This is actually used more often in other languages, because python can usually emulate that behavior with optional arguments. The idea is that the recursion gets a number of initial arguments, that the user doesn't need to provide, which help keep track of the problem.

def sum(lst):
    return sumhelper(lst, 0)

def sumhelper(lst, acc):
    if lst:
        acc += lst[0]
        return sumhelper(lst[0], acc)
    return acc

Here it's used to set a starting parameter to 0, so the user doesn't have to provide it. However, in python you can emulate it by making acc optional:

def sum(lst, acc=0):
    if lst:
        acc += lst[0]
        return sum(lst[0], acc)
    return acc

This is actually used more often in other languages, because python can usually emulate that behavior with optional arguments. The idea is that the recursion gets a number of initial arguments, that the user doesn't need to provide, which help keep track of the problem.

def sum(lst):
    return sumhelper(lst, 0)

def sumhelper(lst, acc):
    if lst:
        acc += lst[0]
        return sumhelper(lst[1:], acc)
    return acc

Here it's used to set a starting parameter to 0, so the user doesn't have to provide it. However, in python you can emulate it by making acc optional:

def sum(lst, acc=0):
    if lst:
        acc += lst[0]
        return sum(lst[1:], acc)
    return acc