
  supported.

* The deprecated "urllib.request.Request" getter and setter methods
  "add_data", "has_data", "get_data", "get_type", "get_host",
  "get_selector", "set_proxy", "get_origin_req_host", and
  "is_unverifiable" have been removed (use direct attribute access
  instead).

* Support for loading the deprecated "TYPE_INT64" has been removed
  from "marshal".  (Contributed by Dan Riti in bpo-15480.)

* "inspect.Signature": positional-only parameters are now required to
  have a valid name.

* "object.__format__()" no longer accepts non-empty format strings, it
  now raises a "TypeError" instead.  Using a non-empty string has been
  deprecated since Python 3.2.  This change has been made to prevent a
  situation where previously working (but incorrect) code would start
  failing if an object gained a __format__ method, which means that
  your code may now raise a "TypeError" if you are using an "'s'"
  format code with objects that do not have a __format__ method that
  handles it.  See bpo-7994 for background.

* "difflib.SequenceMatcher.isbjunk()" and
  "difflib.SequenceMatcher.isbpopular()" were deprecated in 3.2, and
  have now been removed: use "x in sm.bjunk" and "x in sm.bpopular",
  where *sm* is a "SequenceMatcher" object (bpo-13248).


Code Cleanups
-------------

* The unused and undocumented internal "Scanner" class has been
  removed from the "pydoc" module.

* The private and effectively unused "_gestalt" module has been
  removed, along with the private "platform" functions
  "_mac_ver_lookup", "_mac_ver_gstalt", and "_bcd2str", which would
  only have ever been called on badly broken OSX systems (see
  bpo-18393).

* The hardcoded copies of certain "stat" constants that were included
  in the "tarfile" module namespace have been removed.


Porting to Python 3.4
=====================

This section lists previously described changes and other bugfixes
that may require changes to your code.


Changes in ‘python’ Command Behavior
------------------------------------

* In a posix shell, setting the "PATH" environment variable to an
  empty value is equivalent to not setting it at all.  However,
  setting "PYTHONPATH" to an empty value was *not* equivalent to not
  setting it at all: setting "PYTHONPATH" to an empty value was
  equivalent to setting it to ".", which leads to confusion when
  reasoning by analogy to how "PATH" works.  The behavior now conforms
  to the posix convention for "PATH".

* The [X refs, Y blocks] output of a debug ("--with-pydebug") build of
  the CPython interpreter is now off by default.  It can be re-enabled
  using the "-X showrefcount" option.  (Contributed by Ezio Melotti in
  bpo-17323.)

* The python command and most stdlib scripts (as well as "argparse")
  now output "--version" information to "stdout" instead of "stderr"
  (for issue list see Other Improvements above).


Changes in the Python API
-------------------------

* The ABCs defined in "importlib.abc" now either raise the appropriate
  exception or return a default value instead of raising
  "NotImplementedError" blindly. This will only affect code calling
  "super()" and falling through all the way to the ABCs. For
  compatibility, catch both "NotImplementedError" or the appropriate
  exception as needed.

* The module type now initializes the "__package__" and "__loader__"
  attributes to "None" by default. To determine if these attributes
  were set in a backwards-compatible fashion, use e.g.
  "getattr(module, '__loader__', None) is not None".  (bpo-17115.)

* "importlib.util.module_for_loader()" now sets "__loader__" and
  "__package__" unconditionally to properly support reloading. If this
  is not desired then you will need to set these attributes manually.
  You can use "importlib.util.module_to_load()" for module management.

* Import now resets relevant attributes (e.g. "__name__",
  "__loader__", "__package__", "__file__", "__cached__")
  unconditionally when reloading. Note that this restores a pre-3.3
  behavior in that it means a module is re-found when re-loaded
  (bpo-19413).

* Frozen packages no longer set "__path__" to a list containing the
  package name, they now set it to an empty list.  The previous
  behavior could cause the import system to do the wrong thing on
  submodule imports if there was also a directory with the same name
  as the frozen package.  The correct way to determine if a module is
  a package or not is to use "hasattr(module, '__path__')"
  (bpo-18065).

* Frozen modules no longer define a "__file__" attribute. It’s
  semantically incorrect for frozen modules to set the attribute as
  they are not loaded from any explicit location. If you must know
  that a module comes from frozen code then you can see if the
  module’s "__spec__.location" is set to "'frozen'", check if the
  loader is a subclass of "importlib.machinery.FrozenImporter", or if
  Python 2 compatibility is necessary you can use "imp.is_frozen()".

* "py_compile.compile()" now raises "FileExistsError" if the file path
  it would write to is a symlink or a non-regular file. This is to act
  as a warning that import will overwrite those files with a regular
  file regardless of what type of file path they were originally.

* "importlib.abc.SourceLoader.get_source()" no longer raises
  "ImportError" when the source code being loaded triggers a
  "SyntaxError" or "UnicodeDecodeError". As "ImportError" is meant to
  be raised only when source code cannot be found but it should, it
  was felt to be over-reaching/overloading of that meaning when the
  source code is found but improperly structured. If you were catching
  ImportError before and wish to continue to ignore syntax or decoding
  issues, catch all three exceptions now.

