python-peps/peps/pep-0224.rst

PEP: 224
Title: Attribute Docstrings
Author: Marc-André Lemburg <mal@lemburg.com>
Status: Rejected
Type: Standards Track
Content-Type: text/x-rst
Created: 23-Aug-2000
Python-Version: 2.1
Post-History:


Introduction
============

This PEP describes the "attribute docstring" proposal for Python
2.0.  This PEP tracks the status and ownership of this feature.
It contains a description of the feature and outlines changes
necessary to support the feature.  The CVS revision history of
this file contains the definitive historical record.


Rationale
=========

This PEP proposes a small addition to the way Python currently
handles docstrings embedded in Python code.

Python currently only handles the case of docstrings which appear
directly after a class definition, a function definition or as
first string literal in a module.  The string literals are added
to the objects in question under the ``__doc__`` attribute and are
from then on available for introspection tools which can extract
the contained information for help, debugging and documentation
purposes.

Docstrings appearing in locations other than the ones mentioned
are simply ignored and don't result in any code generation.

Here is an example::

    class C:
        "class C doc-string"

        a = 1
        "attribute C.a doc-string (1)"

        b = 2
        "attribute C.b doc-string (2)"

The docstrings (1) and (2) are currently being ignored by the
Python byte code compiler, but could obviously be put to good use
for documenting the named assignments that precede them.

This PEP proposes to also make use of these cases by proposing
semantics for adding their content to the objects in which they
appear under new generated attribute names.

The original idea behind this approach which also inspired the
above example was to enable inline documentation of class
attributes, which can currently only be documented in the class's
docstring or using comments which are not available for
introspection.


Implementation
==============

Docstrings are handled by the byte code compiler as expressions.
The current implementation special cases the few locations
mentioned above to make use of these expressions, but otherwise
ignores the strings completely.

To enable use of these docstrings for documenting named
assignments (which is the natural way of defining e.g. class
attributes), the compiler will have to keep track of the last
assigned name and then use this name to assign the content of the
docstring to an attribute of the containing object by means of
storing it in as a constant which is then added to the object's
namespace during object construction time.

In order to preserve features like inheritance and hiding of
Python's special attributes (ones with leading and trailing double
underscores), a special name mangling has to be applied which
uniquely identifies the docstring as belonging to the name
assignment and allows finding the docstring later on by inspecting
the namespace.

The following name mangling scheme achieves all of the above::

    __doc_<attributename>__

To keep track of the last assigned name, the byte code compiler
stores this name in a variable of the compiling structure.  This
variable defaults to NULL.  When it sees a docstring, it then
checks the variable and uses the name as basis for the above name
mangling to produce an implicit assignment of the docstring to the
mangled name.  It then resets the variable to NULL to avoid
duplicate assignments.

If the variable does not point to a name (i.e. is NULL), no
assignments are made.  These will continue to be ignored like
before.  All classical docstrings fall under this case, so no
duplicate assignments are done.

In the above example this would result in the following new class
attributes to be created::

    C.__doc_a__ == "attribute C.a doc-string (1)"
    C.__doc_b__ == "attribute C.b doc-string (2)"

A patch to the current CVS version of Python 2.0 which implements
the above is available on SourceForge at [1]_.


Caveats of the Implementation
=============================

Since the implementation does not reset the compiling structure
variable when processing a non-expression, e.g. a function
definition, the last assigned name remains active until either the
next assignment or the next occurrence of a docstring.

This can lead to cases where the docstring and assignment may be
separated by other expressions::

   class C:
       "C doc string"

       b = 2

       def x(self):
           "C.x doc string"
            y = 3
            return 1

       "b's doc string"

Since the definition of method "x" currently does not reset the
used assignment name variable, it is still valid when the compiler
reaches the docstring "b's doc string" and thus assigns the string
to ``__doc_b__``.

A possible solution to this problem would be resetting the name
variable for all non-expression nodes in the compiler.


Possible Problems
=================

Even though highly unlikely, attribute docstrings could get
accidentally concatenated to the attribute's value::

   class C:
       x = "text" \
           "x's docstring"

The trailing slash would cause the Python compiler to concatenate
the attribute value and the docstring.

A modern syntax highlighting editor would easily make this
accident visible, though, and by simply inserting empty lines
between the attribute definition and the docstring you can avoid
the possible concatenation completely, so the problem is
negligible.

Another possible problem is that of using triple quoted strings as
a way to uncomment parts of your code.

If there happens to be an assignment just before the start of the
comment string, then the compiler will treat the comment as
docstring attribute and apply the above logic to it.

Besides generating a docstring for an otherwise undocumented
attribute there is no breakage.


Comments from our BDFL
======================

Early comments on the PEP from Guido:

    I "kinda" like the idea of having attribute docstrings (meaning
    it's not of great importance to me) but there are two things I
    don't like in your current proposal:

    1. The syntax you propose is too ambiguous: as you say,
       stand-alone string literal are used for other purposes and could
       suddenly become attribute docstrings.

    2. I don't like the access method either (``__doc_<attrname>__``).

The author's reply:

    ::

        > 1. The syntax you propose is too ambiguous: as you say, stand-alone
        >    string literal are used for other purposes and could suddenly
        >    become attribute docstrings.


    This can be fixed by introducing some extra checks in the
    compiler to reset the "doc attribute" flag in the compiler
    struct.

    ::

        > 2. I don't like the access method either (``__doc_<attrname>__``).

    Any other name will do. It will only have to match these
    criteria:

    * must start with two underscores (to match ``__doc__``)
    * must be extractable using some form of inspection (e.g. by using
      a naming convention which includes some fixed name part)
    * must be compatible with class inheritance (i.e. should be
      stored as attribute)

Later on in March, Guido pronounced on this PEP in March 2001 (on
python-dev). Here are his reasons for rejection mentioned in
private mail to the author of this PEP:

    ...

    It might be useful, but I really hate the proposed syntax.

    ::

        a = 1
        "foo bar"
        b = 1

    I really have no way to know whether "foo bar" is a docstring
    for a or for b.

    ...

    You can use this convention::

        a = 1
        __doc_a__ = "doc string for a"

    This makes it available at runtime.

    ::

       > Are you completely opposed to adding attribute documentation
       > to Python or is it just the way the implementation works ? I
       > find the syntax proposed in the PEP very intuitive and many
       > other users on c.l.p and in private emails have supported it
       > at the time I wrote the PEP.

    It's not the implementation, it's the syntax.  It doesn't
    convey a clear enough coupling between the variable and the
    doc string.


Copyright
=========

This document has been placed in the Public Domain.


References
==========

.. [1] http://sourceforge.net/patch/?func=detailpatch&patch_id=101264&group_id=5470