sum of nested list in Python

python list python-3.3
Jin picture Jin · Feb 17, 2013 · Viewed 32.3k times · Source

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 :

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

The above code gives following error:

object of type 'int' has no len()

I also tried len([L[i]]), still not working.

Anyone can help? It is Python 3.3

Answer

Volatility picture Volatility · Feb 17, 2013

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

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