[Python-bugs-list] [ python-Bugs-467267 ] isinstance depends on import form

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Bugs item #467267, was opened at 2001-10-02 14:56
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https://sourceforge.net/tracker/?func=detail&atid=105470&aid=467267&group_id=5470

Category: Python Interpreter Core
Group: Not a Bug
Status: Closed
Resolution: Invalid
Priority: 5
Submitted By: Luis P Caamano (lcaamano)
Assigned to: Guido van Rossum (gvanrossum)
Summary: isinstance depends on import form

Initial Comment:
isinstance(obj, cls) fails if cls is in a package and 
the import line of the obj creator is different from 
the import line in the code calling isinstance.

It seems to me that the problem is the address test in 
PyObject_IsInstance(), because different import forms 
produce objects with different addresses.

A limited workaround would be to test for class name 
but that doesn't include subclasses or types.  So, 
instead of:

if isinstance(obj, class):

use

if obj.__class__.__name__ == 'classname':

-------Sample--------------------

Python 2.1.1 (#2, Jul 25 2001, 17:54:11) 
[GCC 2.96 20000731 (Red Hat Linux 7.1 2.96-81)] on 
linux2
Type "copyright", "credits" or "license" for more 
information.
>>> from spam.eggs import SpamAndEggs
>>> import spam.eggs
>>> '0x%x' % id(spam.eggs.SpamAndEggs)
'0x8161ce4'
>>> '0x%x' % id(SpamAndEggs)
'0x814c29c'
>>> sag = spam.eggs.SpamAndEggs()
>>> isinstance(sag, SpamAndEggs)
0
>>> isinstance(sag, spam.eggs.SpamAndEggs)
1
>>> sag.__class__
<class spam.eggs.SpamAndEggs at 0x8161ce4>
>>> sag.__class__.__name__
'SpamAndEggs'
>>> SpamAndEggs.__name__
'SpamAndEggs'
>>> spam.eggs.SpamAndEggs.__name__
'SpamAndEggs'
>>> 


----------------------------------------------------------------------

>Comment By: Guido van Rossum (gvanrossum)
Date: 2002-09-16 17:58

Message:
Logged In: YES 
user_id=6380

This problem happens when the same module is loaded twice in
two different ways, one as part of a package, and one as a
top-level module. It is not a bug.

----------------------------------------------------------------------

Comment By: Nobody/Anonymous (nobody)
Date: 2002-09-16 17:21

Message:
Logged In: NO 

I hit this exact problem in Python 2.2.1, and am not sure why 
this is a closed issue.  The behavior seems to be incorrect (it 
broke a runtime event-type checking system I wrote in 
python.) 
 
It is true that if the import statement in the object creator and 
the caller of isinstance are identical, the problem does not 
occur.  Still, this is only a workaround, and the problem 
appears to persist in the general case. 
 
What is the status of this bug?  I would be happy to provide 
example code if needed.  Please email 
 
<beach@verinet.com> 
 
Thanks! 

----------------------------------------------------------------------

Comment By: Guido van Rossum (gvanrossum)
Date: 2001-10-02 17:09

Message:
Logged In: YES 
user_id=6380

OK, closing this as Invalid.

----------------------------------------------------------------------

Comment By: Luis P Caamano (lcaamano)
Date: 2001-10-02 17:01

Message:
Logged In: YES 
user_id=279987

With the site-packages/pkg1.pth file:

>>> from pkg1.spam.eggs import SpamAndEggs
>>> import spam.eggs
>>> '0x%x' % id(SpamAndEggs)
'0x814e174'
>>> '0x%x' % id(spam.eggs.SpamAndEggs)
'0x8168fac'
>>> import pkg1.spam.eggs
>>> '0x%x' % id(pkg1.spam.eggs.SpamAndEggs)
'0x814e174'
>>> 

It is starting to make sense.

--
lpc


----------------------------------------------------------------------

Comment By: Luis P Caamano (lcaamano)
Date: 2001-10-02 16:34

Message:
Logged In: YES 
user_id=279987

After getting your email I went and rechecked things in 
site-packages and the only suspicious thing was a .pth 
file.  I removed the .pth file and now I'm getting the same 
address.  I'm still investigating.   At this moment I have 
no idea why I'm getting different objects.  

Let me investigate a little bit more and I'll get back to 
you.

I apologize for not having verified this better.


--
Luis Caamano

----------------------------------------------------------------------

Comment By: Guido van Rossum (gvanrossum)
Date: 2001-10-02 15:26

Message:
Logged In: YES 
user_id=6380

We need more information: please show all code involved (use
the file upload, please check the checkbox or the upload
won't work; you must be logged in to SourceForge as
yourself).

>From the session you post, it seems that there are two
distinct SpamAndEggs objects, but without seeing your code
it's hard to understand why.

----------------------------------------------------------------------

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