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291 lines
11 KiB
ReStructuredText
=======================
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Extending/Embedding FAQ
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=======================
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.. only:: html
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.. contents::
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.. highlight:: c
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.. XXX need review for Python 3.
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Can I create my own functions in C?
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-----------------------------------
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Yes, you can create built-in modules containing functions, variables, exceptions
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and even new types in C. This is explained in the document
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:ref:`extending-index`.
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Most intermediate or advanced Python books will also cover this topic.
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Can I create my own functions in C++?
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-------------------------------------
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Yes, using the C compatibility features found in C++. Place ``extern "C" {
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... }`` around the Python include files and put ``extern "C"`` before each
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function that is going to be called by the Python interpreter. Global or static
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C++ objects with constructors are probably not a good idea.
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.. _c-wrapper-software:
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Writing C is hard; are there any alternatives?
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----------------------------------------------
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There are a number of alternatives to writing your own C extensions, depending
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on what you're trying to do.
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.. XXX make sure these all work
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`Cython <https://cython.org>`_ and its relative `Pyrex
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<https://www.csse.canterbury.ac.nz/greg.ewing/python/Pyrex/>`_ are compilers
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that accept a slightly modified form of Python and generate the corresponding
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C code. Cython and Pyrex make it possible to write an extension without having
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to learn Python's C API.
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If you need to interface to some C or C++ library for which no Python extension
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currently exists, you can try wrapping the library's data types and functions
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with a tool such as `SWIG <https://www.swig.org>`_. `SIP
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<https://riverbankcomputing.com/software/sip/intro>`__, `CXX
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<https://cxx.sourceforge.net/>`_ `Boost
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<https://www.boost.org/libs/python/doc/index.html>`_, or `Weave
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<https://github.com/scipy/weave>`_ are also
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alternatives for wrapping C++ libraries.
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How can I execute arbitrary Python statements from C?
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-----------------------------------------------------
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The highest-level function to do this is :c:func:`PyRun_SimpleString` which takes
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a single string argument to be executed in the context of the module
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``__main__`` and returns ``0`` for success and ``-1`` when an exception occurred
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(including :exc:`SyntaxError`). If you want more control, use
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:c:func:`PyRun_String`; see the source for :c:func:`PyRun_SimpleString` in
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``Python/pythonrun.c``.
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How can I evaluate an arbitrary Python expression from C?
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---------------------------------------------------------
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Call the function :c:func:`PyRun_String` from the previous question with the
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start symbol :c:data:`Py_eval_input`; it parses an expression, evaluates it and
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returns its value.
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How do I extract C values from a Python object?
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-----------------------------------------------
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That depends on the object's type. If it's a tuple, :c:func:`PyTuple_Size`
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returns its length and :c:func:`PyTuple_GetItem` returns the item at a specified
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index. Lists have similar functions, :c:func:`PyList_Size` and
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:c:func:`PyList_GetItem`.
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For bytes, :c:func:`PyBytes_Size` returns its length and
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:c:func:`PyBytes_AsStringAndSize` provides a pointer to its value and its
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length. Note that Python bytes objects may contain null bytes so C's
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:c:func:`!strlen` should not be used.
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To test the type of an object, first make sure it isn't ``NULL``, and then use
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:c:func:`PyBytes_Check`, :c:func:`PyTuple_Check`, :c:func:`PyList_Check`, etc.
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There is also a high-level API to Python objects which is provided by the
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so-called 'abstract' interface -- read ``Include/abstract.h`` for further
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details. It allows interfacing with any kind of Python sequence using calls
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like :c:func:`PySequence_Length`, :c:func:`PySequence_GetItem`, etc. as well
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as many other useful protocols such as numbers (:c:func:`PyNumber_Index` et
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al.) and mappings in the PyMapping APIs.
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How do I use Py_BuildValue() to create a tuple of arbitrary length?
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-------------------------------------------------------------------
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You can't. Use :c:func:`PyTuple_Pack` instead.
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How do I call an object's method from C?
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----------------------------------------
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The :c:func:`PyObject_CallMethod` function can be used to call an arbitrary
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method of an object. The parameters are the object, the name of the method to
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call, a format string like that used with :c:func:`Py_BuildValue`, and the
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argument values::
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PyObject *
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PyObject_CallMethod(PyObject *object, const char *method_name,
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const char *arg_format, ...);
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This works for any object that has methods -- whether built-in or user-defined.
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You are responsible for eventually :c:func:`Py_DECREF`\ 'ing the return value.
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To call, e.g., a file object's "seek" method with arguments 10, 0 (assuming the
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file object pointer is "f")::
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res = PyObject_CallMethod(f, "seek", "(ii)", 10, 0);
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if (res == NULL) {
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... an exception occurred ...
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}
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else {
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Py_DECREF(res);
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}
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Note that since :c:func:`PyObject_CallObject` *always* wants a tuple for the
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argument list, to call a function without arguments, pass "()" for the format,
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and to call a function with one argument, surround the argument in parentheses,
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e.g. "(i)".
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How do I catch the output from PyErr_Print() (or anything that prints to stdout/stderr)?
