为什么这个return语句在这个python递归函数中抛出一个错误？

Question

I am practicing using recursive functions by summing all the elements in a list.

The function I made was:

def list_sum_recursive(input_list):

    #base case, list is empty
    if input_list == []:
        print("empty")
        return

    #recursive case
    else:
        #grab the first element
        head = input_list[0]
        del input_list[0]
        #return the sum of the head plus the sum of the rest of the list
        return head + list_sum_recursive(input_list)

This function throws this error however:

TypeError: unsupported operand type(s) for +: 'int' and 'NoneType

I did figure out the solution, making the base case return 0 instead of just return .

But now I'm curious what the plain return was, or wasn't, doing to throw an error? And why in python, a language that is pretty flexible and forgiving, such a thing is an issue?

Answer 1

As pointed out in the comments, don't return None in the first section. Return 0 instead.

def list_sum_recursive(input_list):

    #base case, list is empty
    if input_list == []:
    #    print("empty")
        return 0

    #recursive case
    else:
        #grab the first element
        head = input_list[0]
        del input_list[0]
        #return the sum of the head plus the sum of the rest of the list
        return head + list_sum_recursive(input_list)

print(list_sum_recursive([1,2,3]))

Running the program gives us

$ python test.py
6

Answer 2

Just want to give you a more Pythonic version, I hope you don't mind.

def list_sum_recursive(input_list):

    #base case, list is empty
    if not input_list:
        return 0
    #return the sum of the head plus the sum of the rest of the list
    return input_list.pop(0) + list_sum_recursive(input_list)

print(list_sum_recursive([1,2,3]))

Answer 3

def list_sum_recursive(input_list):

    #base case, list is empty
    if input_list == []:
        print("empty")
        return

    #recursive case
    else:
        #grab the first element
        head = input_list[0]
        del input_list[0]
        #return the sum of the head plus the sum of the rest of the list
        x=list_sum_recursive(input_list)
        if(x==None):
            return head + 0
        else:
            return head+x

return 0 instead of none. or you can do this trick.