Why does Python have an __ne__ operator method instead of just __eq__?

SQLAlchemy is a great example. For the uninitiated, SQLAlchemy is a ORM and uses Python expression to generate SQL statements. In a expression such as

meta.Session.query(model.Theme).filter(model.Theme.id == model.Vote.post_id)

the model.Theme.id == model.VoteWarn.post_id does not return a boolean, but a object that eventually produces a SQL query like WHERE theme.id = vote.post_id. The inverse would produce something like WHERE theme.id <> vote.post_id so both methods need to be defined.

Some libraries do fancy things and don't return a bool from these operations. For example, with numpy:

>>> import numpy as np>>> np.array([1,2,5,4,3,4,5,4,4])==4array([False, False, False,  True, False,  True, False,  True,  True], dtype=bool)>>> np.array([1,2,5,4,3,4,5,4,4])!=4array([ True,  True,  True, False,  True, False,  True, False, False], dtype=bool)

When you compare an array to a single value or another array you get back an array of bools of the results of comparing the corresponding elements. You couldn't do this if x!=y was simply equivalent to not (x==y).

More generally, in many valued logic systems, equals and not equals are not necessarily exact inverses of each other.

The obvious example is SQL where True == True, False == False and Null != Null. Although I don't know if there are any specific Python examples I can imagine it being implemented in places.