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Title: Docstring Processing System Framework
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Version: $Revision: 1.7 $
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Last-Modified: $Date: 2002/11/09 20:58:21 $
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Author: David Goodger <goodger@users.sourceforge.net>
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Discussions-To: <doc-sig@python.org>
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Content-Type: text/x-rst
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Post-History: 13-Jun-2001
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Python lends itself to inline documentation. With its built-in
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docstring syntax, a limited form of `Literate Programming`_ is easy to
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do in Python. However, there are no satisfactory standard tools for
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extracting and processing Python docstrings. The lack of a standard
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toolset is a significant gap in Python's infrastructure; this PEP aims
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The issues surrounding docstring processing have been contentious and
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difficult to resolve. This PEP proposes a generic Docstring
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Processing System (DPS) framework, which separates out the components
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(program and conceptual), enabling the resolution of individual issues
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either through consensus (one solution) or through divergence (many).
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It promotes standard interfaces which will allow a variety of plug-in
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components (input context readers, markup parsers, and output format
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The concepts of a DPS framework are presented independently of
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implementation details.
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Road Map to the Docstring PEPs
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==============================
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There are many aspects to docstring processing. The "Docstring PEPs"
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have broken up the issues in order to deal with each of them in
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isolation, or as close as possible. The individual aspects and
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associated PEPs are as follows:
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* Docstring syntax. PEP 287, "reStructuredText Docstring Format"
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[#PEP-287]_, proposes a syntax for Python docstrings, PEPs, and
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* Docstring semantics consist of at least two aspects:
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- Conventions: the high-level structure of docstrings. Dealt with
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in PEP 257, "Docstring Conventions" [#PEP-257]_.
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- Methodology: rules for the informational content of docstrings.
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* Processing mechanisms. This PEP (PEP 256) outlines the high-level
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issues and specification of an abstract docstring processing system
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(DPS). PEP 258, "Docutils Design Specification" [#PEP-258]_, is an
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overview of the design and implementation of one DPS under
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* Output styles: developers want the documentation generated from
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their source code to look good, and there are many different ideas
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about what that means. PEP 258 touches on "Stylist Transforms".
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This aspect of docstring processing has yet to be fully explored.
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By separating out the issues, we can form consensus more easily
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(smaller fights ;-), and accept divergence more readily.
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There are standard inline documentation systems for some other
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languages. For example, Perl has POD_ ("Plain Old Documentation") and
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Java has Javadoc_, but neither of these mesh with the Pythonic way.
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POD syntax is very explicit, but takes after Perl in terms of
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readability. Javadoc is HTML-centric; except for "``@field``" tags,
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raw HTML is used for markup. There are also general tools such as
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Autoduck_ and Web_ (Tangle & Weave), useful for multiple languages.
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There have been many attempts to write auto-documentation systems
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for Python (not an exhaustive list):
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- Marc-Andre Lemburg's doc.py_
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- Daniel Larsson's pythondoc_ & gendoc_
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- Doug Hellmann's HappyDoc_
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- Laurence Tratt's Crystal_
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- Ka-Ping Yee's pydoc_ (pydoc.py is now part of the Python standard
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- Tony Ibbs' docutils_ (Tony has donated this name to the `Docutils
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- Edward Loper's STminus_ formalization and related efforts
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These systems, each with different goals, have had varying degrees of
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success. A problem with many of the above systems was over-ambition
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combined with inflexibility. They provided a self-contained set of
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components: a docstring extraction system, a markup parser, an
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internal processing system and one or more output format writers with
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a fixed style. Inevitably, one or more aspects of each system had
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serious shortcomings, and they were not easily extended or modified,
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preventing them from being adopted as standard tools.
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It has become clear (to this author, at least) that the "all or
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nothing" approach cannot succeed, since no monolithic self-contained
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system could possibly be agreed upon by all interested parties. A
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modular component approach designed for extension, where components
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may be multiply implemented, may be the only chance for success.
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Standard inter-component APIs will make the DPS components
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comprehensible without requiring detailed knowledge of the whole,
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lowering the barrier for contributions, and ultimately resulting in a
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rich and varied system.
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Each of the components of a docstring processing system should be
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developed independently. A "best of breed" system should be chosen,
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either merged from existing systems, and/or developed anew. This
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system should be included in Python's standard library.
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PyDoc & Other Existing Systems
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------------------------------
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PyDoc became part of the Python standard library as of release 2.1.
