assymetry between a == b and a.__eq__(b)

Nick Coghlan ncoghlan at
Sat Dec 4 00:48:26 CET 2004

Steven Bethard wrote:
> Mel Wilson wrote:
>> In article <Qtqrd.177755$HA.59149 at attbi_s01>,
>> Steven Bethard <steven.bethard at> wrote:
>>> I believe what Peter Otten was pointing out is that calling __eq__ is
>>> not the same as using ==, presumably because the code for == checks the
>>> types of the two objects and returns False if they're different before
>>> the __eq__ code ever gets called.
>> Doesn't seem to:
> [snip]
> Check his example:

I suspect that example is due to the rule that "A op B" can be passed to any of 
the following, depending on the operator and the types of A and B:


The latter two get invoked when B is a proper subclass of A (using 'op' for 
commutative operations, and 'rop' for potentially non-commutative ones). This is 
so that subclasses can interact with parent classes correctly.

So, in Steven's original code, B.__eq__(A) was invoked, and returned False 
(since A was the parent class of B, not a subclass).


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