Section on Shorthand Operators not strictly true

[madphysicist]

I am referring to http://pymbook.readthedocs.io/en/latest/operatorsexpressions.html#shorthand-operator. x op= y is not always equivalent to x = x op y. The statement is always true for immutable objects like ints and tuples. Mutable objects, on the other hand, can have a completely different implementation of the shorthand operators since those operators can modify the object in-place. For example, lists define a legitimately different __iadd__ operator (which is basically extend).