# Python Recursion

> Understand recursion in Python, where a function calls itself. Learn it through the classic factorial example, and why you get a RecursionError at 1000 calls.

Author: Flavio Copes | Published: 2021-01-05 | Canonical: https://flaviocopes.com/python-recursion/

A function in [Python](https://flaviocopes.com/python-introduction/) can call itself. That's what recursion is. And it can be pretty useful in many scenarios.

The common way to explain recursion is by using the factorial calculation.

The factorial of a number is the number `n` mutiplied by `n-1`, multiplied by `n-2`... and so on, until reaching the number `1`:

```
3! = 3 * 2 * 1 = 6
4! = 4 * 3 * 2 * 1 = 24
5! = 5 * 4 * 3 * 2 * 1 = 120
```

Using recursion we can write a function that calculates the factorial of any number:

```python
def factorial(n):
    if n == 1: return 1
    return n * factorial(n-1)

print(factorial(3)) #   6
print(factorial(4)) #  24
print(factorial(5)) # 120
```

If inside the `factorial()` function you call `factorial(n)` instead of `factorial(n-1)`, you are going to cause an infinite recursion. Python by default will halt recursions at 1000 calls, and when this limit is reached, you will get a `RecursionError` error.

Recursion is helpful in many places, and it helps us simplify our code when there's no other optimal way to do it, so it's good to know this technique.