def file_diff(file1, file2):
    """
    Returns set of line numbers from file1 which differ from file2
    """
    diff_lines = set()
    with open(file1, "r") as file1open:
        with open(file2, "r") as file2open:
            count = 0
            while(1):
                line1 = file1open.readline()
                line2 = file2open.readline()
                if not line1 or not line2:
                    break
                elif not line1 == line2:
                    diff_lines.add(count)
                count += 1
    return diff_lines

    
    if file_len(file1) < file_len(file2):
        print("WARNING: " + file2 + " has more lines than " + file1)
    elif file_len(file1) < file_len(file2):
        print("WARNING: " + file1 + " has more lines than " + file2)