# Recursive Function (Recursion) in Python

In Python programming, a function is allowed to call itself. A function which calls itself directly or indirectly again and again until some specified condition is satisfied is known as Recursive Function.

A recursive function is a function defined in terms of itself via self-calling expressions. This means that the function will continue to call itself and repeat its behavior until some condition is satisfied to return a value .

Recursion is the process of defining something in terms of itself (Re Occuring). Recursion has many negatives. It repeatedly invokes the mechanism, and consequently increases the overhead of function calls. This repetition can be expensive in terms of both processor time and memory space.

Each recursive call causes another copies of the defined function, these set of copies can consume considerable amount of memory space.

## Recursive Function Example

#### Python program to find factorial of a given number recursively to illustrate recursive function

``````
# Program to find factorial using recursive function

# Recursive function definition
def factorial(n):
if n==0:
return 1
else:
return n * factorial(n-1)

number = int(input('Enter number: '))

# Displaying factorial
if(number< 0):
print('Factorial does not exist!')
else:
print('Factorial of %d is %d' %(number,factorial(number)))
``````

Output

```Run 1:
----------------
Enter number: 13
Factorial of 13 is 6227020800

Run 2:
----------------
Enter number: -9
Factorial does not exist!

Run 3:
----------------
Enter number: 0
Factorial of 0 is 1
```

## One Line Function Definition Code for Factorial Using Recursion in Python

``````
# Program to find factorial using recursive function

# Recursive function definition
def factorial(n):
return 1 if n==0 else n* factorial(n-1)

number = int(input('Enter number: '))

# Displaying factorial
if(number< 0):
print('Factorial does not exist!')
else:
print('Factorial of %d is %d' %(number,factorial(number)))

``````

Output

```Enter number: 5
Factorial of 5 is 120
```