Factorial program in Python

The factorial of a whole number n, written n!, is the product of all whole numbers from 1 to n. For example, 5! = 1 × 2 × 3 × 4 × 5 = 120. By definition, 0! is 1.

The program

n = int(input("Enter a whole number: "))

if n < 0:
    print("Factorial is not defined for negative numbers.")
else:
    result = 1
    for i in range(1, n + 1):
        result *= i
    print(f"{n}! = {result}")

Typed in this example: 6. Press Run to type your own.

Enter a whole number: 6
6! = 720

How it works

• result starts at 1, not 0. If it started at 0, every multiplication would give 0.
• range(1, n + 1) gives the numbers 1, 2, ..., n. The end value of range() is not included, so you need n + 1.
• result *= i is a short way to write result = result * i.
• For n = 0, range(1, 1) is empty, so the loop does not run and the program prints 0! = 1, which is correct.
• Factorials grow very fast. Python integers have no size limit, so even 50! is printed exactly.

Another way: math.factorial()

The math module has a built-in function for this:

import math

print(math.factorial(6))
print(math.factorial(20))

720
2432902008176640000

math.factorial() raises a ValueError for negative numbers. The same calculation written as a function that calls itself is on the factorial using recursion page.