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----------------------------------------------------------------------------------------
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In Python code, define an object that supports the ``write()`` method. Assign
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this object to :data:`sys.stdout` and :data:`sys.stderr`. Call print_error, or
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just allow the standard traceback mechanism to work. Then, the output will go
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wherever your ``write()`` method sends it.
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The easiest way to do this is to use the :class:`io.StringIO` class:
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.. code-block:: pycon
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>>> import io, sys
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>>> sys.stdout = io.StringIO()
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>>> print('foo')
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>>> print('hello world!')
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>>> sys.stderr.write(sys.stdout.getvalue())
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foo
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hello world!
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A custom object to do the same would look like this:
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.. code-block:: pycon
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>>> import io, sys
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>>> class StdoutCatcher(io.TextIOBase):
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... def __init__(self):
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... self.data = []
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... def write(self, stuff):
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... self.data.append(stuff)
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...
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>>> import sys
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>>> sys.stdout = StdoutCatcher()
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>>> print('foo')
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>>> print('hello world!')
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>>> sys.stderr.write(''.join(sys.stdout.data))
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foo
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hello world!
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How do I access a module written in Python from C?
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--------------------------------------------------
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You can get a pointer to the module object as follows::
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module = PyImport_ImportModule("<modulename>");
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If the module hasn't been imported yet (i.e. it is not yet present in
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:data:`sys.modules`), this initializes the module; otherwise it simply returns
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the value of ``sys.modules["<modulename>"]``. Note that it doesn't enter the
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module into any namespace -- it only ensures it has been initialized and is
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stored in :data:`sys.modules`.
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You can then access the module's attributes (i.e. any name defined in the
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module) as follows::
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attr = PyObject_GetAttrString(module, "<attrname>");
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Calling :c:func:`PyObject_SetAttrString` to assign to variables in the module
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also works.
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How do I interface to C++ objects from Python?
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----------------------------------------------
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Depending on your requirements, there are many approaches. To do this manually,
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begin by reading :ref:`the "Extending and Embedding" document
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<extending-index>`. Realize that for the Python run-time system, there isn't a
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whole lot of difference between C and C++ -- so the strategy of building a new
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Python type around a C structure (pointer) type will also work for C++ objects.
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For C++ libraries, see :ref:`c-wrapper-software`.
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I added a module using the Setup file and the make fails; why?
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--------------------------------------------------------------
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Setup must end in a newline, if there is no newline there, the build process
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fails. (Fixing this requires some ugly shell script hackery, and this bug is so
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minor that it doesn't seem worth the effort.)
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How do I debug an extension?
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----------------------------
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When using GDB with dynamically loaded extensions, you can't set a breakpoint in
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your extension until your extension is loaded.
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In your ``.gdbinit`` file (or interactively), add the command:
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.. code-block:: none
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br _PyImport_LoadDynamicModule
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Then, when you run GDB:
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.. code-block:: shell-session
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$ gdb /local/bin/python
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gdb) run myscript.py
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gdb) continue # repeat until your extension is loaded
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gdb) finish # so that your extension is loaded
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gdb) br myfunction.c:50
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gdb) continue
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I want to compile a Python module on my Linux system, but some files are missing. Why?
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--------------------------------------------------------------------------------------
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Most packaged versions of Python don't include the
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:file:`/usr/lib/python2.{x}/config/` directory, which contains various files
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required for compiling Python extensions.
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For Red Hat, install the python-devel RPM to get the necessary files.
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For Debian, run ``apt-get install python-dev``.
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How do I tell "incomplete input" from "invalid input"?
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------------------------------------------------------
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Sometimes you want to emulate the Python interactive interpreter's behavior,
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where it gives you a continuation prompt when the input is incomplete (e.g. you
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typed the start of an "if" statement or you didn't close your parentheses or
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triple string quotes), but it gives you a syntax error message immediately when
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the input is invalid.
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In Python you can use the :mod:`codeop` module, which approximates the parser's
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behavior sufficiently. IDLE uses this, for example.
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The easiest way to do it in C is to call :c:func:`PyRun_InteractiveLoop` (perhaps
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in a separate thread) and let the Python interpreter handle the input for
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you. You can also set the :c:func:`PyOS_ReadlineFunctionPointer` to point at your
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custom input function. See ``Modules/readline.c`` and ``Parser/myreadline.c``
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for more hints.
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How do I find undefined g++ symbols __builtin_new or __pure_virtual?
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--------------------------------------------------------------------
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To dynamically load g++ extension modules, you must recompile Python, relink it
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using g++ (change LINKCC in the Python Modules Makefile), and link your
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extension module using g++ (e.g., ``g++ -shared -o mymodule.so mymodule.o``).
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Can I create an object class with some methods implemented in C and others in Python (e.g. through inheritance)?
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----------------------------------------------------------------------------------------------------------------
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Yes, you can inherit from built-in classes such as :class:`int`, :class:`list`,
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:class:`dict`, etc.
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The Boost Python Library (BPL, https://www.boost.org/libs/python/doc/index.html)
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provides a way of doing this from C++ (i.e. you can inherit from an extension
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class written in C++ using the BPL).
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