* "functools.update_wrapper()" and "functools.wraps()" now correctly
  set the "__wrapped__" attribute to the function being wrapped, even
  if that function also had its "__wrapped__" attribute set. This
  means "__wrapped__" attributes now correctly link a stack of
  decorated functions rather than every "__wrapped__" attribute in the
  chain referring to the innermost function. Introspection libraries
  that assumed the previous behaviour was intentional can use
  "inspect.unwrap()" to access the first function in the chain that
  has no "__wrapped__" attribute.

* "inspect.getfullargspec()" has been reimplemented on top of
  "inspect.signature()" and hence handles a much wider variety of
  callable objects than it did in the past. It is expected that
  additional builtin and extension module callables will gain
  signature metadata over the course of the Python 3.4 series. Code
  that assumes that "inspect.getfullargspec()" will fail on non-Python
  callables may need to be adjusted accordingly.

* "importlib.machinery.PathFinder" now passes on the current working
  directory to objects in "sys.path_hooks" for the empty string. This
  results in "sys.path_importer_cache" never containing "''", thus
  iterating through "sys.path_importer_cache" based on "sys.path" will
  not find all keys. A module’s "__file__" when imported in the
  current working directory will also now have an absolute path,
  including when using "-m" with the interpreter (except for
  "__main__.__file__" when a script has been executed directly using a
  relative path) (Contributed by Brett Cannon in bpo-18416).  is
  specified on the command-line) (bpo-18416).

* The removal of the *strict* argument to "HTTPConnection" and
  "HTTPSConnection" changes the meaning of the remaining arguments if
  you are specifying them positionally rather than by keyword.  If
  you’ve been paying attention to deprecation warnings your code
  should already be specifying any additional arguments via keywords.

* Strings between "from __future__ import ..." statements now *always*
  raise a "SyntaxError".  Previously if there was no leading
  docstring, an interstitial string would sometimes be ignored.  This
  brings CPython into compliance with the language spec; Jython and
  PyPy already were. (bpo-17434).

* "ssl.SSLSocket.getpeercert()" and "ssl.SSLSocket.do_handshake()" now
  raise an "OSError" with "ENOTCONN" when the "SSLSocket" is not
  connected, instead of the previous behavior of raising an
  "AttributeError".  In addition, "getpeercert()" will raise a
  "ValueError" if the handshake has not yet been done.

* "base64.b32decode()" now raises a "binascii.Error" when the input
  string contains non-b32-alphabet characters, instead of a
  "TypeError".  This particular "TypeError" was missed when the other
  "TypeError"s were converted.  (Contributed by Serhiy Storchaka in
  bpo-18011.)  Note: this change was also inadvertently applied in
  Python 3.3.3.

* The "file" attribute is now automatically closed when the creating
  "cgi.FieldStorage" instance is garbage collected. If you were
  pulling the file object out separately from the "cgi.FieldStorage"
  instance and not keeping the instance alive, then you should either
  store the entire "cgi.FieldStorage" instance or read the contents of
  the file before the "cgi.FieldStorage" instance is garbage
  collected.

* Calling "read" or "write" on a closed SSL socket now raises an
  informative "ValueError" rather than the previous more mysterious
  "AttributeError" (bpo-9177).

* "slice.indices()" no longer produces an "OverflowError" for huge
  values.  As a consequence of this fix, "slice.indices()" now raises
  a "ValueError" if given a negative length; previously it returned
  nonsense values (bpo-14794).

* The "complex" constructor, unlike the "cmath" functions, was
  incorrectly accepting "float" values if an object’s "__complex__"
  special method returned one.  This now raises a "TypeError".
  (bpo-16290.)

* The "int" constructor in 3.2 and 3.3 erroneously accepts "float"
  values for the *base* parameter.  It is unlikely anyone was doing
  this, but if so, it will now raise a "TypeError" (bpo-16772).

* Defaults for keyword-only arguments are now evaluated *after*
  defaults for regular keyword arguments, instead of before.
  Hopefully no one wrote any code that depends on the previous buggy
  behavior (bpo-16967).

* Stale thread states are now cleared after "fork()".  This may cause
  some system resources to be released that previously were
  incorrectly kept perpetually alive (for example, database
  connections kept in thread-local storage).  (bpo-17094.)

* Parameter names in "__annotations__" dicts are now mangled properly,
  similarly to "__kwdefaults__". (Contributed by Yury Selivanov in
  bpo-20625.)

* "hashlib.hash.name" now always returns the identifier in lower case.
  Previously some builtin hashes had uppercase names, but now that it
  is a formal public interface the naming has been made consistent
  (bpo-18532).

* Because "unittest.TestSuite" now drops references to tests after
  they are run, test harnesses that reuse a "TestSuite" to re-run a
  set of tests may fail.   Test suites should not be re-used in this
  fashion since it means state is retained between test runs, breaking
  the test isolation that "unittest" is designed to provide.  However,
  if the lack of isolation is considered acceptable, the old behavior
  can be restored by creating a "TestSuite" subclass that defines a
  "_removeTestAtIndex" method that does nothing (see
  "TestSuite.__iter__()") (bpo-11798).

