Why doesn't NotImplemented raise a TypeError?
comparison-operators, python, python-3.x
Solution
When you return `NotImplemented` you indicate that you do not know if `__ne__` should return `True` or `False`.
Normally, Python will then swap the operands; if `a != b` results in `NotImplemented`, it'll try `b != a` instead. That'll fail here too, since you use the same type on both sides of the operator. For the `!=` operator, Python will then fall back to comparing their memory addresses, and these are not the same (two separate instances), so False is returned.
See the `do_richcompare` C function for details.
You'll have to raise `TypeError()` manually if that is your expected outcome.
Problem
Suppose I define a class `A` and I don't want anyone to write an inequality of that class without getting away. ``` class A(): def __ne__(self, other): return NotImplemented print(A() != A()) ``` But this prints out `True` and doesn't raise a `TypeError` although I have deliberately "turned off" the `!=` operator?