[ python-Bugs-718532 ] inspect, class instances and __getattr__

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Bugs item #718532, was opened at 2003-04-09 22:01
Message generated for change (Comment added) made by sschwarzer
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Category: Python Library
>Group: Python 2.4
Status: Open
Resolution: None
Priority: 5
Submitted By: Stefan Schwarzer (sschwarzer)
Assigned to: Raymond Hettinger (rhettinger)
Summary: inspect, class instances and __getattr__

Initial Comment:
inspect.isclass(class_instance) fails if the
corresponding class uses a "wildcard" implementation of
__getattr__.

Example:

Python 2.2.2 (#1, Nov 13 2002, 22:53:57) 
[GCC 2.95.4 20020320 [FreeBSD]] on freebsd4
Type "help", "copyright", "credits" or "license" for
more information.
>>> import inspect
>>> class X:
...     def __getattr__(self, name):
...             if name == 'foo':
...                     return 1   
...             if name == 'bar':
...                     return 2
...             else:
...                     return "default"
... 
>>> x = X()
>>> inspect.isclass(x)
1

The problematic expression in inspect.isclass is
hasattr(object, '__bases__') which returns a true value.

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>Comment By: Stefan Schwarzer (sschwarzer)
Date: 2005-01-28 17:44

Message:
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Hi Facundo

The problem still exists in both Python 2.3.4 and 2.4.

A possible test case is:

import inspect
import unittest

class TestIsclass(unittest.TestCase):
    def test_instance_with_getattr(self):
        class Cls:
            def __getattr__(self, name):
                return "not important"
        obj = Cls()
        # obj is not a class
        self.failIf(inspect.isclass(obj))


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Comment By: Facundo Batista (facundobatista)
Date: 2005-01-15 18:50

Message:
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Please, could you verify if this problem persists in Python 2.3.4
or 2.4?

If yes, in which version? Can you provide a test case?

If the problem is solved, from which version?

Note that if you fail to answer in one month, I'll close this bug
as "Won't fix".

Thank you! 

.    Facundo

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Comment By: Raymond Hettinger (rhettinger)
Date: 2003-04-15 12:40

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Ping, if this change is made, will isclass() still be able to 
find extension classes?

The addition of the hasattr(object, '__bases__') was made 
by you in ver 1.11 about two years ago. 


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Comment By: Walter Dörwald (doerwalter)
Date: 2003-04-15 12:01

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type(object) in (types.TypeType, types.ClassType)
won't work with custom metaclasses.
isinstance(object, (type, types.ClassType))
would be better.


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Comment By: Stefan Schwarzer (sschwarzer)
Date: 2003-04-15 10:01

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Hello Raymond, thanks for your reply. In fact, I'm also not
sure if it counts as a bug. I also suggested a patch (handle
__getattr__ requests for __bases__ with an AttributeError)
for for the SF project which causes/d the problem.

I think, if there's a better way to decide on "class-ness"
than now, the code in inspect should be changed.
Fortunately, it doesn't have to be backward-compatible,
because the module is always distributed with a certain
interpreter version.

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Comment By: Raymond Hettinger (rhettinger)
Date: 2003-04-15 02:36

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Hmm.  I'm not sure that counts as a bug.  In an OO 
language, it's a feature that objects can be made to look 
like and be substituable for other types.  In this case, 
you've taught your object to be able to fake some classlike 
behavior (having a __bases__ attribute).

OTOH, inspect could have a stronger test for classhood:    
type(object) in (types.TypeType, types.ClassType)

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