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Python Sets Comparisons

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Python Sets Comparisons

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This week I lost some time playing with Python's sets.

After digging into Python source code, I finally discovered there is what seems to be little bug. Anyway, it has been "fixed" in Python 3, fortunately. I did not find if it was reported somewhere, but since it's fixed, it's not a big deal.

Python 2.7.1+ (default, Apr 20 2011, 10:53:33) 
[GCC 4.5.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> class A(object):
...     def __eq__(self, other):
...             return True
... 
>>> A() == A()
True
>>> [A()] == [A()]
True
>>> set([A()]) == set([A()])
False

This clearly did not make any sense to me. I've then tested under Python 3.2:

Python 3.2.1a0 (default, May  4 2011, 19:59:25) 
[GCC 4.6.1 20110428 (prerelease)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> class A(object):
...     def __eq__(self, other):
...             return True
... 
>>> set([A()]) == set([A()])
Traceback (most recent call last):
  File "", line 1, in 
TypeError: unhashable type: 'A'

 

At least, raising an error is saner. It actually helped me to understand what I needed to do to have my sets working correctly with Python 2:

Python 2.7.1+ (default, Apr 20 2011, 10:53:33) 
[GCC 4.5.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> class A(object):
...     def __eq__(self, other):
...             return True
...     def __hash__(self):
...             return 123456789
... 
>>> set([A()]) == set([A()])
True

Source: http://julien.danjou.info/blog/2011/python-sets-comparisons

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