PEP: 257
Title: Docstring Conventions
Version: $Revision$
Last-Modified: $Date$
Authors: David Goodger <goodger@python.org>,
         Guido van Rossum <guido@python.org>
Discussions-To: doc-sig@python.org
Status: Active
Type: Informational
Content-Type: text/x-rst
Created: 29-May-2001
Post-History: 13-Jun-2001


Abstract
========

This PEP documents the semantics and conventions associated with
Python docstrings.


Rationale
=========

The aim of this PEP is to standardize the high-level structure of
docstrings: what they should contain, and how to say it (without
touching on any markup syntax within docstrings).  The PEP contains
conventions, not laws or syntax.

    "A universal convention supplies all of maintainability, clarity,
    consistency, and a foundation for good programming habits too.
    What it doesn't do is insist that you follow it against your will.
    That's Python!"

    -- Tim Peters on comp.lang.python, 2001-06-16

If you violate these conventions, the worst you'll get is some dirty
looks.  But some software (such as the Docutils_ docstring processing
system [1]_ [2]_) will be aware of the conventions, so following them
will get you the best results.


Specification
=============

What is a Docstring?
--------------------

A docstring is a string literal that occurs as the first statement in
a module, function, class, or method definition.  Such a docstring
becomes the ``__doc__`` special attribute of that object.

All modules should normally have docstrings, and all functions and
classes exported by a module should also have docstrings.  Public
methods (including the ``__init__`` constructor) should also have
docstrings.  A package may be documented in the module docstring of
the ``__init__.py`` file in the package directory.

String literals occurring elsewhere in Python code may also act as
documentation.  They are not recognized by the Python bytecode
compiler and are not accessible as runtime object attributes (i.e. not
assigned to ``__doc__``), but two types of extra docstrings may be
extracted by software tools:

1. String literals occurring immediately after a simple assignment at
   the top level of a module, class, or ``__init__`` method are called
   "attribute docstrings".

2. String literals occurring immediately after another docstring are
   called "additional docstrings".

Please see PEP 258, "Docutils Design Specification" [2]_, for a
detailed description of attribute and additional docstrings.

XXX Mention docstrings of 2.2 properties.

For consistency, always use ``"""triple double quotes"""`` around
docstrings.  Use ``r"""raw triple double quotes"""`` if you use any
backslashes in your docstrings.  For Unicode docstrings, use
``u"""Unicode triple-quoted strings"""``.

There are two forms of docstrings: one-liners and multi-line
docstrings.


One-line Docstrings
--------------------

One-liners are for really obvious cases.  They should really fit on
one line.  For example::

    def kos_root():
        """Return the pathname of the KOS root directory."""
        global _kos_root
        if _kos_root: return _kos_root
        ...

Notes:

- Triple quotes are used even though the string fits on one line.
  This makes it easy to later expand it.

- The closing quotes are on the same line as the opening quotes.  This
  looks better for one-liners.

- There's no blank line either before or after the docstring.

- The docstring is a phrase ending in a period.  It prescribes the
  function or method's effect as a command ("Do this", "Return that"),
  not as a description; e.g. don't write "Returns the pathname ...".

- The one-line docstring should NOT be a "signature" reiterating the
  function/method parameters (which can be obtained by introspection).
  Don't do::

      def function(a, b):
          """function(a, b) -> list"""

  This type of docstring is only appropriate for C functions (such as
  built-ins), where introspection is not possible.  However, the
  nature of the *return value* cannot be determined by introspection,
  so it should be mentioned.  The preferred form for such a docstring
  would be something like::

      def function(a, b):
          """Do X and return a list."""

  (Of course "Do X" should be replaced by a useful description!)


Multi-line Docstrings
----------------------

Multi-line docstrings consist of a summary line just like a one-line
docstring, followed by a blank line, followed by a more elaborate
description.  The summary line may be used by automatic indexing
tools; it is important that it fits on one line and is separated from
the rest of the docstring by a blank line.  The summary line may be on
the same line as the opening quotes or on the next line.  The entire
docstring is indented the same as the quotes at its first line (see
example below).

Insert a blank line before and after all docstrings (one-line or
multi-line) that document a class -- generally speaking, the class's
methods are separated from each other by a single blank line, and the
docstring needs to be offset from the first method by a blank line;
for symmetry, put a blank line between the class header and the
docstring.  Docstrings documenting functions or methods generally
don't have this requirement, unless the function or method's body is
written as a number of blank-line separated sections -- in this case,
treat the docstring as another section, and precede it with a blank
line.

The docstring of a script (a stand-alone program) should be usable as
its "usage" message, printed when the script is invoked with incorrect
or missing arguments (or perhaps with a "-h" option, for "help").
Such a docstring should document the script's function and command
line syntax, environment variables, and files.  Usage messages can be
fairly elaborate (several screens full) and should be sufficient for a
new user to use the command properly, as well as a complete quick
reference to all options and arguments for the sophisticated user.

