Replace head of a file with the contents of another file

Question

I have two files:

File A:

line A_1
line A_2
line A_3
...
line A_n

File B:

line B_1
line B_2
...
line B_n
line B_n+1
...
line B_m

I want the first n lines of file A to replace the first n lines of file B ( m > n ). After this file B should look like this:

File B:

line A_1
...
line A_n
line B_n+1
...
line B_m

What is the most pythonic solution to this?

Answer 1

with open("newB.txt","wb") as fout:
    with open("B.txt") as b:
        for lineA,lineB in zip(open("A.txt"),b):
            fout.write(lineA)
        fout.writelines(b)

maybe ... I guess...

Answer 2

with open(A) as fp_A:
    for i, line in enumerate(fp_A):
        if i <= n:
            print(line)
        else:
            break
with open(B) as fp_B:
    for i, line in enumerate(fp_B):
        if i > n:
            print(line)