[Python-bugs-list] [ python-Bugs-693121 ] Set == non-Set is a TypeError

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Bugs item #693121, was opened at 2003-02-25 14:37
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Category: Python Library
Group: Python 2.3
>Status: Closed
>Resolution: Fixed
Priority: 5
Submitted By: Chris Reedy (creedy)
Assigned to: Tim Peters (tim_one)
Summary: Set == non-Set is a TypeError

Initial Comment:
Comparing Sets to non-Sets results in a TypeError. For
example:

>>> from sets import Set
>>> x = Set([1])
>>> x == 2
TypeError: Binary operation only permitted between sets

This seems to be inconsistent  with other Python
behavior. For example:

>>> (1,2,3) == 2
0
>>> "abcd" == 2
0

Assuming that the standard behavior is what is desired,
the implementation of __eq__ and other comparison
operators in sets.py should be changed to return
NotImplemented when the other object is not a Set.

Note: Looking at the code, I'm not sure whether the
implementation of __lt__, __le__, etc. should also be
changed to not return a Type Error.


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>Comment By: Tim Peters (tim_one)
Date: 2003-03-01 19:32

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This turned out to be messier than I hoped, because Set 
also implements __cmp__, so returning NotImplented 
didn't do a lick of good.  See the checkin msg and new code 
comments for discussions of subtleties.

Lib/sets.py; new revision: 1.43
Checking in Lib/test/test_sets.py; new revision: 1.21 
Misc/NEWS; new revision: 1.685

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Comment By: Raymond Hettinger (rhettinger)
Date: 2003-03-01 13:02

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That makes sense.  What I don't understand is how to 
implement it.  Any change I make to __eq__ and __ne__ 
causes test_cmp to fail.

The new tests are attached.

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Comment By: Guido van Rossum (gvanrossum)
Date: 2003-02-28 21:48

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Yes, that's the right solution. In general, more objects
should implement this behavior: __eq__, __ne__ return
NotImplemented, the others continue to raise TypeError.

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Comment By: Tim Peters (tim_one)
Date: 2003-02-28 21:45

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Short course: returning NotImplemented (== falling back to 
address comparison, unless the other comparand wants  to 
handle it) instead for __eq__ and __ne__ would be fine.  
That would be similar to what was done for datetime 
objects in 2.3a2.  The other 4 relationals shouldn't be 
changed.

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Comment By: Raymond Hettinger (rhettinger)
Date: 2003-02-28 21:08

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The OP added this note at the behest of 
comp.lang.python.  The thread is at:
http://tinyurl.com/6n91 and includes
some comments by Tim.

Assigning to Guido to decide whether he wants to keep 
that behavior.

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