My solution almost same, but the output is in different order...
does order matter?
Here is my solution
from itertools import product
L = ['a', 'b']
k = 4
L2 = range(1, k+1)
print [x+ str(y) for x,y in list(product(L,L2))]
output:
['a1', 'a2', 'a3', 'a4', 'b1', 'b2', 'b3', 'b4']