Add two lists of different lengths in python, start from the right

Edit (2020-18-03):

>>> P = [3, 0, 2, 1]
>>> Q = [8, 7]
>>> from itertools import zip_longest
>>> [x+y for x,y in zip_longest(reversed(P), reversed(Q), fillvalue=0)][::-1]
[3, 0, 10, 8]

Obviously, if you choose a convention where the coefficients are ordered the opposite way, you can just use

P = [1, 2, 0, 3]
Q = [7, 8]
[x+y for x,y in zip_longest(P, Q, fillvalue=0)]

I believe a simple for loop is far simpler than a comprehension with zip_longest...

P = [3, 0, 2, 1]
Q = [8, 7]

A, B = sorted([P, Q], key=len)

for i, x in enumerate(reversed(A), 1):
   B[-i] += x

#print(B)

If you need to keep P unchanged, copy it first. Also, if Q is much smaller than P, this will be more effective.