PEP: 7 Title: Style Guide for C Code Author: Guido van Rossum
<guido@python.org>, Barry Warsaw <barry@python.org> Status: Active Type:
Process 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 <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.
    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); /* incorrect */

    Instead:

        return albatross; /* correct */

-   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, braces should
    be formatted as shown:

        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 */
        }

    It's OK to put operators at ends of lines, especially to be
    consistent with surrounding code. (See
    PEP 8 <pep8-operator-linebreak> for a longer discussion.)

-   Vertically align line continuation characters in multi-line macros.

-   Macros intended to be used as a statement should use the
    do { ... } while (0) macro idiom, without a final semicolon.
    Example:

        #define ADD_INT_MACRO(MOD, INT)                                   \
            do {                                                          \
                if (PyModule_AddIntConstant((MOD), (#INT), (INT)) < 0) {  \
                    goto error;                                           \
                }                                                         \
            } while (0)

        // To be used like a statement with a semicolon:
        ADD_INT_MACRO(m, SOME_CONSTANT);

-   #undef file local macros after use.

-   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.
-   Macro parameters should use ALL_CAPS style, so they are easily
    distinguishable from C variables and struct members.

Documentation Strings

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

-   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.