The docstring for a module should generally list the classes,
exceptions and functions (and any other objects) that are exported by
the module, with a one-line summary of each.  (These summaries
generally give less detail than the summary line in the object's
docstring.)  The docstring for a package (i.e., the docstring of the
package's ``__init__.py`` module) should also list the modules and
subpackages exported by the package.

The docstring for a function or method should summarize its behavior
and document its arguments, return value(s), side effects, exceptions
raised, and restrictions on when it can be called (all if applicable).
Optional arguments should be indicated.  It should be documented
whether keyword arguments are part of the interface.

The docstring for a class should summarize its behavior and list the
public methods and instance variables.  If the class is intended to be
subclassed, and has an additional interface for subclasses, this
interface should be listed separately (in the docstring).  The class
constructor should be documented in the docstring for its ``__init__``
method.  Individual methods should be documented by their own
docstring.

If a class subclasses another class and its behavior is mostly
inherited from that class, its docstring should mention this and
summarize the differences.  Use the verb "override" to indicate that a
subclass method replaces a superclass method and does not call the
superclass method; use the verb "extend" to indicate that a subclass
method calls the superclass method (in addition to its own behavior).

*Do not* use the Emacs convention of mentioning the arguments of
functions or methods in upper case in running text.  Python is case
sensitive and the argument names can be used for keyword arguments, so
the docstring should document the correct argument names.  It is best
to list each argument on a separate line.  For example::

    def complex(real=0.0, imag=0.0):
        """Form a complex number.

        Keyword arguments:
        real -- the real part (default 0.0)
        imag -- the imaginary part (default 0.0)

        """
        if imag == 0.0 and real == 0.0: return complex_zero
        ...

The BDFL [3]_ recommends inserting a blank line between the last
paragraph in a multi-line docstring and its closing quotes, placing
the closing quotes on a line by themselves.  This way, Emacs'
``fill-paragraph`` command can be used on it.


Handling Docstring Indentation
------------------------------

Docstring processing tools will strip a uniform amount of indentation
from the second and further lines of the docstring, equal to the
minimum indentation of all non-blank lines after the first line.  Any
indentation in the first line of the docstring (i.e., up to the first
newline) is insignificant and removed.  Relative indentation of later
lines in the docstring is retained.  Blank lines should be removed
from the beginning and end of the docstring.

Since code is much more precise than words, here is an implementation
of the algorithm::

    def trim(docstring):
        if not docstring:
            return ''
        # Convert tabs to spaces (following the normal Python rules)
        # and split into a list of lines:
        lines = docstring.expandtabs().splitlines()
        # Determine minimum indentation (first line doesn't count):
        indent = sys.maxint
        for line in lines[1:]:
            stripped = line.lstrip()
            if stripped:
                indent = min(indent, len(line) - len(stripped))
        # Remove indentation (first line is special):
        trimmed = [lines[0].strip()]
        if indent < sys.maxint:
            for line in lines[1:]:
                trimmed.append(line[indent:].rstrip())
        # Strip off trailing and leading blank lines:
        while trimmed and not trimmed[-1]:
            trimmed.pop()
        while trimmed and not trimmed[0]:
            trimmed.pop(0)
        # Return a single string:
        return '\n'.join(trimmed)

The docstring in this example contains two newline characters and is
therefore 3 lines long.  The first and last lines are blank::

    def foo():
        """
        This is the second line of the docstring.
        """

To illustrate::

    >>> print repr(foo.__doc__)
    '\n    This is the second line of the docstring.\n    '
    >>> foo.__doc__.splitlines()
    ['', '    This is the second line of the docstring.', '    ']
    >>> trim(foo.__doc__)
    'This is the second line of the docstring.'

Once trimmed, these docstrings are equivalent::

    def foo():
        """A multi-line
        docstring.
        """

    def bar():
        """
        A multi-line
        docstring.
        """


References and Footnotes
========================

.. [1] PEP 256, Docstring Processing System Framework, Goodger
   (http://www.python.org/peps/pep-0256.html)

.. [2] PEP 258, Docutils Design Specification, Goodger
   (http://www.python.org/peps/pep-0258.html)

.. [3] Guido van Rossum, Python's creator and Benevolent Dictator For
   Life.

.. _Docutils: http://docutils.sourceforge.net/

.. _Python Style Guide:
   http://www.python.org/doc/essays/styleguide.html

.. _Doc-SIG: http://www.python.org/sigs/doc-sig/


Copyright
=========

This document has been placed in the public domain.


Acknowledgements
================

The "Specification" text comes mostly verbatim from the `Python Style
Guide`_ essay by Guido van Rossum.

This document borrows ideas from the archives of the Python Doc-SIG_.
Thanks to all members past and present.



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