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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 style guidelines for Python code [1]_.
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 versions before 3.6 use ANSI/ISO standard C (the 1989 version
of the standard). This means (amongst many other things) that all
declarations must be at the top of a block (not necessarily at the
top of function).
* Python versions greater than or equal to 3.6 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
Future C99 features may be added to this list in the future
depending on compiler support (mostly significantly MSVC).
* Don't use GCC extensions (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).
* Only use C++ style // one-line comments in Python 3.6 or later.
* No compiler warnings with major compilers (gcc, VC++, a few others).
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, like this::
PyAPI_FUNC(PyObject *) PyObject_Repr(PyObject *);
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.
References
==========
.. [1] PEP 8, "Style Guide for Python Code", van Rossum, Warsaw
(http://www.python.org/dev/peps/pep-0008)
Copyright
=========
This document has been placed in the public domain.
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