def nested_sum(L):

    sum=0
    for i in range(len(L)):
       if (len(L[i])>1):
          sum=sum+nested_sum(L[i])
       else:
          sum=sum+L[i]
    return sum

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

def nested_sum(a) :
    total = 0
    for item in a :
        try:
            total += item
        except TypeError:
            total += nested_sum(item)
    return total

rec = lambda x: sum(map(rec, x)) if isinstance(x, list) else x

result = rec(a)

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

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

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

def add_all(t):
    total = 0
    for i in t:
        if type(i) == list: # check whether i is list or not
            total = total + add_all(i)
        else:
            total += i
    return total

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

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

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