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	PEP: 7
	Title: Style Guide for C Code
	Version: $Revision$
	Last-Modified: $Date$
	Author: Guido van Rossum <guido@python.org>, Barry Warsaw <barry@python.org>
	Status: Active
	Type: Process
	Content-Type: text/x-rst
	Created: 05-Jul-2001
	Post-History:


	Introduction
	============

	This document gives coding conventions for the C code comprising the C
	implementation of Python. Please see the companion informational PEP
	describing :pep:`style guidelines for Python code <8>`.

	Note, rules are there to be broken. Two good reasons to break a
	particular rule:

	1. When applying the rule would make the code less readable, even for
	someone who is used to reading code that follows the rules.

	2. To be consistent with surrounding code that also breaks it (maybe
	for historic reasons) -- although this is also an opportunity to
	clean up someone else's mess (in true XP style).


	C dialect
	=========

	* Python 3.11 and newer versions use C11 without `optional features
	<https://en.wikipedia.org/wiki/C11_%28C_standard_revision%29#Optional_features>`_.
	The public C API should be compatible with C++.

	* Python 3.6 to 3.10 use C89 with several select C99 features:

	- Standard integer types in ``<stdint.h>`` and ``<inttypes.h>``. We
	require the fixed width integer types.
	- ``static inline`` functions
	- designated initializers (especially nice for type declarations)
	- intermingled declarations
	- booleans
	- C++-style line comments

	* Python versions before 3.6 used ANSI/ISO standard C (the 1989 version
	of the standard). This meant (amongst many other things) that all
	declarations must be at the top of a block (not necessarily at the
	top of function).

	* Don't use compiler-specific extensions, such as those of GCC or MSVC
	(e.g. don't write multi-line strings without trailing backslashes).

	* All function declarations and definitions must use full prototypes
	(i.e. specify the types of all arguments).

	* No compiler warnings with major compilers (gcc, VC++, a few others).

	* ``static inline`` functions should be preferred over macros in new
	code.


	Code lay-out
	============

	* Use 4-space indents and no tabs at all.

	* No line should be longer than 79 characters. If this and the
	previous rule together don't give you enough room to code, your code
	is too complicated -- consider using subroutines.

	* No line should end in whitespace. If you think you need significant
	trailing whitespace, think again -- somebody's editor might delete
	it as a matter of routine.

	* Function definition style: function name in column 1, outermost
	curly braces in column 1, blank line after local variable
	declarations. ::

	static int
	extra_ivars(PyTypeObject type, PyTypeObject base)
	{
	int t_size = PyType_BASICSIZE(type);
	int b_size = PyType_BASICSIZE(base);

	assert(t_size >= b_size); /* type smaller than base! */
	...
	return 1;
	}

	* Code structure: one space between keywords like ``if``, ``for`` and
	the following left paren; no spaces inside the paren; braces are
	required everywhere, even where C permits them to be omitted, but do
	not add them to code you are not otherwise modifying. All new C
	code requires braces. Braces should be formatted as shown::

	if (mro != NULL) {
	...
	}
	else {
	...
	}

	* The return statement should not get redundant parentheses::

	return albatross; /* correct */
	return(albatross); /* incorrect */

	* Function and macro call style: ``foo(a, b, c)`` -- no space before
	the open paren, no spaces inside the parens, no spaces before
	commas, one space after each comma.

	* Always put spaces around assignment, Boolean and comparison
	operators. In expressions using a lot of operators, add spaces
	around the outermost (lowest-priority) operators.

	* Breaking long lines: if you can, break after commas in the outermost
	argument list. Always indent continuation lines appropriately,
	e.g.::

	PyErr_Format(PyExc_TypeError,
	"cannot create '%.100s' instances",
	type->tp_name);

	* When you break a long expression at a binary operator, the
	operator goes at the end of the previous line, and braces should be
	formatted as shown. E.g.::

	if (type->tp_dictoffset != 0 && base->tp_dictoffset == 0 &&
	type->tp_dictoffset == b_size &&
	(size_t)t_size == b_size + sizeof(PyObject *))
	{
	return 0; /* "Forgive" adding a __dict__ only */
	}

	* Put blank lines around functions, structure definitions, and major
	sections inside functions.

	* Comments go before the code they describe.

	* All functions and global variables should be declared static unless
	they are to be part of a published interface

	* For external functions and variables, we always have a declaration
	in an appropriate header file in the "Include" directory, which uses
	the ``PyAPI_FUNC()`` macro and ``PyAPI_DATA()`` macro, like this::

	PyAPI_FUNC(PyObject ) PyObject_Repr(PyObject );

	PyAPI_DATA(PyTypeObject) PySuper_Type;


	Naming conventions
	==================

	* Use a ``Py`` prefix for public functions; never for static
	functions. The ``Py_`` prefix is reserved for global service
	routines like ``Py_FatalError``; specific groups of routines
	(e.g. specific object type APIs) use a longer prefix,
	e.g. ``PyString_`` for string functions.

	* Public functions and variables use MixedCase with underscores, like
	this: ``PyObject_GetAttr``, ``Py_BuildValue``, ``PyExc_TypeError``.

	* Occasionally an "internal" function has to be visible to the loader;
	we use the ``_Py`` prefix for this, e.g.: ``_PyObject_Dump``.

	* Macros should have a MixedCase prefix and then use upper case, for
	example: ``PyString_AS_STRING``, ``Py_PRINT_RAW``.


	Documentation Strings
	=====================

	* Use the ``PyDoc_STR()`` or ``PyDoc_STRVAR()`` macro for docstrings
	to support building Python without docstrings (``./configure
	--without-doc-strings``).

	For C code that needs to support versions of Python older than 2.3,
	you can include this after including ``Python.h``::

	#ifndef PyDoc_STR
	#define PyDoc_VAR(name) static char name[]
	#define PyDoc_STR(str) (str)
	#define PyDoc_STRVAR(name, str) PyDoc_VAR(name) = PyDoc_STR(str)
	#endif

	* The first line of each function docstring should be a "signature
	line" that gives a brief synopsis of the arguments and return value.
	For example::

	PyDoc_STRVAR(myfunction__doc__,
	"myfunction(name, value) -> bool\n\n\
	Determine whether name and value make a valid pair.");

	Always include a blank line between the signature line and the text
	of the description.

	If the return value for the function is always None (because there
	is no meaningful return value), do not include the indication of the
	return type.

	* When writing multi-line docstrings, be sure to always use backslash
	continuations, as in the example above, or string literal
	concatenation::

	PyDoc_STRVAR(myfunction__doc__,
	"myfunction(name, value) -> bool\n\n"
	"Determine whether name and value make a valid pair.");

	Though some C compilers accept string literals without either::

	/* BAD -- don't do this! */
	PyDoc_STRVAR(myfunction__doc__,
	"myfunction(name, value) -> bool\n\n
	Determine whether name and value make a valid pair.");

	not all do; the MSVC compiler is known to complain about this.


	Copyright
	=========

	This document has been placed in the public domain.



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