Swapping list elements in python where the expressions contain function calls

It has to do with the order of the operations.

We can subclass lists to instrument them to show what they're doing.

class TracingList(list):    def __getitem__(self, key):        value = super().__getitem__(key)        print(self, "reading", key, "=", value)        return value    def __setitem__(self, key, value):        print(self, "writing", key, "=", value)        super().__setitem__(key, value)    def index(self, value):        index = super().index(value)        print(self, "finding index of", value, "=", index)        return indexarr = TracingList([4, 3, 2, 1])arr[0], arr[arr.index(min(arr))] = min(arr), arr[0]print(arr)print("===")arr = TracingList([4, 3, 2, 1])arr[0], arr[arr.index(min(arr))] = arr[arr.index(min(arr))], arr[0]print(arr)print("===")arr = TracingList([4, 3, 2, 1])b = arr.index(min(arr))arr[0], arr[b] = arr[b], arr[0]print(arr)

prints out

[4, 3, 2, 1] reading 0 = 4[4, 3, 2, 1] writing 0 = 1[1, 3, 2, 1] finding index of 1 = 0[1, 3, 2, 1] writing 0 = 4[4, 3, 2, 1]===[4, 3, 2, 1] finding index of 1 = 3[4, 3, 2, 1] reading 3 = 1[4, 3, 2, 1] reading 0 = 4[4, 3, 2, 1] writing 0 = 1[1, 3, 2, 1] finding index of 1 = 0[1, 3, 2, 1] writing 0 = 4[4, 3, 2, 1]===[4, 3, 2, 1] finding index of 1 = 3[4, 3, 2, 1] reading 3 = 1[4, 3, 2, 1] reading 0 = 4[4, 3, 2, 1] writing 0 = 1[1, 3, 2, 1] writing 3 = 4[1, 3, 2, 4]