heapify returns NoneType

Question

I have

b = heapify([5,4,9,1])

and if I do a

type(b)

It says it's NoneType instead of list type, anyone know what I'm doing wrong??

Answer 1

The heapify() method transforms the list in-place. This means that it alters the list, but does not returned the modified list. As agf mentions below, heapify() returns None to protect you from this mistake. Therefore, if you do

lst = [5,4,9,1]
heapify(lst)
type(lst)

you will see that lst is now heapified. See the library reference for more info.

Answer 2

heapify mutates the list passed to it; just like how l.sort() does.

>>> import heapq
>>> l = [9, 8, 7, 6]
>>> heapq.heapify(l)
>>> l
[6, 8, 7, 9]