Compare two files and create file with unique lines

compare_select_unique.py

def find_unique(a, b):
    return [item for item in b if item not in a]


a = open("file1.txt", "rb").readlines()
b = open("file2.txt", "rb").readlines()

with open("out.txt", "wb") as f:
    unique_lines = find_unique(a, b)
    for line in unique_lines:
        f.write(line)