Bugs item #1249615, was opened at 2005-08-01 16:54 Message generated for change (Comment added) made by doerwalter You can respond by visiting: https://sourceforge.net/tracker/?func=detail&atid=105470&aid=1249615&group_id=5470
Please note that this message will contain a full copy of the comment thread, including the initial issue submission, for this request, not just the latest update. Category: Python Interpreter Core Group: Python 2.4 Status: Open Resolution: None Priority: 5 Submitted By: Hugh Gibson (hgibson50) Assigned to: Nobody/Anonymous (nobody) Summary: isinstance() fails depending on how modules imported Initial Comment: I have found an inconsistency with instance type checking dependent on how modules are imported. Using Windows 2000, Python 2.4.1. Source files are attached. Unzip preserving paths. To run, open a cmd window and set PythonPath to the location of the "system" folder. Run ServerRun.py. The program creates two class instances in two different modules, then calls a class function on one instance which takes as parameter the second instance. A call to isinstance to check if the parameter is of the correct class fails. If a parameter is passed to ServerRun.py then the class module is imported in a different way (specifying the path to the class) and the isinstance check succeeds. The output shows that before the call to the class function, an isinstance() call succeeds in both cases. There is obviously an easy fix in code, but I think that isinstance checking should not depend on how modules are imported. And, if I am using an incorrect module import sequence, that sequence should be disallowed. Sample output (also showing Windows 2000 version) is: Microsoft Windows 2000 [Version 5.00.2195] (C) Copyright 1985-2000 Microsoft Corp. C:\system\server>set pythonpath=c:\system C:\system\server>serverrun ServerRun: oNew.__class__ == <class 'Stream.Stream.CElement'> Stream.CElement == <class 'Stream.Stream.CElement'> isinstance(oNew, Stream.CElement) == True AddField(): oNew.__class__ == <class 'Stream.Stream.CElement'> CElement == <class 'server.Stream.Stream.CElement'> isinstance(oNew, CElement) == False C:\system\server>serverrun 1 ServerRun: oNew.__class__ == <class 'server.Stream.Stream.CElement'> Stream.CElement == <class 'server.Stream.Stream.CElement'> isinstance(oNew, Stream.CElement) == True AddField(): oNew.__class__ == <class 'server.Stream.Stream.CElement'> CElement == <class 'server.Stream.Stream.CElement'> isinstance(oNew, CElement) == True C:\system\server> ---------------------------------------------------------------------- >Comment By: Walter Dörwald (doerwalter) Date: 2005-08-02 15:14 Message: Logged In: YES user_id=89016 The problem is not isinstance() per se, but the fact that you import one Python script as two different modules. A simple demo might look like this: class Foo: pass import bug print isinstance(Foo(), bug.Foo) Put this in bug.py and run "python bug.py" and you'll get: True False So I think this bug should be closed. ---------------------------------------------------------------------- Comment By: Hugh Gibson (hgibson50) Date: 2005-08-02 15:00 Message: Logged In: YES user_id=732318 The case I found here is particularly worrying because I'm doing the isinstance() check in the module that contains the class. The success of the check depends on how that module was imported into other modules. The two cases are: from server.Stream import Stream from Stream import Stream We found this problem in a large application under development. Unit testing of the class succeeded because of the way the class module was imported. But the application itself failed because of the way one module imported the class module. The failure was obvious in our application but clearly there may be subtle failure modes - for example, exception handling may use isinstance() internally to see if an exception is an instance of a given class. I've modified the code and added some more testing which shows clearly that it's not the class object itself which is the problem, but when you have a class method which accepts an instance of the class as a parameter. Any test of the class of that parameter will fail if the first instance is created in one module and the second instance is created in another module, and they import the class module differently. Note that it's not restricted to isinstance() - checking __class__ fails as well. Also, in both of the modules that import the class module, isinstance() checking succeeds. There is a false sense of security that isinstance() checking will be OK. Modifed code attached. Hugh ---------------------------------------------------------------------- Comment By: Reinhold Birkenfeld (birkenfeld) Date: 2005-08-02 11:19 Message: Logged In: YES user_id=1188172 This has bitten me too at one time. When you import a module from a package starting from different sys.path entries, you will end up with two different modules in sys.modules. This seems to be intended behaviour, but I don't know enough about it to close this. ---------------------------------------------------------------------- You can respond by visiting: https://sourceforge.net/tracker/?func=detail&atid=105470&aid=1249615&group_id=5470 _______________________________________________ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
