Fixing a surprising Python UnboundLocalError

Question

Hi so I am trying to write a code that sorts an array from a given node such that it has a max heap property. The code is as follows:

def left_child(index):
    return (index * 2) + 1

def right_child(index):
    return (index + 1) * 2

def parent(index):
    return (index - 1) // 2

def max_child_index(parent_index, array):
    left = left_child(parent_index)
    right = right_child(parent_index)
    if left < len(array) and array[left] >= array[right]:
        return left
    elif left < len(array) and array[left] < array[right]:
        return right

def max_heapify(array, index):
    swapped = True
    while swapped == True:
        swapped = False
        max_child_index = max_child_index(index, array)
        if array[index] < array[max_child_index]:
            array[index], array[max_child_index] = array[max_child_index], array[index]
            print(array)
            index = max_child_index
            swapped = True

So I then tested the code with the following:

array = [1, 14, 9, 10, 2, 8, 3, 7, 4, 16]
max_heapify(array, 0)
assert array == [14, 10, 9, 7, 2, 8, 3, 1, 4, 16]

but then the following error was shown:

UnboundLocalError                         Traceback (most recent call last)
<ipython-input-17-ba04a5f4816a> in <module>
      1 array = [1, 14, 9, 10, 2, 8, 3, 7, 4, 16]
----> 2 max_heapify(array, 0)
      3 print(array)

<ipython-input-16-ebac3db736fd> in max_heapify(array, index)
      3     while swapped == True:
      4         swapped = False
----> 5         max_child_index = max_child_index(index, array)
      6         if array[index] < array[max_child_index]:
      7             array[index], array[max_child_index] = array[max_child_index], array[index]

UnboundLocalError: local variable 'max_child_index' referenced before assignment

What I do not get is at the line in max_heapify where Python says the error occurs, that line is the first time when the local variable max_child_index is mentioned, declared and assigned a value. So, why does Python think that I have already used it?

Answer 1

Your function and variable have the same name, which causes an error.

max_child_index = max_child_index(...)

You should change one of them.