[Python-Dev] Data descriptor doc/implementation inconsistency

Benjamin Peterson benjamin at python.org
Mon Jan 11 01:07:35 CET 2010

Consider this program:

class Descr(object):
    def __init__(self, name):
        self.name = name
    def __set__(self, instance, what):
        instance.__dict__[self.name] = what

class X(object):
    attr = Descr("attr")

x = X()
x.attr = 42

It gives in output:

<__main__.Descr object at 0x7fe1c9b28150>

The documentation [1] says that Descr is a data descriptor because it
defines the __set__ method. It also states that data descriptors
always override the value in the instance dictionary. So, the second
line should also be the descriptor object according to the

My question is: Is this a doc bug or a implementation bug? If the
former, it will be the description of a data descriptor much less
consistent, since it will require that a __get__ method be present,
too. If the latter, the fix may break some programs relying on the
ability to "cache" a value in the instance dictionary.

[1] http://docs.python.org/reference/datamodel#invoking-descriptors


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