[ python-Bugs-1174712 ] subclassing ModuleType and another built-in type

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Bugs item #1174712, was opened at 2005-04-01 10:22
Message generated for change (Comment added) made by mwh
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https://sourceforge.net/tracker/?func=detail&atid=105470&aid=1174712&group_id=5470

Category: Python Interpreter Core
Group: None
Status: Open
Resolution: None
Priority: 5
Submitted By: Armin Rigo (arigo)
Assigned to: Nobody/Anonymous (nobody)
Summary: subclassing ModuleType and another built-in type

Initial Comment:
class X(types.ModuleType, str): pass
X('name')
-> segfault

This buggy subclassing goes through typeobject.c's checks because PyModuleObject looks exactly like a user-defined subclass of 'object': it has a PyObject_HEAD followed only by the dict, as specified by tp_dictoffset.

A fix would be to forbid any subclassing to move the tp_dictoffset of a non-heap type.

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>Comment By: Michael Hudson (mwh)
Date: 2005-04-03 14:39

Message:
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> This might point to the need for cleaning up typeobject.c's
> darker corners...

No kidding.  Maybe Samuele, you and I can gang up on Guido at 
EuroPython (is he sprinting?  Perhaps we should ask).

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Comment By: Armin Rigo (arigo)
Date: 2005-04-02 13:27

Message:
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The proposed fix is not good enough.  If another built-in C type similar to PyModuleObject is defined (say by a C extension module), then it would become possible to subclass both this new type and ModuleType.  This might lead to unwanted behavior if e.g. one parent class allows rebinding __dict__ and the other not (and crashes if __dict__ is rebound).

This might point to the need for cleaning up typeobject.c's darker corners...

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You can respond by visiting: 
https://sourceforge.net/tracker/?func=detail&atid=105470&aid=1174712&group_id=5470