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Sat Apr 4 03:54:51 CEST 2009 · https://mail.python.org/pipermail/python-dev/2009-April/088186.html

Paul Moore wrote:
> 2009/4/3 R. David Murray <rdmurray at bitdance.com>:
>> a == b
>>
>> So, python calls a.__eq__(b)
>>
>> Now, that function does:
>>
>> a.key == b
>>
>> Since b is an object with an __eq__ method, python calls
>> b.__eq__(a.key).
> 
> That's the bit I can't actually find documented anywhere.

It doesn't quite work the way RDM desribed it - he missed a step.

a == b

So, python calls a.__eq__(b)

Now, that function does:

a.key == b

which first calls a.key.__eq__(b) # This step was missing

Since str has no idea what an Element is, that returns NotImplemented.

Since __eq__ is defined as being commutative, the interpreter then tries
b.__eq__(a.key).

That function does:

b.key == a.key

which calls b.key.__eq__(a.key)

which is a well defined string comparison and returns the expected answer.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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[Python-Dev] Getting values stored inside sets