Your first C API extension module

This tutorial will take you through creating a simple Python extension module written in C or C++.

We will use the low-level Python C API directly. For easier ways to create extension modules, see the recommended third party tools.

The tutorial assumes basic knowledge about Python: you should be able to define functions in Python code before starting to write them in C. See The Python Tutorial for an introduction to Python itself.

The tutorial should be approachable for anyone who can write a basic C library. While we will mention several concepts that a C beginner would not be expected to know, like static functions or linkage declarations, understanding these is not necessary for success.

We will focus on giving you a “feel” of what Python’s C API is like. It will not teach you important concepts, like error handling and reference counting, which are covered in later chapters.

We will assume that you use a Unix-like system (including macOS and Linux), or Windows. On other systems, you might need to adjust some details – for example, a system command name.

You need to have a suitable C compiler and Python development headers installed. On Linux, headers are often in a package like python3-dev or python3-devel.

You need to be able to install Python packages. This tutorial uses pip (pip install), but you can substitute any tool that can build and install pyproject.toml-based projects, like uv (uv pip install). Preferably, have a virtual environment activated.

Note

This tutorial uses APIs that were added in CPython 3.15. To create an extension that’s compatible with earlier versions of CPython, please follow an earlier version of this documentation.

This tutorial uses C syntax added in C11 and C++20. If your extension needs to be compatible with earlier standards, please follow tutorials in documentation for Python 3.14 or below.

What we’ll do

Let’s create an extension module called spam [1], which will include a Python interface to the C standard library function system(). This function is defined in stdlib.h. It takes a C string as argument, runs the argument as a system command, and returns a result value as an integer. A manual page for system() might summarize it this way:

#include <stdlib.h>
int system(const char *command);

Note that like many functions in the C standard library, this function is already exposed in Python. In production, use os.system() or subprocess.run() rather than the module you’ll write here.

We want this function to be callable from Python as follows:

>>> import spam
>>> status = spam.system("whoami")
User Name
>>> status
0

Note

The system command whoami prints out your username. It’s useful in tutorials like this one because it has the same name on both Unix and Windows.

Start with the headers

Begin by creating a directory for this tutorial, and switching to it on the command line. Then, create a file named spammodule.c in your directory. [2]

In this file, we’ll include two headers: Python.h to pull in all declarations of the Python C API, and stdlib.h for the system() function. [3]

Add the following lines to spammodule.c:

#include <Python.h>
#include <stdlib.h>     // for system()

Be sure to put stdlib.h, and any other standard library includes, after Python.h. On some systems, Python may define some pre-processor definitions that affect the standard headers.

Running your build tool

With only the includes in place, your extension won’t do anything. Still, it’s a good time to compile it and try to import it. This will ensure that your build tool works, so that you can make and test incremental changes as you follow the rest of the text.

CPython itself does not come with a tool to build extension modules; it is recommended to use a third-party project for this. In this tutorial, we’ll use meson-python. (If you want to use another one, see Appendix: Other build tools.)

meson-python requires defining a “project” using two extra files.

First, add pyproject.toml with these contents:

[build-system]
build-backend = 'mesonpy'
requires = ['meson-python']

[project]
# Placeholder project information
# (change this before distributing the module)
name = 'sampleproject'
version = '0'

Then, create meson.build containing the following:

project('sampleproject', 'c')

py = import('python').find_installation(pure: false)

py.extension_module(
   'spam',          # name of the importable Python module
   'spammodule.c',  # the C source file
   install: true,
)

Note

See meson-python documentation for details on configuration.

Now, build install the project in the current directory (.) via pip:

python -m pip install .

Tip

If you don’t have pip installed, run python -m ensurepip, preferably in a virtual environment. (Or, if you prefer another tool that can build and install pyproject.toml-based projects, use that.)

Note that you will need to run this command again every time you change your extension. Unlike Python, C has an explicit compilation step.

When your extension is compiled and installed, start Python and try to import it. This should fail with the following exception:

>>> import spam
Traceback (most recent call last):
   ...
ImportError: dynamic module does not define module export function (PyModExport_spam or PyInit_spam)

Module export hook

The exception you got when you tried to import the module told you that Python is looking for a “module export function”, also known as a module export hook. Let’s define one.

First, add a prototype below the #include lines:


PyMODEXPORT_FUNC PyModExport_spam(void);

Tip

The prototype is not strictly necessary, but some modern compilers emit warnings without it. It’s generally better to add the prototype than to disable the warning.

The PyMODEXPORT_FUNC macro declares the function’s return type, and adds any special linkage declarations needed to make the function visible and usable when CPython loads it.

After the prototype, add the function itself. For now, make it return NULL:

PyMODEXPORT_FUNC
PyModExport_spam(void)
{
   return NULL;
}

Compile and load the module again. You should get a different error this time.

>>> import spam
Traceback (most recent call last):
   ...
SystemError: module export hook for module 'spam' failed without setting an exception

Simply returning NULL is not correct behavior for an export hook, and CPython complains about it. That’s good – it means that CPython found the function! Let’s now make it do something useful.

