Understanding return [0,size-1][nums[0]<nums[size-1]] in Python

Question

While working on a simple coding question, writing a function findPeakElement, I came across the following code:

def findPeakElement(self, nums):
     size = len(nums)
     for x in range(1,size-1):
         if nums[x] > nums[x-1] and nums[x] > nums[x+1]:
            return x

     return [0,size-1][nums[0]<nums[size-1]]

What is the meaning of last line?

Answer 1

That last row is an obscure way of writing an if then else expression.

• [0, size-1] creates a list of two elements.
• nums[0] < nums[size-1] returns either True or False
• when used as a list index, this True/False is implicitly converted into 1 or 0.
• and by that, either size-1 or 0 is picked from the list.

A clearer way to write it is:

return size - 1 if nums[0] < nums[size - 1] else 0