PEP 224 – Attribute Docstrings
- Marc-André Lemburg <mal at lemburg.com>
- Standards Track
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.
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
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.
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:
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 .
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
A possible solution to this problem would be resetting the name variable for all non-expression nodes in the compiler.
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.
This document has been placed in the Public Domain.
Last modified: 2023-09-09 17:39:29 GMT