Python 2.6 / 3.0: Determining if a method is inherited (Fixed)

Fuzzyman fuzzyman at gmail.com
Sun Oct 5 21:35:17 CEST 2008


Hello all,

Sorry - my messages aren't showing up via google groups, so I'm kind
of posting on faith...

Anyway, I solved my problem (I think)...

import sys

if sys.version_info[0] == 3:
    def _has_method(cls, name):
        for B in cls.__mro__:
            if B is object:
                continue
            if name in B.__dict__:
                return True
        return False
else:
    def _has_method(cls, name):
        for B in cls.mro():
            if B is object:
                continue
            if name in B.__dict__:
                return True
        return False

I've reposted the question that this is the answer to below...


I may well be being dumb (it has happened before), but I'm struggling
to fix some code breakage with Python 2.6.

I have some code that looks for the '__lt__' method on a class:

if hasattr(clr, '__lt__'):

However - in Python 2.6 object has grown a default implementation of
'__lt__', so this test always returns True.

>>> class X(object): pass
...
>>> X.__lt__
<method-wrapper '__lt__' of type object at 0xa15cf0>
>>> X.__lt__ == object.__lt__
False

So how do I tell if the X.__lt__ is inherited from object? I can look
in the '__dict__' of the class - but that doesn't tell me if X
inherits '__lt__' from a base class other than object. (Looking inside
the method wrapper repr with a regex is not an acceptable answer...)

Some things I have tried:

>>> X.__lt__.__self__
<class '__main__.X'>
>>> dir(X.__lt__)
['__call__', '__class__', '__cmp__', '__delattr__', '__doc__',
'__format__', '__getattribute__', '__hash__', '__init__', '__name__',
'__new__', '__objclass__', '__reduce__', '__reduce_ex__', '__repr__',
'__self__', '__setattr__', '__sizeof__', '__str__',
'__subclasshook__']
>>> X.__lt__.__func__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'method-wrapper' object has no attribute '__func__'


Hmmm... I can get this working with Python 2.6 with:

if '__lt__' in dir(cls):

The default implementation of '__lt__' doesn't appear in the dir of
classes. However this fails with Python 3 where the default
implementation *does* appear in the output of 'dir'. Any suggestions?

Michael Foord
--
http://www.ironpythoninaction.com/



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