[docs] [issue23317] Incorrect description of descriptor invocation in Python Language Reference

Justin Eldridge report at bugs.python.org
Sun Jan 25 18:16:03 CET 2015

New submission from Justin Eldridge:

The section titled "Invoking Descriptors" in the Python Language Reference [1]

    Class Binding
        If binding to a new-style class, A.x is transformed into the call: 
        A.__dict__['x'].__get__(None, A).

This suggests that __get__ is looked up on the instance of x, when I believe
that it is actually looked up on the type. That is, it's my understanding that
A.x invokes:

    type(A.__dict__['x']).__get__(A.__dict__['x'], None, Foo))

Here's some Python 3.4 code demonstrating this:

    class A:

    class Descriptor:
        def __get__(self, obj, cls):
    A.x = Descriptor()

    def replacement_get(obj, cls):
        print("This is a replacement.")
    A.__dict__['x'].__get__ = replacement_get

Now, writing:

    >>> A.x

    >>> A.__dict__['x'].__get__(None, A)
    This is a replacement!

    >>> type(A.__dict__['x']).__get__(A.__dict__['x'], None, A)

The documentation makes a similar statement about instance binding that also
appears to be incorrect. This is the case in all versions of the document I
could find.

What I believe to be the actual behavior is implied by a later section in the
same document, which states that the implicit invocation of special methods is
only guaranteed to work correctly if the special method is defined on the type,
not the instance.  This suggests that the statements in "Invoking Descriptors"
aren't quite correct, and while the true behavior is a little more verbose, I
think it would be worthwhile to update the documentation so as to avoid

[1]: https://docs.python.org/2/reference/datamodel.html#invoking-descriptors

assignee: docs at python
components: Documentation
messages: 234676
nosy: Justin.Eldridge, docs at python
priority: normal
severity: normal
status: open
title: Incorrect description of descriptor invocation in Python Language Reference
type: enhancement
versions: Python 2.7, Python 3.2, Python 3.3, Python 3.4, Python 3.5, Python 3.6

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