239 lines
9.1 KiB
Plaintext
239 lines
9.1 KiB
Plaintext
PEP: 257
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Title: Docstring Conventions
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Version: $Revision$
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Last-Modified: $Date$
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Author: dgoodger@bigfoot.com (David Goodger),
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guido@digicool.com (Guido van Rossum)
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Discussions-To: doc-sig@python.org
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Status: Draft
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Type: Informational
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Created: 29-May-2001
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Post-History: 13-Jun-2001
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Abstract
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This PEP documents the semantics and conventions associated with
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Python docstrings.
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Rationale
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The aim of this PEP is to standardize the high-level structure of
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docstrings: what they should contain, and how to say it (without
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touching on any markup syntax within docstrings). The PEP
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contains conventions, not laws or syntax.
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"A universal convention supplies all of maintainability,
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clarity, consistency, and a foundation for good programming
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habits too. What it doesn't do is insist that you follow it
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against your will. That's Python!"
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--Tim Peters on comp.lang.python, 2001-06-16
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If you violate the conventions, the worst you'll get is some dirty
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looks. But some software (such as the Docstring Processing System
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[1]) will be aware of the conventions, so following them will get
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you the best results.
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Specification
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What is a Docstring?
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--------------------
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A docstring is a string literal that occurs as the first statement
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in a module, function, class, or method definition. Such a
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docstring becomes the __doc__ special attribute of that object.
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All modules should normally have docstrings, and all functions and
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classes exported by a module should also have docstrings. Public
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methods (including the __init__ constructor) should also have
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docstrings. A package may be documented in the module docstring
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of the __init__.py file in the package directory.
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String literals occuring elsewhere in Python code may also act as
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documentation. They are not recognized by the Python bytecode
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compiler and are not accessible as runtime object attributes
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(i.e. not assigned to __doc__), but two types of extra docstrings
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are recognized by software tools:
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1. String literals occuring immediately after a simple assignment
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at the top level of a module, class, or __init__ method
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are called "attribute docstrings".
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2. String literals occuring immediately after another docstring
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are called "additional docstrings".
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Please see PEP 258 "DPS Generic Implementation Details" [2] for a
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detailed description of attribute and additional docstrings.
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For consistency, always use """triple double quotes""" around
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docstrings. Use r"""raw triple double quotes""" if you use any
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backslashes in your docstrings.
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There are two forms of docstrings: one-liners and multi-line
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docstrings.
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One-line Docstrings
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--------------------
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One-liners are for really obvious cases. They should really fit
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on one line. For example::
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def kos_root():
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"""Return the pathname of the KOS root directory."""
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global _kos_root
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if _kos_root: return _kos_root
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...
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Notes:
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- Triple quotes are used even though the string fits on one line.
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This makes it easy to later expand it.
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- The closing quotes are on the same line as the opening quotes.
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This looks better for one-liners.
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- There's no blank line either before or after the docstring.
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- The docstring is a phrase ending in a period. It prescribes the
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function's effect as a command ("Do this", "Return that"), not
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as a description: e.g. don't write "Returns the pathname ..."
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- The one-line docstring should NOT be a "signature" reiterating
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the function parameters (which can be obtained by
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introspection). Don't do::
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def function(a, b):
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"""function(a, b) -> list"""
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This type of docstring is only appropriate for C functions (such
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as built-ins), where introspection is not possible.
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Multi-line Docstrings
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----------------------
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Multi-line docstrings consist of a summary line just like a
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one-line docstring, followed by a blank line, followed by a more
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elaborate description. The summary line may be used by automatic
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indexing tools; it is important that it fits on one line and is
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separated from the rest of the docstring by a blank line.
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The entire docstring is indented the same as the quotes at its
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first line (see example below). Docstring processing tools will
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strip an amount of indentation from the second and further lines
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of the docstring equal to the indentation of the first non-blank
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line after the first line of the docstring. Relative indentation
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of later lines in the docstring is retained.
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Insert a blank line before and after all docstrings (one-line or
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multi-line) that document a class -- generally speaking, the
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class's methods are separated from each other by a single blank
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line, and the docstring needs to be offset from the first method
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by a blank line; for symmetry, put a blank line between the class
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header and the docstring. Docstrings documenting functions
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generally don't have this requirement, unless the function's body
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is written as a number of blank-line separated sections -- in this
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case, treat the docstring as another section, and precede it with
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a blank line.
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The docstring of a script (a stand-alone program) should be usable
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as its "usage" message, printed when the script is invoked with
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incorrect or missing arguments (or perhaps with a "-h" option, for
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"help"). Such a docstring should document the script's function
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and command line syntax, environment variables, and files. Usage
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messages can be fairly elaborate (several screens full) and should
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be sufficient for a new user to use the command properly, as well
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as a complete quick reference to all options and arguments for the
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sophisticated user.
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The docstring for a module should generally list the classes,
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exceptions and functions (and any other objects) that are exported
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by the module, with a one-line summary of each. (These summaries
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generally give less detail than the summary line in the object's
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docstring.) The docstring for a package (i.e., the docstring of
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the package's __init__.py module) should also list the modules and
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subpackages exported by the package.
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The docstring for a function or method should summarize its
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behavior and document its arguments, return value(s), side
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effects, exceptions raised, and restrictions on when it can be
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called (all if applicable). Optional arguments should be
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indicated. It should be documented whether keyword arguments are
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part of the interface.
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The docstring for a class should summarize its behavior and list
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the public methods and instance variables. If the class is
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intended to be subclassed, and has an additional interface for
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subclasses, this interface should be listed separately (in the
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docstring). The class constructor should be documented in the
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docstring for its __init__ method. Individual methods should be
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documented by their own docstring.
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If a class subclasses another class and its behavior is mostly
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inherited from that class, its docstring should mention this and
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summarize the differences. Use the verb "override" to indicate
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that a subclass method replaces a superclass method and does not
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call the superclass method; use the verb "extend" to indicate that
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a subclass method calls the superclass method (in addition to its
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own behavior).
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*Do not* use the Emacs convention of mentioning the arguments of
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functions or methods in upper case in running text. Python is
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case sensitive and the argument names can be used for keyword
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arguments, so the docstring should document the correct argument
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names. It is best to list each argument on a separate line. For
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example::
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def complex(real=0.0, imag=0.0):
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"""Form a complex number.
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Keyword arguments:
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real -- the real part (default 0.0)
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imag -- the imaginary part (default 0.0)
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"""
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if imag == 0.0 and real == 0.0: return complex_zero
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...
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The BDFL [3] recommends inserting a blank line between the last
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paragraph in a multi-line docstring and its closing quotes,
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placing the closing quotes on a line by themselves. This way,
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Emacs' fill-paragraph command can be used on it.
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References and Footnotes
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[1] http://python.sf.net/peps/pep-0256.html
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[2] http://python.sf.net/peps/pep-0258.html
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[3] Guido van Rossum, Python's creator and Benevolent Dictator For
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Life.
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[4] http://www.python.org/doc/essays/styleguide.html
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[5] http://www.python.org/sigs/doc-sig/
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Copyright
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This document has been placed in the public domain.
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Acknowledgements
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The "Specification" text comes mostly verbatim from the Python
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Style Guide by Guido van Rossum [4].
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This document borrows ideas from the archives of the Python
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Doc-SIG [5]. Thanks to all members past and present.
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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End:
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