A factorial is the product of an integer and all the integers below it; for example, the factorial of 4 (written as “4!”) is 4 * 3 * 2 * 1 = 24. Here is a simple Python program that calculates the factorial of a given number:

```
def factorial(n):
if n == 0:
return 1
else:
return n * factorial(n-1)
number = int(input("Enter a number: "))
print(factorial(number))
```

## Here is how the program works:

- We define a function
`factorial()`

that takes an integer`n`

as an argument. - The function contains a base case, which is
`if n == 0: return 1`

. This is important because the factorial of 0 is 1, and we want to make sure our program terminates. - If
`n`

is not 0, the function returns`n * factorial(n-1)`

. This is known as recursion, and it means that the function will call itself with a smaller value`n`

until the base case is reached. - Outside the function, we ask the user to enter a number, which is stored in the variable
`number`

. - We call the
`factorial()`

function and pass`number`

it as an argument. The result of the function is printed on the screen.

## Here is a more detailed explanation of the Python program for calculating the factorial of a number:

1) First, we define a function called `factorial()`

that takes an integer `n`

as an argument. The function definition is as follows:

```
def factorial(n):
```

2) Next, we add a conditional statement to check if `n`

is equal to 0. If it is, the function returns 1. This is the base case for our function, which means that it will stop calling itself once the value of `n`

reaches 0. The base case is important because it allows our program to terminate and avoid an infinite loop.

```
if n == 0:
return 1
```

3) If `n`

is not 0, the function returns `n * factorial(n-1)`

. This line of code uses recursion, which means that the function calls itself with a smaller value of `n`

until the base case is reached. In this case, the function will multiply `n`

by the result of `factorial(n-1)`

, which is the factorial of `n-1`

.

```
else:
return n * factorial(n-1)
```

4) Outside the function, we ask the user to enter a number, which is stored in the variable `number`

. We use the `int()`

function to convert the user input from a string to an integer.

```
number = int(input("Enter a number: "))
```

5) Finally, we call the `factorial()`

function and pass `number`

as an argument. The result of the function is printed to the screen using the `print()`

function.

```
print(factorial(number))
```

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