* "unittest" now uses "argparse" for command line parsing.  There are
  certain invalid command forms that used to work that are no longer
  allowed; in theory this should not cause backward compatibility
  issues since the disallowed command forms didn’t make any sense and
  are unlikely to be in use.

* The "re.split()", "re.findall()", and "re.sub()" functions, and the
  "group()" and "groups()" methods of "match" objects now always
  return a *bytes* object when the string to be matched is a *bytes-
  like object*.  Previously the return type matched the input type, so
  if your code was depending on the return value being, say, a
  "bytearray", you will need to change your code.

* "audioop" functions now raise an error immediately if passed string
  input, instead of failing randomly later on (bpo-16685).

* The new *convert_charrefs* argument to "HTMLParser" currently
  defaults to "False" for backward compatibility, but will eventually
  be changed to default to "True".  It is recommended that you add
  this keyword, with the appropriate value, to any "HTMLParser" calls
  in your code (bpo-13633).

* Since the *digestmod* argument to the "hmac.new()" function will in
  the future have no default, all calls to "hmac.new()" should be
  changed to explicitly specify a *digestmod* (bpo-17276).

* Calling "sysconfig.get_config_var()" with the "SO" key, or looking
  "SO" up in the results of a call to "sysconfig.get_config_vars()" is
  deprecated.  This key should be replaced by "EXT_SUFFIX" or
  "SHLIB_SUFFIX", depending on the context (bpo-19555).

* Any calls to "open" functions that specify "U" should be modified.
  "U" is ineffective in Python3 and will eventually raise an error if
  used. Depending on the function, the equivalent of its old Python2
  behavior can be achieved using either a *newline* argument, or if
  necessary by wrapping the stream in "TextIOWrapper" to use its
  *newline* argument (bpo-15204).

* If you use "pyvenv" in a script and desire that pip *not* be
  installed, you must add "--without-pip" to your command invocation.

* The default behavior of "json.dump()" and "json.dumps()" when an
  indent is specified has changed: it no longer produces trailing
  spaces after the item separating commas at the ends of lines.  This
  will matter only if you have tests that are doing white-space-
  sensitive comparisons of such output (bpo-16333).

* "doctest" now looks for doctests in extension module "__doc__"
  strings, so if your doctest test discovery includes extension
  modules that have things that look like doctests in them you may see
  test failures you’ve never seen before when running your tests
  (bpo-3158).

* The "collections.abc" module has been slightly refactored as part of
  the Python startup improvements.  As a consequence of this, it is no
  longer the case that importing "collections" automatically imports
  "collections.abc".  If your program depended on the (undocumented)
  implicit import, you will need to add an explicit "import
  collections.abc" (bpo-20784).


Changes in the C API
--------------------

* "PyEval_EvalFrameEx()", "PyObject_Repr()", and "PyObject_Str()",
  along with some other internal C APIs, now include a debugging
  assertion that ensures they are not used in situations where they
  may silently discard a currently active exception. In cases where
  discarding the active exception is expected and desired (for
  example, because it has already been saved locally with
  "PyErr_Fetch()" or is being deliberately replaced with a different
  exception), an explicit "PyErr_Clear()" call will be needed to avoid
  triggering the assertion when invoking these operations (directly or
  indirectly) and running against a version of Python that is compiled
  with assertions enabled.

* "PyErr_SetImportError()" now sets "TypeError" when its **msg**
  argument is not set. Previously only "NULL" was returned with no
  exception set.

* The result of the "PyOS_ReadlineFunctionPointer" callback must now
  be a string allocated by "PyMem_RawMalloc()" or
  "PyMem_RawRealloc()", or "NULL" if an error occurred, instead of a
  string allocated by "PyMem_Malloc()" or "PyMem_Realloc()"
  (bpo-16742)

* "PyThread_set_key_value()" now always set the value. In Python 3.3,
  the function did nothing if the key already exists (if the current
  value is a non-"NULL" pointer).

* The "f_tstate" (thread state) field of the "PyFrameObject" structure
  has been removed to fix a bug: see bpo-14432 for the rationale.


Changed in 3.4.3
================


PEP 476: Enabling certificate verification by default for stdlib http clients
-----------------------------------------------------------------------------

"http.client" and modules which use it, such as "urllib.request" and
"xmlrpc.client", will now verify that the server presents a
certificate which is signed by a CA in the platform trust store and
whose hostname matches the hostname being requested by default,
significantly improving security for many applications.

For applications which require the old previous behavior, they can
pass an alternate context:

   import urllib.request
   import ssl

   # This disables all verification
   context = ssl._create_unverified_context()

   # This allows using a specific certificate for the host, which doesn't need
   # to be in the trust store
   context = ssl.create_default_context(cafile="/path/to/file.crt")

   urllib.request.urlopen("https://invalid-cert", context=context)
