Why does __ne__ exist?
> Let's put it this way. Suppose that __eq__ existed and __ne__ didn't,
> just like with __contains__. Go ahead: sell the notion of __ne__.
> Pitch it, show why we absolutely need to allow this. Make sure you
> mention the potential confusion when subclassing. Be sure to show why
> it's okay for "not in" to force to boolean but "==" should allow any
> return value.
__ne__ and __eq__ are important for building mask arrays in NumPy,
which allow complex indexing operations. A lot of NumPy's design was
inspired by MATLAB, so being able to index the same way as in MATLAB
is a pretty killer feature.
Indexing an array using mask arrays like this is idiomatic:
some_arr = np.array([-1, 0, 1, 2, 3, 4, 5, 2, -1, 3, -1, 6, 7, 3])
valid = some_arr[some_arr != -1]
Anybody with familiarity with NumPy appreciates that this is possible.
I imagine that ORMs like Django or SqlAlchemy also override __ne__ to
provide nice APIs.
Finally (and perhaps least imporant), there is a performance hit if
only allowing __eq__ and then taking its inverse.