Python file write

from math import sqrt


def is_prime(n):
    assert n > 0
    for i in range(2, int (sqrt(n)) + 1):
        if n % i == 0:
            return False
    return True if n != 1 else False


def main():
    file_names = ['a.txt', 'b.txt', 'c.txt']
    file_list = []
    try:
        for filename in file_names:
            file_list.append(open(filename, 'w', encoding='utf-8'))
        for n in range(1, 10000):
            if is_prime(n):
                if n < 100:
                    file_list[0].write(str(n) + "\r\n")
                elif n < 1000:
                    file_list[1].write(str(n) + "\r\n")
                else:
                    file_list[2].write(str(n) + "\r\n")
    except IOError:
        print("Error!")
    finally:
        for f in file_list:
            f.close()
    print("Complete!")


if __name__ == "__main__":
    main()