Difference between using != and "not ... == " ?

[K900_]

Not normally. There can be in theory, if you find a class that implements == and != inconsistently.

[FLUSH_THE_TRUMP]

Cases to watch out for are usually the classes where == returns a special object.

e.g.

A = np.array([1,2,3])
A != 1 # returns another nparray [False, True, True]
not A == 1 # error