What is the order of evaluation in python when using pop(), list[-1] and +=?

Question

a = [1, 2, 3]
a[-1] += a.pop()

This results in [1, 6].

a = [1, 2, 3]
a[0] += a.pop()

This results in

Answer 1

For you specific example

a[-1] += a.pop() #is the same as 
a[-1] = a[-1] + a.pop() # a[-1] = 3 + 3

Order:

1. evaluate a[-1] after =
2. pop(), decreasing the length of a
3. addition
4. assignment

The thing is, that a[-1] becomes the value of a[1] (was a[2]) after the pop(), but this happens before the assignment.

a[0] = a[0] + a.pop() 

Works as expected

1. evaluate a[0] after =
2. pop()
3. addition
4. assignment

This example shows, why you shouldn't manipulate a list while working on it (commonly said for loops). Always work on copys in this case.