difference between `x in list` and `list.index(x)` for instances of a new-style class

[Riccardo Murri]

bpgbaires at gmail.com writes:

> On 28 dic, 20:12, Riccardo Murri <riccardo.mu... at gmail.com> wrote:
>
>> The list `self.base` contains "canonical" forms of the graphs and the
>> `graph` object must compare equal to some item of the list, which
>> indeed it does::
>>
>>   (Pydb) p graph == self.base[27]  
>>   True
>>
>>   (Pydb) p graph in self.base
>>   True
>>
>> However, I cannot directly get the index of the canonical graph (the
>> number "27" above was found by manual inspection)::
>>
>>   (Pydb) self.base.index(graph)
>>   *** ValueError: list.index(x): x not in list
>>
>> All graphs are instances of a `Graph` new-style class that implements
>> comparison operators `__eq__` and `__ne__`, but no other rich-compare
>> stuff.
>>
>> I'm using Python 2.5::
>>
>>   Python 2.5 (release25-maint, Dec  9 2006, 16:17:58)
>>   [GCC 4.1.2 20061115 (prerelease) (Debian 4.1.1-20)] on linux2
>>
>> So my question is: what are the implementation differences between `x
>> in list` and `list.index(x)` and why can one report that an item is in
>> the list while the other cannot find its index?  Should I add
>> something to the `Graph` class so that `index` works?
>
> (I've checked on 2.5.1 but I don't see any relevant differences with
> the 2.5 version). Looking at the source for both methods, they only
> use the __eq__ operator, but there is a slight difference: while one
> evaluates list[i]==x, the other reverses the operands. If your __eq__
> is not reflexive, that could explain the difference.
>

That was indeed the reason: a bug in Graph.__eq__ broke reflexivity in
certain cases.

Thank you very much!!

-- 
Riccardo Murri, via Galeazzo Alessi 61, 00176 Roma