[Solved] To preallocate or not to preallocate lists in Python

When should and shouldn’t I preallocate a list of lists in python?
For example, I have a function that takes 2 lists and creates a lists of lists out of it.
Quite like, but not exactly, matrix multiplication.

Should I preallocate the result,

X = Len(M)
Y = Len(F)
B = [[None for y in range(Y)] for x in range(X)]
for x in range(X):
    for y in range(Y):
        B[x][y] = foo(M[x], F[y])
return B

or dynamically create it as I go?

B = []
for m in M:
    B.append([])
    for f in F:
        B[-1].append(foo(m, f))
return B

Preallocating seems unnecessary and perhaps slower, but dynamically looks obfuscated. In particular, B[-1].append(...) seems illegible.