You can assign to a slice:
a[:0] = b
Demo:
>>> a = [1,2,3]
>>> b = [4,5,6]
>>> a[:0] = b
>>> a
[4, 5, 6, 1, 2, 3]
Essentially, list.extend()
is an assignment to the list[len(list):]
slice.
You can 'insert' another list at any position, just address the empty slice at that location:
>>> a = [1,2,3]
>>> b = [4,5,6]
>>> a[1:1] = b
>>> a
[1, 4, 5, 6, 2, 3]