python-peps / pep-0007.txt

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PEP: 7
Title: Style Guide for C Code
Version: $Revision$
Last-Modified: $Date$
Author: Guido van Rossum <>
Status: Active
Type: Process
Content-Type: text/x-rst
Created: 05-Jul-2001


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

* 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).

* 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).

* Never use C++ style // one-line comments.

* 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 may be
  omitted where C permits but when present, they should be formatted
  as shown::

      if (mro != NULL) {
      else {

* The return statement should *not* get redundant parentheses::

      return Py_None; /* correct */
      return(Py_None); /* 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,

                   "cannot create '%.100s' instances",

* When you break a long expression at a binary operator, the
  operator goes at the end of the previous line, 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

  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)

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

      "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

      "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! */
      "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.


.. [1] PEP 8, "Style Guide for Python Code", van Rossum, Warsaw


This document has been placed in the public domain.

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