[Python-bugs-list] [ python-Bugs-535444 ] super() broken with classmethods
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Wed, 27 Mar 2002 05:06:09 -0800
Bugs item #535444, was opened at 2002-03-26 22:13
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Category: Python Interpreter Core
Group: Python 2.2.1 candidate
Status: Open
Resolution: None
Priority: 5
Submitted By: Phillip J. Eby (pje)
Assigned to: Nobody/Anonymous (nobody)
Summary: super() broken with classmethods
Initial Comment:
Using super() in a classmethod breaks in Python 2.2.
Apparently, when super looks up an attribute from the
__mro__ sequence, it calls the found descriptor's
__get__ with the descriptor itself as the 'type'
argument, which breaks horribly with class methods
(which always binds to the type argument).
Presumably, the fix is to pass a NULL type argument,
which should work with the recent fixes to the
classmethod's __get__ logic. In other words, this code
in the super_getattro function Objects/typeobject.c:
tmp = f(res, su->obj, res);
should probably actually read:
tmp = f(res, su->obj, NULL);
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>Comment By: Phillip J. Eby (pje)
Date: 2002-03-27 13:06
Message:
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class cm1(object):
def cmm(klass):
print klass
cmm = classmethod(cmm)
class cm2(cm1):
def cmm(klass):
super(cm2,klass).cmm()
cmm = classmethod(cmm)
cm2.cmm()
The above code prints "<classmethod object at 0x00A9B930>",
demonstrating that super(cm2,klass).cmm is bound improperly.
(It should print <class '__main__.cm2'>, analagous to how
calling cm1.cmm() directly prints <class '__main__.cm1'>.)
You can more specifically verify this like so:
>>> cm1.cmm.im_self
<class '__main__.cm1'>
>>> cm2.cmm.im_self
<class '__main__.cm2'>
>>> super(cm2,cm2).cmm.im_self
<classmethod object at 0x00A9B930>
>>>
The last item's im_self should of course be <class
'__main__.cm2'>. As I said, the problem is that
super_getattro incorrectly asks the classmethod descriptor
to bind to *itself*, rather than to a type.
Note that if you use the pure Python example version of
"super" defined in the python.org/2.2/descrintro.html
document, the above examples work correctly as long as you
use a version of Python that has the "classmethod core dump"
problem fixed. However, the builtin super() never works
correctly for classmethods, whether the "classmethod core
dump" is fixed or not.
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Comment By: Martin v. Löwis (loewis)
Date: 2002-03-27 12:26
Message:
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Can you give an example of how to break it? Please also
report what your example does when you run it.
----------------------------------------------------------------------
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