[Python-ideas] PEP 3155 - Qualified name for classes and functions

[Eric Snow]

On Oct 29, 2011 4:22 PM, "Antoine Pitrou" <solipsis at pitrou.net> wrote:
>
>
> Hello,
>
> I would like to propose the following PEP for discussion and, if
> possible, acceptance. I think the proposal shouldn't be too
> controversial (I find it quite simple and straightforward myself :-)).
>
> I also have a draft implementation that's quite simple
> (http://hg.python.org/features/pep-3155).
>
> Regards
>
> Antoine.
>
>
>
> PEP: 3155
> Title: Qualified name for classes and functions
> Version: $Revision$
> Last-Modified: $Date$
> Author: Antoine Pitrou <solipsis at pitrou.net>
> Status: Draft
> Type: Standards Track
> Content-Type: text/x-rst
> Created: 2011-10-29
> Python-Version: 3.3
> Post-History:
> Resolution: TBD
>
>
> Rationale
> =========
>
> Python's introspection facilities have long had poor support for nested
> classes.  Given a class object, it is impossible to know whether it was
> defined inside another class or at module top-level; and, if the former,
> it is also impossible to know in which class it was defined.  While
> use of nested classes is often considered poor style, the only reason
> for them to have second class introspection support is a lousy pun.
>
> Python 3 adds insult to injury by dropping what was formerly known as
> unbound methods.  In Python 2, given the following definition::
>
>    class C:
>        def f():
>            pass
>
> you can then walk up from the ``C.f`` object to its defining class::
>
>    >>> C.f.im_class
>    <class '__main__.C'>
>
> This possibility is gone in Python 3::
>
>    >>> C.f.im_class
>    Traceback (most recent call last):
>      File "<stdin>", line 1, in <module>
>    AttributeError: 'function' object has no attribute 'im_class'
>    >>> dir(C.f)
>    ['__annotations__', '__call__', '__class__', '__closure__', '__code__',
>    '__defaults__', '__delattr__', '__dict__', '__dir__', '__doc__',
>    '__eq__', '__format__', '__ge__', '__get__', '__getattribute__',
>    '__globals__', '__gt__', '__hash__', '__init__', '__kwdefaults__',
>    '__le__', '__lt__', '__module__', '__name__', '__ne__', '__new__',
>    '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__',
>    '__str__', '__subclasshook__']
>
> This limits again the introspection capabilities available to the user.
> It can produce actual issues when porting software to Python 3, for
example
> Twisted Core where the issue of introspecting method objects came up
> several times.  It also limits pickling support [1]_.
>
>
> Proposal
> ========
>
> This PEP proposes the addition of a ``__qname__`` attribute to functions
> and classes.  For top-level functions and classes, the ``__qname__``
> attribute is equal to the ``__name__`` attribute.  For nested classed,
> methods, and nested functions, the ``__qname__`` attribute contains a
> dotted path leading to the object from the module top-level.
>
> The repr() and str() of functions and classes is modified to use
``__qname__``
> rather than ``__name__``.
>
> Example with nested classes
> ---------------------------
>
> >>> class C:
> ...   def f(): pass
> ...   class D:
> ...     def g(): pass
> ...
> >>> C.__qname__
> 'C'
> >>> C.f.__qname__
> 'C.f'
> >>> C.D.__qname__
> 'C.D'
> >>> C.D.g.__qname__
> 'C.D.g'
>
> Example with nested functions
> -----------------------------
>
> >>> def f():
> ...   def g(): pass
> ...   return g
> ...
> >>> f.__qname__
> 'f'
> >>> f().__qname__
> 'f.g'
>
>
> Limitations
> ===========
>
> With nested functions (and classes defined inside functions), the dotted
> path will not be walkable programmatically as a function's namespace is
not
> available from the outside.  It will still be more helpful to the human
> reader than the bare ``__name__``.
>

If it helps, I have a patch that adds f_func to frame objects [1].  It
points to the function that was called, so __qname__ could reference that
function.

-eric

[1] see http://bugs.python.org/issue12857
> As the ``__name__`` attribute, the ``__qname__`` attribute is computed
> statically and it will not automatically follow rebinding.
>
>
> References
> ==========
>
> .. [1] "pickle should support methods":
>   http://bugs.python.org/issue9276
>
> Copyright
> =========
>
> This document has been placed in the public domain.
>
>
>
> ..
>   Local Variables:
>   mode: indented-text
>   indent-tabs-mode: nil
>   sentence-end-double-space: t
>   fill-column: 70
>   coding: utf-8
>   End:
>
>
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