classification
Title: Set == non-Set is a TypeError
Type: Stage:
Components: Library (Lib) Versions: Python 2.3
process
Status: closed Resolution: fixed
Dependencies: Superseder:
Assigned To: tim_one Nosy List: creedy, gvanrossum, rhettinger, tim_one
Priority: normal Keywords:

Created on 2003-02-25 19:37 by creedy, last changed 2003-03-02 00:32 by tim_one.

Files
File name Uploaded Description Edit Remove
test_sets.diff rhettinger, 2003-03-01 17:51 New tests
Messages
msg14833 (view) Author: Chris Reedy (creedy) Date: 2003-02-25 19:37 
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.
msg14834 (view) Author: Raymond Hettinger (rhettinger) Date: 2003-03-01 02:08 
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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.
msg14835 (view) Author: Tim Peters (tim_one) Date: 2003-03-01 02:45 
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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.
msg14836 (view) Author: Guido van Rossum (gvanrossum) Date: 2003-03-01 02:48 
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user_id=6380

Yes, that's the right solution. In general, more objects
should implement this behavior: __eq__, __ne__ return
NotImplemented, the others continue to raise TypeError.
msg14837 (view) Author: Raymond Hettinger (rhettinger) Date: 2003-03-01 18:02 
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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.
msg14838 (view) Author: Tim Peters (tim_one) Date: 2003-03-02 00:32 
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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
History
Date User Action Args
2003-02-25 19:37:45creedycreate