From https://docs.python.org/3/tutorial/datastructures.html#more-on-lists :

    list.extend(iterable)
        Extend the list by appending all the items from the iterable.
        Equivalent to a[len(a):] = iterable.

The "equivalent" is not very good. Consider 

    def gen():
        yield 1
        yield 2
        raise Exception

Using `a.extend(gen())` would mutate `a`. Using slice assignment would still consume the generator, but `a` would not be modified.

I propose a different example to use to describe the behaviour of extend:

    for x in iterable:
        a.append(x)