[Patches] [ python-Patches-995939 ] Avoid calling tp_compare with different types

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Patches item #995939, was opened at 2004-07-22 14:14
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Category: Core (C code)
Group: None
Status: Open
Resolution: None
Priority: 5
Submitted By: Dima Dorfman (ddorfman)
Assigned to: Nobody/Anonymous (nobody)
Summary: Avoid calling tp_compare with different types

Initial Comment:
C implementations of the tp_compare slot usually expect both of their 
arguments to have the type for which that slot is defined (this isn't 
documented anywhere that I can find, but many core types assume this, 
and extension authors are likely to do the same). A problem occurs if a 
user-defined nb_coerce slot (__coerce__) returns objects with different 
types. Avoid the problem by refusing to call a non-Python tp_compare 
unless the arguments have the same type. Indiscriminately calling 
tp_compare with different types is wrong as long as there are 
implementations that don't check the type of the second argument, but 
Python implementations should be allowed to receive a different type.

Other options for fixing the problem:
  - Make PyNumber_CoerceEx require that the results have the same 
type. This would prevent a user-defined __coerce__ from 
communicating an arbitrary object to a user-defined __cmp__. 
Furthermore, there are uses for coerce besides comparison, and those 
uses might not require identical types.
  - Change tp_compare implementations to check the type of the 
second argument. This might be the most desirable long-term solution, 
but as I understand it, tp_richcompare is preferred over tp_compare for 
new code, and old (current) code is likely assuming that the types are 
the same.

Addresses bug #980352


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