How to Get Factorial in Python?

Before version 2.6 of Python, you had to calculate the factorial by writing your own function/code.

From version 2.6 and above, the factorial can be calculated by using the built-in function math.factorial(x) which is part of the math module.

In this tutorial, I will show you examples of getting the factorial both ways: Using the math module’s factorial function and writing custom functions for getting the factorial.

How to use the factorial function?

The built-in factorial function can be used as follows:

The function returns the factorial of argument x.

An example of factorial function

In this example, I used four different integer values to get the factorial by using the math.factorial function.

See the code and output below:

#Factorial function demo



from math import factorial

print ("The factorial of 10 = " ,factorial(10))

print ("The factorial of 20 = " ,factorial(20))

print ("The factorial of 25 = " ,factorial(25))

print ("The factorial of 100 = " ,factorial(30))

Output:

What if a non-integer is passed to factorial function?

The example shows using a float number in the factorial function. See what it results in?

#Facotial function demo

from math import factorial
print ("The factorial of float number 11.5 = " ,factorial(11.5))

The result:

Writing a custom function for getting factorial

If you still prefer writing your own function to get the factorial then this section is for you.

Here, I will give three different functions for getting the factorial of a number.

The aim of each function is to multiply the numbers downwards until it reach 1 from the given number:

Function Number 1:

#Custom Factorial function demo

def factorial(x):

    #If 0 is passed as argument then result should be 1

    if x == 0:

        return 1

    else:

        return x * factorial(x-1)



print ("Factorial of 0 = ", factorial(0))

print ("Factorial of 1 = ", factorial(1))

print ("Factorial of 2 = ", factorial(2))

print ("Factorial of 3 = ", factorial(3))

print ("Factorial of 4 = ", factorial(4))

print ("Factorial of 5 = ", factorial(5))

print ("Factorial of 6 = ", factorial(6))

print ("Factorial of 7 = ", factorial(7))

The output:

Using for loop and range example

In the following custom function for getting the factorial, a for loop and range is used. The factorial of 0 and other numbers is as shown in the output below:

#Custom Factorial function #2 demo

def factorial(n):

    x = 1

    for i in range(n,0,-1):

        x = x * i

    return x



print ("Factorial of 0 = ", factorial(0))

print ("Factorial of 1 = ", factorial(1))

print ("Factorial of 17 = ", factorial(17))

print ("Factorial of 19 = ", factorial(19))

print ("Factorial of 21 = ", factorial(21))

print ("Factorial of 23 = ", factorial(23))

print ("Factorial of 25 = ", factorial(25))

print ("Factorial of 27 = ", factorial(27))

The output:

Using while loop to calculate factorial

This function uses a while loop for calculating the factorial of the given number:

#Custom Factorial function by while loop

def factorial(number):
    x = 1
    while number >= 1:
        x = x * number
        number = number - 1
    return x

print ("0 = ", factorial(0))

print ("1 = ", factorial(1))

print ("11 = ", factorial(11))

print ("15 = ", factorial(15))

print ("17 = ", factorial(17))

print ("19 = ", factorial(19))

print ("21 = ", factorial(21))

print ("23 = ", factorial(23))

The result: