Recursively-defined Functions

In Python

Let $f(n)$ return the result of $$ \sum _{i=0}^ni $$ $f(n)$ can also be defined recursively as so: $$ f(n) = \begin{cases} 0, & n=0 \\ f(n-1)+n, & n>0 \end{cases} $$

def f(n: int) -> int:
    """Return the sum of the numbers between 0 and n, inclusive.

    Preconditions:
        - n >= 0

    >>> f(4)
    10
    """
    if n == 0:
        return 0
    else:
        return f(n - 1) + n