Swapping elements in lists in python -

i have list , need swap 1st element in list maximum element in list.

but why code 1 work while code 2 doesn't:

code 1:

a = list.index(max(list)) list[0], list[a] = list[a], list[0] 

code 2:

list[0], list[list.index(max(list))] = list[list.index(max(list))], list[0] 

i thought python first evaluate right-hand side before assigning names on left?

python stores results in multiple targets left right, executing assignment target expression in order.

so second version comes down this:

temp = list[list.index(max(list))],list[0] list[0] = temp[0] list[list.index(max(list))] = temp[1] 

note list.index(max(list)) executed after list[0] altered, , that's stored maximum value.

this documented in assignment statements documenation:

• if target list comma-separated list of targets: object must iterable same number of items there targets in target list, , items assigned, from left right, corresponding targets.

from there on out if each target single target, documentation follows applies left right each target:

assignment of object single target recursively defined follows.

[...]

• if target subscription: primary expression in reference evaluated. should yield either mutable sequence object (such list) or mapping object (such dictionary). next, subscript expression evaluated.

if changed order of assignments code work:

list[list.index(max(list))], list[0] = list[0], list[list.index(max(list))] 

because list[list.index(max(list))] assigned first.