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Can some explain how this expression is evaluated in python?

Can someone explain what's going on "behind the scenes" in the below expression?

arr[i], mx = mx, max(arr[i], mx)

Take for example :

arr[i] = 5
mx = -1

So in the "first" part of the expression arr[i] gets set to -1, but what I'm having trouble grasping is why arr[i] remains 5 when evaluating the second part of the expression.

1 Answer

Chris Freeman
Chris Freeman
Treehouse Moderator 67,989 Points
# Given
arr[i] = 5
mx = -1
arr[i], mx = mx, max(arr[i], mx)
# becomes
arr[i], mx = -1, max(5, -1)
# becomes
arr[i], mx = -1, 5
# tuple on right is assigned to tuple on left
# arr[i] becomes -1
# mx becomes 5
>>> arr = [0, 5, 10, 15]
>>> i = 1
>>> arr[i]
>>> mx = -1
>>> arr[i], mx = mx, max(arr[i], mx)
>>> arr
[0, -1, 10, 15]
>>> mx

Post back if you need more help. Good luck!!

That explains everything. Thank you so much!!