The slot table

Rather than NULL, the export hook should return the information needed to create a module. Let’s start with the basics: the name and docstring.

The information should be defined in a static array of PyModuleDef_Slot entries, which are essentially key-value pairs. Define this array just before your export hook:

static PyModuleDef_Slot spam_slots[] = {
   {Py_mod_name, "spam"},
   {Py_mod_doc, "A wonderful module with an example function"},
   {0, NULL}
};

For both Py_mod_name and Py_mod_doc, the values are C strings – that is, NUL-terminated, UTF-8 encoded byte arrays.

Note the zero-filled sentinel entry at the end. If you forget it, you’ll trigger undefined behavior.

The array is defined as static – that is, not visible outside this .c file. This will be a common theme. CPython only needs to access the export hook; all global variables and all other functions should generally be static, so that they don’t clash with other extensions.

Return this array from your export hook instead of NULL:

PyMODEXPORT_FUNC
PyModExport_spam(void)
{
   return spam_slots;
}

Now, recompile and try it out:

>>> import spam
>>> print(spam)
<module 'spam' from '/home/encukou/dev/cpython/spam.so'>

You have an extension module! Try help(spam) to see the docstring.

The next step will be adding a function.

Exposing a function

To expose the system() C function directly to Python, we’ll need to write a layer of glue code to convert arguments from Python objects to C values, and the C return value back to Python.

One of the simplest ways to write glue code is a “METH_O” function, which takes two Python objects and returns one. All Python objects – regardless of the Python type – are represented in C as pointers to the PyObject structure.

Add such a function above the slots array:

static PyObject *
spam_system(PyObject *self, PyObject *arg)
{
   Py_RETURN_NONE;
}

For now, we ignore the arguments, and use the Py_RETURN_NONE macro, which expands to a return statement that properly returns a Python None object.

Recompile your extension to make sure you don’t have syntax errors. We haven’t yet added spam_system to the module, so you might get a warning that spam_system is unused.

Method definitions

To expose the C function to Python, you will need to provide several pieces of information in a structure called PyMethodDef [4]:

  • ml_name: the name of the Python function;

  • ml_doc: a docstring;

  • ml_meth: the C function to be called; and

  • ml_flags: a set of flags describing details like how Python arguments are passed to the C function. We’ll use METH_O here – the flag that matches our spam_system function’s signature.

Because modules typically create several functions, these definitions need to be collected in an array, with a zero-filled sentinel at the end. Add this array just below the spam_system function:


static PyMethodDef spam_methods[] = {
    {
        .ml_name="system",
        .ml_meth=spam_system,
        .ml_flags=METH_O,
        .ml_doc="Execute a shell command.",
    },
    {NULL, NULL, 0, NULL}        /* Sentinel */
};

As with module slots, a zero-filled sentinel marks the end of the array.

Next, we’ll add the method to the module. Add a Py_mod_methods slot to your PyMethodDef array:


static PyModuleDef_Slot spam_slots[] = {
    {Py_mod_name, "spam"},
    {Py_mod_doc, "A wonderful module with an example function"},
    {Py_mod_methods, spam_methods},
    {0, NULL}
};

Recompile your extension again, and test it. Be sure to restart the Python interpreter, so that import spam picks up the new version of the module.

You should now be able to call the function:

>>> import spam
>>> print(spam.system)
<built-in function system>
>>> print(spam.system('whoami'))
None

Note that our spam.system does not yet run the whoami command; it only returns None.

Check that the function accepts exactly one argument, as specified by the METH_O flag:

>>> print(spam.system('too', 'many', 'arguments'))
Traceback (most recent call last):
   ...
TypeError: spam.system() takes exactly one argument (3 given)

Returning an integer

Now, let’s take a look at the return value. Instead of None, we’ll want spam.system to return a number – that is, a Python int object. Eventually this will be the exit code of a system command, but let’s start with a fixed value, say, 3.

The Python C API provides a function to create a Python int object from a C int value: PyLong_FromLong(). [5]

To call it, replace the Py_RETURN_NONE with the following 3 lines:

static PyObject *
spam_system(PyObject *self, PyObject *arg)
{
   int status = 3;
   PyObject *result = PyLong_FromLong(status);
   return result;
}

Recompile, restart the Python interpreter again, and check that the function now returns 3:

>>> import spam
>>> spam.system('whoami')
3

Accepting a string

Finally, let’s handle the function argument.

Our C function, spam_system(), takes two arguments. The first one, PyObject *self, will be set to the spam module object. This isn’t useful in our case, so we’ll ignore it.

The other one, PyObject *arg, will be set to the object that the user passed from Python. We expect that it should be a Python string. In order to use the information in it, we will need to convert it to a C value – in this case, a C string (const char *).

