[Numpy-discussion] str == int puzzlement

[Friedrich Romstedt]

2010/8/1 Matthew Brett <matthew.brett at gmail.com>:
>> Maybe it would be better to raise a ValueError, which is not caught by
>> the evaluation mechanism, to prevent such stuff.
>
> Sorry that this is not yet clear to me, but, is it true then that:
>
> The only situation where array.__eq__ sensibly falls back to python
> __eq__ is for the individual elements of object arrays?

Btw, I changed my opinion on that.  The rationale was to inform the
user about that numpy cannot do that.  This is based on that Python
cannot do better.  But right this assumtion isn't true, since maybe
the other operand knows quite well how to compare the pears to the
apples, for whatever reason.  So better stick to the documentation:

"A rich comparison method may return the singleton NotImplemented if
it does not implement the operation for a given pair of arguments."

And since at the moment, the functionality is in fact
"NotImplemented", this is the way to go at the moment.  Of course,
there is room for change, e.g., by breaking it down to the scalar
level, if the r.h.s. is not a numpy.ndarray, and returning an array of
Falses else.  But even this is error-prone, because, the r.h.s. may be
a descendant-class instance of numpy.ndarray, knowing better what to
do, but being no longer asked.

So I come to the conclusion, that in fact all "implementations" of the
__eq__ for not-comparable dtypes aren't.  They are just based on too
many assumptions.  They would be a replacement for a real
implementation, for a real functionality.

To come to your question, Matthew, I think I would tend to give the
general recommendation to not mix dtypes which not really compare to
each other.  It's just error-prone.  In fact, when you want to compare
by Python's implementation of __eq__, which should in fact be the
builtin cmp(), then object arrays are the way to go.  But then you
cannot use a single non-object ndarray.

To summarise, numpy simply does not take part in __eq__ calculations
where it does not know what to do.

Python does the rest to ask the others about what they think about the
operation.  But the others should be ndarray-aware.

Friedrich