Following system colour scheme Selected dark colour scheme Selected light colour scheme

Python Enhancement Proposals

PEP 216 – Docstring Format

Moshe Zadka <moshez at>


Table of Contents


This PEP has been withdrawn.


It has been superseded by PEP 287.


Named Python objects, such as modules, classes and functions, have a string attribute called __doc__. If the first expression inside the definition is a literal string, that string is assigned to the __doc__ attribute.

The __doc__ attribute is called a documentation string, or docstring. It is often used to summarize the interface of the module, class or function. However, since there is no common format for documentation string, tools for extracting docstrings and transforming those into documentation in a standard format (e.g., DocBook) have not sprang up in abundance, and those that do exist are for the most part unmaintained and unused.

Perl Documentation

In Perl, most modules are documented in a format called POD – Plain Old Documentation. This is an easy-to-type, very low level format which integrates well with the Perl parser. Many tools exist to turn POD documentation into other formats: info, HTML and man pages, among others. However, in Perl, the information is not available at run-time.

Java Documentation

In Java, special comments before classes and functions function to document the code. A program to extract these, and turn them into HTML documentation is called javadoc, and is part of the standard Java distribution. However, the only output format that is supported is HTML, and JavaDoc has a very intimate relationship with HTML.

Python Docstring Goals

Python documentation string are easy to spot during parsing, and are also available to the runtime interpreter. This double purpose is a bit problematic, sometimes: for example, some are reluctant to have too long docstrings, because they do not want to take much space in the runtime. In addition, because of the current lack of tools, people read objects’ docstrings by “print”ing them, so a tendency to make them brief and free of markups has sprung up. This tendency hinders writing better documentation-extraction tools, since it causes docstrings to contain little information, which is hard to parse.

High Level Solutions

To counter the objection that the strings take up place in the running program, it is suggested that documentation extraction tools will concatenate a maximum prefix of string literals which appear in the beginning of a definition. The first of these will also be available in the interactive interpreter, so it should contain a few summary lines.

Docstring Format Goals

These are the goals for the docstring format, as discussed ad nauseam in the doc-sig.

  1. It must be easy to type with any standard text editor.
  2. It must be readable to the casual observer.
  3. It must not contain information which can be deduced from parsing the module.
  4. It must contain sufficient information so it can be converted to any reasonable markup format.
  5. It must be possible to write a module’s entire documentation in docstrings, without feeling hampered by the markup language.

Docstring Contents

For requirement 5. above, it is needed to specify what must be in docstrings.

At least the following must be available:

  1. A tag that means “this is a Python something, guess what”

    Example: In the sentence “The POP3 class”, we need to markup “POP3” so. The parser will be able to guess it is a class from the contents of the poplib module, but we need to make it guess.

  2. Tags that mean “this is a Python class/module/class var/instance var…”

    Example: The usual Python idiom for singleton class A is to have _A as the class, and A a function which returns _A objects. It’s usual to document the class, nonetheless, as being A. This requires the strength to say “The class A” and have A hyperlinked and marked-up as a class.

  3. An easy way to include Python source code/Python interactive sessions
  4. Emphasis/bold
  5. List/tables

Docstring Basic Structure

The documentation strings will be in StructuredTextNG ( Since StructuredText is not yet strong enough to handle (a) and (b) above, we will need to extend it. I suggest using [<optional description>:python identifier]. E.g.: [class:POP3], [:POP3.list], etc. If the description is missing, a guess will be made from the text.

Unresolved Issues

Is there a way to escape characters in ST? If so, how? (example: * at the beginning of a line without being bullet symbol)

Is my suggestion above for Python symbols compatible with ST-NG? How hard would it be to extend ST-NG to support it?

How do we describe input and output types of functions?

What additional constraint do we enforce on each docstring? (module/class/function)?

What are the guesser rules?

Rejected Suggestions

XML – it’s very hard to type, and too cluttered to read it comfortably.


Last modified: 2024-04-14 20:08:31 GMT