There’s a slight type mismatch here: Python’s str objects store Unicode text, but C strings are arrays of bytes. So, we’ll need to encode the data, and we’ll use the UTF-8 encoding for it. (UTF-8 might not always be correct for system commands, but it’s what str.encode() uses by default, and the C API has special support for it.)

The function to encode a Python string into a UTF-8 buffer is named PyUnicode_AsUTF8() [6]. Call it like this:

static PyObject *
spam_system(PyObject *self, PyObject *arg)
{
   const char *command = PyUnicode_AsUTF8(arg);
   int status = 3;
   PyObject *result = PyLong_FromLong(status);
   return result;
}

If PyUnicode_AsUTF8() is successful, command will point to the resulting array of bytes. This buffer is managed by the arg object, which means we don’t need to free it, but we must follow some rules:

  • We should only use the buffer inside the spam_system function. When spam_system returns, arg and the buffer it manages might be garbage-collected.

  • We must not modify it. This is why we use const.

If PyUnicode_AsUTF8() was not successful, it returns a NULL pointer. When calling any Python C API, we always need to handle such error cases. The way to do this in general is left for later chapters of this documentation. For now, be assured that we are already handling errors from PyLong_FromLong() correctly.

For the PyUnicode_AsUTF8() call, the correct way to handle errors is returning NULL from spam_system. Add an if block for this:

static PyObject *
spam_system(PyObject *self, PyObject *arg)
{
   const char *command = PyUnicode_AsUTF8(arg);
   if (command == NULL) {
      return NULL;
   }
   int status = 3;
   PyObject *result = PyLong_FromLong(status);
   return result;
}

That’s it for the setup. Now, all that is left is calling the C library function system() with the char * buffer, and using its result instead of the 3:

static PyObject *
spam_system(PyObject *self, PyObject *arg)
{
   const char *command = PyUnicode_AsUTF8(arg);
   if (command == NULL) {
      return NULL;
   }
   int status = system(command);
   PyObject *result = PyLong_FromLong(status);
   return result;
}

Compile your module, restart Python, and test. This time, you should see your username – the output of the whoami system command:

>>> import spam
>>> result = spam.system('whoami')
User Name
>>> result
0

You might also want to test error cases:

>>> import spam
>>> result = spam.system('nonexistent-command')
sh: line 1: nonexistent-command: command not found
>>> result
32512

>>> spam.system(3)
Traceback (most recent call last):
   ...
TypeError: bad argument type for built-in operation

The result

Congratulations! You have written a complete Python C API extension module, and completed this tutorial!

Here is the entire source file, for your convenience:

/// Includes

#include <Python.h>
#include <stdlib.h>     // for system()

/// Implementation of spam.system

static PyObject *
spam_system(PyObject *self, PyObject *arg)
{
   const char *command = PyUnicode_AsUTF8(arg);
   if (command == NULL) {
      return NULL;
   }
   int status = system(command);
   PyObject *result = PyLong_FromLong(status);
   return result;
}

/// Module method table

static PyMethodDef spam_methods[] = {
    {
        .ml_name="system",
        .ml_meth=spam_system,
        .ml_flags=METH_O,
        .ml_doc="Execute a shell command.",
    },
    {NULL, NULL, 0, NULL}        /* Sentinel */
};

/// Module slot table

static PyModuleDef_Slot spam_slots[] = {
    {Py_mod_name, "spam"},
    {Py_mod_doc, "A wonderful module with an example function"},
    {Py_mod_methods, spam_methods},
    {0, NULL}
};

/// Export hook prototype

PyMODEXPORT_FUNC PyModExport_spam(void);

/// Module export hook

PyMODEXPORT_FUNC
PyModExport_spam(void)
{
   return spam_slots;
}

Appendix: Other build tools

You should be able to follow this tutorial – except the Running your build tool section itself – with a build tool other than meson-python.

The Python Packaging User Guide has a list of recommended tools; be sure to choose one for the C language.

Workaround for missing PyInit function

If your build tool output complains about missing PyInit_spam, add the following function to your module for now:

// A workaround
void *PyInit_spam(void) { return NULL; }

This is a shim for an old-style initialization function, which was required in extension modules for CPython 3.14 and below. Current CPython does not need it, but some build tools may still assume that all extension modules need to define it.

If you use this workaround, you will get the exception SystemError: initialization of spam failed without raising an exception instead of ImportError: dynamic module does not define module export function.

Compiling directly

Using a third-party build tool is heavily recommended, as it will take care of various details of your platform and Python installation, of naming the resulting extension, and, later, of distributing your work.

If you are building an extension for as specific system, or for yourself only, you might instead want to run your compiler directly. The way to do this is system-specific; be prepared for issues you will need to solve yourself.

Linux

On Linux, the Python development package may include a python3-config command that prints out the required compiler flags. If you use it, check that it corresponds to the CPython interpreter you’ll use to load the module. Then, start with the following command:

gcc --shared $(python3-config --cflags --ldflags) spammodule.c -o spam.so

This should generate a spam.so file that you need to put in a directory on sys.path.

Footnotes