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

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.