http://docs.python.org/3/reference/simple_stmts.html#assignment-statements

The docs says:

"If the target is a slicing: The primary expression in the reference is evaluated. It should yield a mutable sequence object (such as a list). The assigned object should be a sequence object of the same type."

This seems wrong, because the assigned object can be any iterable:

a = [4, 5, 6]
a[0:0] = range(1, 4)
# a is now [1, 2, 3, 4, 5, 6]

Since range objects have a known length, that example is not enough to show 'any iterable'. However, generators do not even have a __length_hint__ and they work too.

a = [1,2,3]
a[0:1] = (i for i in range(4))
print(a)
>>> 
[0, 1, 2, 3, 2, 3]