factorial.py

def factorial(n):

    if n < 0:
        return "Factorial is not defined for negative numbers."
    elif n == 0:
        return 1
    else:
        fact = 1
        for i in range(1, n + 1):
            fact *= i
        return fact

num = int(input("Enter a non-negative integer: "))

result = factorial(num)
print("The factorial of", num, "is", result)