A function that merges two arrays together at a given length creating one array array

merge_arrays.py

def merge_arrays(a, b, c):
    index = 0
    b_index = 0
    b_len = len(b)
    d = []
    for i in a:
        d.append(i)

        index += 1
        if index == c:
            if b_index < b_len:
                d.append(b[b_index])
                b_index += 1
                index = 0
    while b_index < b_len:
        d.append(b[b_index])
        b_index += 1
    return d