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It extracts and displays docstrings from within the Python interactive
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interpreter, from the shell command line, and from a GUI window into a
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web browser (HTML). Although a very useful tool, PyDoc has several
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deficiencies, including:
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- In the case of the GUI/HTML, except for some heuristic hyperlinking
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of identifier names, no formatting of the docstrings is done. They
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are presented within ``<p><small><tt>`` tags to avoid unwanted line
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wrapping. Unfortunately, the result is not attractive.
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- PyDoc extracts docstrings and structural information (class
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identifiers, method signatures, etc.) from imported module objects.
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There are security issues involved with importing untrusted code.
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Also, information from the source is lost when importing, such as
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comments, "additional docstrings" (string literals in non-docstring
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contexts; see PEP 258 [#PEP-258]_), and the order of definitions.
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The functionality proposed in this PEP could be added to or used by
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PyDoc when serving HTML pages. The proposed docstring processing
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system's functionality is much more than PyDoc needs in its current
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form. Either an independent tool will be developed (which PyDoc may
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or may not use), or PyDoc could be expanded to encompass this
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functionality and *become* the docstring processing system (or one
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such system). That decision is beyond the scope of this PEP.
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Similarly for other existing docstring processing systems, their
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authors may or may not choose compatibility with this framework.
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However, if this framework is accepted and adopted as the Python
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standard, compatibility will become an important consideration in
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these systems' future.
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The docstring processing system framework is broken up as follows:
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1. Docstring conventions. Documents issues such as:
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- What should be documented where.
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- First line is a one-line synopsis.
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PEP 257 [#PEP-257]_ documents some of these issues.
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2. Docstring processing system design specification. Documents
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- High-level spec: what a DPS does.
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- Command-line interface for executable script.
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- Docstring extraction rules.
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- Readers, which encapsulate the input context.
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- Document tree: the intermediate internal data structure. The
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output of the Parser and Reader, and the input to the Writer all
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share the same data structure.
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- Transforms, which modify the document tree.
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- Writers for output formats.
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- Distributors, which handle output management (one file, many
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files, or objects in memory).
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These issues are applicable to any docstring processing system
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implementation. PEP 258 [#PEP-258]_ documents these issues.
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3. Docstring processing system implementation.
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4. Input markup specifications: docstring syntax. PEP 287 [#PEP-287]_
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proposes a standard syntax.
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5. Input parser implementations.
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6. Input context readers ("modes": Python source code, PEP, standalone
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text file, email, etc.) and implementations.
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7. Stylists: certain input context readers may have associated
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stylists which allow for a variety of output document styles.
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8. Output formats (HTML, XML, TeX, DocBook, info, etc.) and writer
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Components 1, 2/3/5, and 4 are the subject of individual companion
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PEPs. If there is another implementation of the framework or
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syntax/parser, additional PEPs may be required. Multiple
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implementations of each of components 6 and 7 will be required; the
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PEP mechanism may be overkill for these components.
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A SourceForge project has been set up for this work at
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http://docutils.sourceforge.net/.
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References and Footnotes
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========================
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.. [#PEP-287] PEP 287, reStructuredText Docstring Format, Goodger
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(http://www.python.org/peps/pep-0287.html)
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.. [#PEP-257] PEP 257, Docstring Conventions, Goodger, Van Rossum
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(http://www.python.org/peps/pep-0257.html)
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.. [#PEP-258] PEP 258, Docutils Design Specification, Goodger
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(http://www.python.org/peps/pep-0258.html)
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.. _Literate Programming: http://www.literateprogramming.com/
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.. _POD: http://www.perldoc.com/perl5.6/pod/perlpod.html
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.. _Javadoc: http://java.sun.com/j2se/javadoc/
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http://www.helpmaster.com/hlp-developmentaids-autoduck.htm
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.. _Web: http://www-cs-faculty.stanford.edu/~knuth/cweb.html
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http://www.lemburg.com/files/python/SoftwareDescriptions.html#doc.py
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.. _gendoc: http://starship.python.net/crew/danilo/pythondoc/
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.. _HappyDoc: http://happydoc.sourceforge.net/
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.. _Crystal: http://www.btinternet.com/~tratt/comp/python/crystal/
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.. _pydoc: http://www.python.org/doc/current/lib/module-pydoc.html
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.. _docutils: http://homepage.ntlworld.com/tibsnjoan/docutils/
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.. _Docutils project: http://docutils.sourceforge.net/
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.. _STMinus: http://www.cis.upenn.edu/~edloper/pydoc/
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.. _Python Doc-SIG: http://www.python.org/sigs/doc-sig/
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This document has been placed in the public domain.
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This document borrows ideas from the archives of the `Python
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Doc-SIG`_. Thanks to all members past & present.
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