
``popen2`` --- Subprocesses with accessible I/O streams
*******************************************************

Deprecated since version 2.6: This module is obsolete.  Use the
``subprocess`` module.  Check especially the *Replacing Older
Functions with the subprocess Module* section.

This module allows you to spawn processes and connect to their
input/output/error pipes and obtain their return codes under Unix and
Windows.

The ``subprocess`` module provides more powerful facilities for
spawning new processes and retrieving their results.  Using the
``subprocess`` module is preferable to using the ``popen2`` module.

The primary interface offered by this module is a trio of factory
functions. For each of these, if *bufsize* is specified,  it specifies
the buffer size for the I/O pipes.  *mode*, if provided, should be the
string ``'b'`` or ``'t'``; on Windows this is needed to determine
whether the file objects should be opened in binary or text mode.  The
default value for *mode* is ``'t'``.

On Unix, *cmd* may be a sequence, in which case arguments will be
passed directly to the program without shell intervention (as with
``os.spawnv()``). If *cmd* is a string it will be passed to the shell
(as with ``os.system()``).

The only way to retrieve the return codes for the child processes is
by using the ``poll()`` or ``wait()`` methods on the ``Popen3`` and
``Popen4`` classes; these are only available on Unix.  This
information is not available when using the ``popen2()``,
``popen3()``, and ``popen4()`` functions, or the equivalent functions
in the ``os`` module. (Note that the tuples returned by the ``os``
module's functions are in a different order from the ones returned by
the ``popen2`` module.)

popen2.popen2(cmd[, bufsize[, mode]])

   Executes *cmd* as a sub-process.  Returns the file objects
   ``(child_stdout, child_stdin)``.

popen2.popen3(cmd[, bufsize[, mode]])

   Executes *cmd* as a sub-process.  Returns the file objects
   ``(child_stdout, child_stdin, child_stderr)``.

popen2.popen4(cmd[, bufsize[, mode]])

   Executes *cmd* as a sub-process.  Returns the file objects
   ``(child_stdout_and_stderr, child_stdin)``.

   New in version 2.0.

On Unix, a class defining the objects returned by the factory
functions is also available.  These are not used for the Windows
implementation, and are not available on that platform.

class class popen2.Popen3(cmd[, capturestderr[, bufsize]])

   This class represents a child process.  Normally, ``Popen3``
   instances are created using the ``popen2()`` and ``popen3()``
   factory functions described above.

   If not using one of the helper functions to create ``Popen3``
   objects, the parameter *cmd* is the shell command to execute in a
   sub-process.  The *capturestderr* flag, if true, specifies that the
   object should capture standard error output of the child process.
   The default is false.  If the *bufsize* parameter is specified, it
   specifies the size of the I/O buffers to/from the child process.

class class popen2.Popen4(cmd[, bufsize])

   Similar to ``Popen3``, but always captures standard error into the
   same file object as standard output.  These are typically created
   using ``popen4()``.

   New in version 2.0.


Popen3 and Popen4 Objects
=========================

Instances of the ``Popen3`` and ``Popen4`` classes have the following
methods:

Popen3.poll()

   Returns ``-1`` if child process hasn't completed yet, or its status
   code (see ``wait()``) otherwise.

Popen3.wait()

   Waits for and returns the status code of the child process.  The
   status code encodes both the return code of the process and
   information about whether it exited using the ``exit`` system call
   or died due to a signal.  Functions to help interpret the status
   code are defined in the ``os`` module; see section *Process
   Management* for the ``W*()`` family of functions.

The following attributes are also available:

Popen3.fromchild

   A file object that provides output from the child process.  For
   ``Popen4`` instances, this will provide both the standard output
   and standard error streams.

Popen3.tochild

   A file object that provides input to the child process.

Popen3.childerr

   A file object that provides error output from the child process, if
   *capturestderr* was true for the constructor, otherwise ``None``.
   This will always be ``None`` for ``Popen4`` instances.

Popen3.pid

   The process ID of the child process.


Flow Control Issues
===================

Any time you are working with any form of inter-process communication,
control flow needs to be carefully thought out.  This remains the case
with the file objects provided by this module (or the ``os`` module
equivalents).

When reading output from a child process that writes a lot of data to
standard error while the parent is reading from the child's standard
output, a deadlock can occur.  A similar situation can occur with
other combinations of reads and writes.  The essential factors are
that more than ``_PC_PIPE_BUF`` bytes are being written by one process
in a blocking fashion, while the other process is reading from the
first process, also in a blocking fashion.

There are several ways to deal with this situation.

The simplest application change, in many cases, will be to follow this
model in the parent process:

   import popen2

   r, w, e = popen2.popen3('python slave.py')
   e.readlines()
   r.readlines()
   r.close()
   e.close()
   w.close()

with code like this in the child:

   import os
   import sys

   # note that each of these print statements
   # writes a single long string

   print >>sys.stderr, 400 * 'this is a test\n'
   os.close(sys.stderr.fileno())
   print >>sys.stdout, 400 * 'this is another test\n'

In particular, note that ``sys.stderr`` must be closed after writing
all data, or ``readlines()`` won't return.  Also note that
``os.close()`` must be used, as ``sys.stderr.close()`` won't close
``stderr`` (otherwise assigning to ``sys.stderr`` will silently close
it, so no further errors can be printed).

Applications which need to support a more general approach should
integrate I/O over pipes with their ``select()`` loops, or use
separate threads to read each of the individual files provided by
whichever ``popen*()`` function or ``Popen*`` class was used.

See also:

   Module ``subprocess``
      Module for spawning and managing subprocesses.
