Extending/Embedding FAQ
***********************


Can I create my own functions in C?
===================================

Yes, you can create built-in modules containing functions, variables,
exceptions and even new types in C.  This is explained in the document
Extending and Embedding the Python Interpreter.

Most intermediate or advanced Python books will also cover this topic.


Can I create my own functions in C++?
=====================================

Yes, using the C compatibility features found in C++.  Place "extern
"C" { ... }" around the Python include files and put "extern "C""
before each function that is going to be called by the Python
interpreter.  Global or static C++ objects with constructors are
probably not a good idea.


Writing C is hard; are there any alternatives?
==============================================

There are a number of alternatives to writing your own C extensions,
depending on what you’re trying to do.

Cython and its relative Pyrex are compilers that accept a slightly
modified form of Python and generate the corresponding C code.  Cython
and Pyrex make it possible to write an extension without having to
learn Python’s C API.

If you need to interface to some C or C++ library for which no Python
extension currently exists, you can try wrapping the library’s data
types and functions with a tool such as SWIG.  SIP, CXX Boost, or
Weave are also alternatives for wrapping C++ libraries.


How can I execute arbitrary Python statements from C?
=====================================================

The highest-level function to do this is "PyRun_SimpleString()" which
takes a single string argument to be executed in the context of the
module "__main__" and returns "0" for success and "-1" when an
exception occurred (including "SyntaxError").  If you want more
control, use "PyRun_String()"; see the source for
"PyRun_SimpleString()" in "Python/pythonrun.c".


How can I evaluate an arbitrary Python expression from C?
=========================================================

Call the function "PyRun_String()" from the previous question with the
start symbol "Py_eval_input"; it parses an expression, evaluates it
and returns its value.


How do I extract C values from a Python object?
===============================================

That depends on the object’s type.  If it’s a tuple, "PyTuple_Size()"
returns its length and "PyTuple_GetItem()" returns the item at a
specified index.  Lists have similar functions, "PyList_Size()" and
"PyList_GetItem()".

For bytes, "PyBytes_Size()" returns its length and
"PyBytes_AsStringAndSize()" provides a pointer to its value and its
length.  Note that Python bytes objects may contain null bytes so C’s
"strlen()" should not be used.

To test the type of an object, first make sure it isn’t "NULL", and
then use "PyBytes_Check()", "PyTuple_Check()", "PyList_Check()", etc.

There is also a high-level API to Python objects which is provided by
the so-called ‘abstract’ interface – read "Include/abstract.h" for
further details.  It allows interfacing with any kind of Python
sequence using calls like "PySequence_Length()",
"PySequence_GetItem()", etc. as well as many other useful protocols
such as numbers ("PyNumber_Index()" et al.) and mappings in the
PyMapping APIs.


How do I use Py_BuildValue() to create a tuple of arbitrary length?
===================================================================

You can’t.  Use "PyTuple_Pack()" instead.


How do I call an object’s method from C?
========================================

The "PyObject_CallMethod()" function can be used to call an arbitrary
method of an object.  The parameters are the object, the name of the
method to call, a format string like that used with "Py_BuildValue()",
and the argument values:

   PyObject *
   PyObject_CallMethod(PyObject *object, const char *method_name,
                       const char *arg_format, ...);

This works for any object that has methods – whether built-in or user-
defined. You are responsible for eventually "Py_DECREF()"‘ing the
return value.

To call, e.g., a file object’s “seek” method with arguments 10, 0
(assuming the file object pointer is “f”):

   res = PyObject_CallMethod(f, "seek", "(ii)", 10, 0);
   if (res == NULL) {
           ... an exception occurred ...
   }
   else {
           Py_DECREF(res);
   }

Note that since "PyObject_CallObject()" *always* wants a tuple for the
argument list, to call a function without arguments, pass “()” for the
format, and to call a function with one argument, surround the
argument in parentheses, e.g. “(i)”.


How do I catch the output from PyErr_Print() (or anything that prints to stdout/stderr)?
========================================================================================

In Python code, define an object that supports the "write()" method.
Assign this object to "sys.stdout" and "sys.stderr".  Call
print_error, or just allow the standard traceback mechanism to work.
Then, the output will go wherever your "write()" method sends it.

