Comparing two file's characters

Using the previous code that I made, this compares the characters used in both files and returns which characters one has that the other does not. 

def compare():
    file1 = characters()
    file2 = characters()
    in_file1_not_file2 = []
    in_file2_not_file1 = []
    for x in file1:
        if x not in file2:
            in_file1_not_file2.append(x)
    for x in file2:
        if x not in file1:
            in_file2_not_file1.append(x)
    print("Characters in first file but not in second file:", in_file1_not_file2)
    print("Characters in second file but not in first file:", in_file2_not_file1)