what's wrong with my code? recursively finding sum of nested list

Question

I'm trying to make a function that returns the sum of all integers in a tree list in the format:

element 1 is an integer
element 2 is another treelist or None
element 3 is another treelist or None

ex: [1,[1,None,None],None]

so basically i want my function to add up all the integers in that list and return 2.

This is what i made so far..

def sum_nested(t):
    sum = 0
    for i in t:
        if type(i) == type(None):
           sum += 0
        if type(i) == list:
           return sum_nested(i)
        else:
            sum = sum + i

    return sum

However, I get the error:

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

cant seem to figure out what to do when i hit that None type..any suggestions?

`sum += 0` equals `sum`, so that `if` block is superfluous. – Joel Cornett Mar 13 '14 at 19:18 — Joel Cornett, Mar 13 '14 at 19:18

Joel Cornett · Answer 1 · 2014-03-13T19:47:04.843

2

How about:

def sum_nested(seq):

    # Don't use `sum` as a variable name, as it shadows the builtin `sum()`
    total = 0
    for item in seq:

        # This will allow you to sum *any* iterable (tuples, for example)
        if hasattr(item, '__iter__'): 

            # Use the `+=` syntactic sugar
            total += sum_nested(item) 
        else:

            # `None or 0` evaluates to 0 :)
            total += item or 0 

    return total

edited Mar 13 '14 at 19:47

answered Mar 13 '14 at 19:23

Joel Cornett

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hey this works so thank you but what i don't understand is how you made the 2nd last statement "total += item or 0. How does it know when to add 0 to the the total? – user3050527 Mar 13 '14 at 21:41
@user3050527: `None`, `''`, `{}`, `0`, `[]`, and of course `False` are "false-y" values in Python. This means that they all evaluate to false inside of a logical expression. `A or B` is such an expression. This [answer](http://stackoverflow.com/a/13960667/1142167) explains it pretty clearly. – Joel Cornett Mar 13 '14 at 22:44

BoshWash · Answer 2 · 2014-03-13T19:35:24.457

1

Try this:

t = [1, [1, None, None], None]
def sum_nested(t):
    sum = 0
    for i in t:
        if type(i) is int:
            sum += 1
        elif type(i) == list:
           sum += sum_nested(i)

    return sum

print(sum_nested(t))

if you want to test if something is None the shortest form is:

if i:

But in your case that's not really necessary since you're not changing sum anyways

edited Mar 13 '14 at 19:35

answered Mar 13 '14 at 19:16

BoshWash

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score 1 · Answer 3 · answered Mar 13 '14 at 19:44

def sum_nested(seq):
    total = 0
    for item in seq:
        if hasattr(item, '__iter__'): 
            total += sum_nested(item)
            continue
        try:
            total += item
        except TypeError:
            continue
    return total

print sum_nested([2, ['text', [None, 1], [int, [True, None], 5]]])
# True counts as 1

what's wrong with my code? recursively finding sum of nested list

3 Answers3