CODEWITHSUNDEEP
pythonrecursion
Maxime Roussin-Bélanger
Maxime Roussin-Bélanger

Reputation: 650

function inside a function, python recursive on a list

def minimizeMaximumPair(lst):
    lst.sort()

    def compute(aList):
        if len(aList) != 0:
            return [(aList[0], lst[len(aList) - 1])].extend(compute(aList[1:len(aList) - 1]))
        return []

    return compute(lst)

When I get the the last recursion step I get an 

TypeError: 'NoneType' object is not iterable

I tried returning nothing, and a []

Upvotes: 4

Views: 690

Answers (3)

John La Rooy
John La Rooy

Reputation: 304175

Instead of list.extend which returns None, you can use list.__iadd__

__iadd__ also extends the list inplace, but returns the list afterward

if you have an aversion to using special methods, you can use iadd from the operator module

from operator import iadd

...

def compute(aList):
    if len(aList) != 0:
        return iadd([(aList[0], aList[-1])], compute(aList[1: -1]))
    return []

Upvotes: 3

BrunoRB
BrunoRB

Reputation: 909

That's because extend is a function that doesn't return anything, it simply changes the list in-place. A possible solution is to use + instead:

return [(aList[0], lst[len(aList) - 1])] + compute(aList[1:len(aList) - 1])

Upvotes: 1

Zizouz212
Zizouz212

Reputation: 4998

The issue is when you call .extend().

Your compute function tries to return the value of .extend(), which is None. Similar to .sort(), .extend() modifies the object itself, rather than returning a modified copy of the object. This is known as mutability. Here's some working code:

def compute(aList):
    if len(aList) != 0:
        out = [(aList[0], lst[len(aList) - 1])]
        out.extend(compute(aList[1:len(aList) - 1]))
        return out
    return []

Upvotes: 4

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