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

RHS first and then LHS. And at any side, the evaluation order is left to right.

a[-1] += a.pop() is same as, a[-1] = a[-1] + a.pop()

a = [1,2,3]
a[-1] = a[-1] + a.pop() # a = [1, 6]

See how the behavior changes when we change the order of the operations at RHS,

a = [1,2,3]
a[-1] = a.pop() + a[-1] # a = [1, 5]