The easiest way to do this is to use the "io.StringIO" class:

   >>> import io, sys
   >>> sys.stdout = io.StringIO()
   >>> print('foo')
   >>> print('hello world!')
   >>> sys.stderr.write(sys.stdout.getvalue())
   foo
   hello world!

A custom object to do the same would look like this:

   >>> import io, sys
   >>> class StdoutCatcher(io.TextIOBase):
   ...     def __init__(self):
   ...         self.data = []
   ...     def write(self, stuff):
   ...         self.data.append(stuff)
   ...
   >>> import sys
   >>> sys.stdout = StdoutCatcher()
   >>> print('foo')
   >>> print('hello world!')
   >>> sys.stderr.write(''.join(sys.stdout.data))
   foo
   hello world!


How do I access a module written in Python from C?
==================================================

You can get a pointer to the module object as follows:

   module = PyImport_ImportModule("<modulename>");

If the module hasn’t been imported yet (i.e. it is not yet present in
"sys.modules"), this initializes the module; otherwise it simply
returns the value of "sys.modules["<modulename>"]".  Note that it
doesn’t enter the module into any namespace – it only ensures it has
been initialized and is stored in "sys.modules".

You can then access the module’s attributes (i.e. any name defined in
the module) as follows:

   attr = PyObject_GetAttrString(module, "<attrname>");

Calling "PyObject_SetAttrString()" to assign to variables in the
module also works.


How do I interface to C++ objects from Python?
==============================================

Depending on your requirements, there are many approaches.  To do this
manually, begin by reading the “Extending and Embedding” document.
Realize that for the Python run-time system, there isn’t a whole lot
of difference between C and C++ – so the strategy of building a new
Python type around a C structure (pointer) type will also work for C++
objects.

For C++ libraries, see Writing C is hard; are there any alternatives?.


I added a module using the Setup file and the make fails; why?
==============================================================

Setup must end in a newline, if there is no newline there, the build
process fails.  (Fixing this requires some ugly shell script hackery,
and this bug is so minor that it doesn’t seem worth the effort.)


How do I debug an extension?
============================

When using GDB with dynamically loaded extensions, you can’t set a
breakpoint in your extension until your extension is loaded.

In your ".gdbinit" file (or interactively), add the command:

   br _PyImport_LoadDynamicModule

Then, when you run GDB:

   $ gdb /local/bin/python
   gdb) run myscript.py
   gdb) continue # repeat until your extension is loaded
   gdb) finish   # so that your extension is loaded
   gdb) br myfunction.c:50
   gdb) continue


I want to compile a Python module on my Linux system, but some files are missing. Why?
======================================================================================

Most packaged versions of Python omit some files required for
compiling Python extensions.

For Red Hat, install the python3-devel RPM to get the necessary files.

For Debian, run "apt-get install python3-dev".


How do I tell “incomplete input” from “invalid input”?
======================================================

Sometimes you want to emulate the Python interactive interpreter’s
behavior, where it gives you a continuation prompt when the input is
incomplete (e.g. you typed the start of an “if” statement or you
didn’t close your parentheses or triple string quotes), but it gives
you a syntax error message immediately when the input is invalid.

In Python you can use the "codeop" module, which approximates the
parser’s behavior sufficiently.  IDLE uses this, for example.

The easiest way to do it in C is to call "PyRun_InteractiveLoop()"
(perhaps in a separate thread) and let the Python interpreter handle
the input for you. You can also set the
"PyOS_ReadlineFunctionPointer()" to point at your custom input
function. See "Modules/readline.c" and "Parser/myreadline.c" for more
hints.


How do I find undefined g++ symbols __builtin_new or __pure_virtual?
====================================================================

To dynamically load g++ extension modules, you must recompile Python,
relink it using g++ (change LINKCC in the Python Modules Makefile),
and link your extension module using g++ (e.g., "g++ -shared -o
mymodule.so mymodule.o").


Can I create an object class with some methods implemented in C and others in Python (e.g. through inheritance)?
================================================================================================================

Yes, you can inherit from built-in classes such as "int", "list",
"dict", etc.

The Boost Python Library (BPL,
https://www.boost.org/libs/python/doc/index.html) provides a way of
doing this from C++ (i.e. you can inherit from an extension class
written in C++ using the BPL).
