Problem comparing object graphs and trees

[Peter Otten]

raphael.marvie at gmail.com wrote:

> </pre>
> $ cat cmp.py
> 
> class A:
>     def __init__(self, b):
>         self.b = b
>     def __cmp__(self, other):
>         return self.b == other.b
> 
> class B:
>     pass
> 
> if __name__ == '__main__':
>     b = B()
>     a1 = A(b)
>     a2 = A(b)
>     print a1 == a2
>     print a1 == a1
> 
> $ python cmp.py
> False
> False
> </pre>
> 
> I swear I am not drunk, but why isn't a1 == a2 and worse why isn't a1
> == a1? Does someone have a clue and can explain to me this suprising
> behavior? (python 2.4.3 on Ubuntu 6.06).

__cmp__() must return 0 for equal objects:

>>> 1 .__cmp__(0), 1 .__cmp__(1), 1 .__cmp__(2)
(1, 0, -1)

You might have a look at rich comparison before you proceed:

>>> class A(object):
...     def __init__(self, b):
...             self.b = b
...     def __eq__(self, other):
...             return self.b == other.b
...
>>> A(1) == A(1)
True
>>> A(1) == A(42)
False

Peter