sum of nested list in Python

Question

I try to sum a list of nested elements

e.g, numbers=[1,3,5,6,[7,8]] should produce sum=30

I wrote the following code :

Answer 1

A simple solution would be to use nested loops.

def nested_sum(t):

    sum=0
    for i in t:
        if isinstance(i, list):
            for j in i:
                sum +=j
        else:
            sum += i        
    return sum

Answer 2

You need to use isinstance to check whether an element is a list or not. Also, you might want to iterate over the actual list, to make things simpler.

def nested_sum(L):
    total = 0  # don't use `sum` as a variable name
    for i in L:
        if isinstance(i, list):  # checks if `i` is a list
            total += nested_sum(i)
        else:
            total += i
    return total

Answer 3

An example using filter and map and recursion:

def islist(x): 
    return isinstance(x, list)

def notlist(x): 
    return not isinstance(x, list)

def nested_sum(seq):
    return sum(filter(notlist, seq)) + map(nested_sum, filter(islist, seq))

And here is an example using reduce and recursion

from functools import reduce


def nested_sum(seq):
    return reduce(lambda a,b: a+(nested_sum(b) if isinstance(b, list) else b), seq)

An example using plain old recursion:

def nested_sum(seq):
    if isinstance(seq[0], list):
        head = nested_sum(seq[0])
    else:
        head = seq[0]
    return head + nested_sum(seq[1:])

An example using simulated recursion:

def nested_sum(seq):
    stack = []
    stack.append(seq)
    result = 0
    while stack:
        item = stack.pop()
        if isinstance(item, list):
            for e in item:
                stack.append(e)
        else:
            result += item
    return result

Adjustment for handling self-referential lists is left as an exercise for the reader.

Answer 4

I would sum the flattened list:

def flatten(L):
    '''Flattens nested lists or tuples with non-string items'''
    for item in L:
        try:
            for i in flatten(item):
                yield i
        except TypeError:
            yield item


>>> sum(flatten([1,3,5,6,[7,8]]))
30

Answer 5

L = [1, 2, 3, [4, 5, 6], 5, [7, 8, 9]]
total = 0 # assign any var 
for a in L: # assign index and start to iterate using if else
    if (isinstance(a, list)): # since its a list you are basically repeating the prev step
        for b in a:
            total += b
    else:
        total += a
print(total)

Answer 6

One alternative solution with list comprehension:

>>> sum( sum(x) if isinstance(x, list) else x for x in L )
30

Edit: And for lists with more than two levels(thx @Volatility):

def nested_sum(L):
    return sum( nested_sum(x) if isinstance(x, list) else x for x in L )