[Python-bugs-list] [ python-Bugs-535444 ] super() broken with classmethods
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Mon, 01 Apr 2002 12:50:17 -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: Guido van Rossum (gvanrossum)
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: Michael Hudson (mwh)
Date: 2002-04-01 20:50
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OK, you've found the right place, good.
The bug/patch guidlines are probably confusing because,
unless you're part of the Python project I think you can't
attach a file to a report you didn't submit. Generally I
prefer patches to be attached to bugs, but that's just my
opinion, I don't know what other developers think. I also
think the whole bug/patch division is misguided, but that's
another rant entirely.
I think you're doing fine! Now we just wait for Guido to
stop changing nappies :)
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Comment By: Phillip J. Eby (pje)
Date: 2002-04-01 19:46
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I was indeed just asking where to put it, and how to
*insert* the test. Anyway, I found that most of what was
needed for the test was already in
test_descr.classmethods(), there were just a few conditions
using super() that needed adding. I've uploaded the patch
for test_descr to the patch #537536 for this bug.
By the way, the bug/patch submission guidelines were a
little unclear to me; specifically whether I was supposed to
put the patch with the bug or the bug with the patch or
upload everything to both or what. Hope my ignorance hasn't
inconvenienced anyone; this is my first time submitting
Python bugs and fixes. Also, I hadn't worked with the test
framework used in Python's test suite before, although I've
done quite a bit with unittest in my own code.
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Comment By: Michael Hudson (mwh)
Date: 2002-04-01 09:36
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Assign to Guido.
Bearing in mind that I haven't even tried to understand this
bug, the fact that you can't come up with a test case stands
against you... if you're just asking where to put it, stick
it in test_descr.
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Comment By: Phillip J. Eby (pje)
Date: 2002-03-31 23:17
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Patch #537536 submitted to fix this. "make test" output is
same as before the patch. If someone can give me an idea of
where/how to insert a regression test for the bug being
fixed, I'll submit a patch for that, too. Thanks.
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Comment By: Michael Hudson (mwh)
Date: 2002-03-31 20:19
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Unless someone can come up with a obviously correct patch
(and convince Guido that it's obviously correct) very soon,
this isn't going to go into 2.2.1.
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Comment By: Phillip J. Eby (pje)
Date: 2002-03-27 13:38
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Ugh. I just realized that my "presumable fix" is actually
wrong. I checked back on my "Python super" workaround, and
realized I modified Guido's example slightly, to call
__get__(self.__obj__,starttype), instead of
__get__(self.__obj__). This implies that the fix to
super_getattro is a little more complicated, since
super_getattro doesn't have a C variable equivalent to
starttype in the Python version of super. :(
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Comment By: Phillip J. Eby (pje)
Date: 2002-03-27 13:06
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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
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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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