python: using list slice as target of a for loop

Question

I found this code snippet to be very interesting.

a = [0, 1, 2, 3]

for a[-1] in a:
    print(a)

Output is as follows:

[0,          


        

Answer 1

What happens is

         0 SETUP_LOOP              24 (to 26)
          2 LOAD_GLOBAL              0 (a)
          4 GET_ITER
    >>    6 FOR_ITER                16 (to 24)
          8 LOAD_GLOBAL              0 (a)     //ARRAY (TOS1)
         10 LOAD_CONST               2 (-1)    //DEST (TOS)
         12 STORE_SUBSCR                       //ARRAY[DEST] = TOS2*
        14 LOAD_GLOBAL              1 (print)
         16 LOAD_GLOBAL              0 (a)
         18 CALL_FUNCTION            1
         20 POP_TOP
         22 JUMP_ABSOLUTE            6
    >>   24 POP_BLOCK
    >>   26 LOAD_CONST               0 (None)
         28 RETURN_VALUE

*So if someone could clarify that TOS2 is actually the 'visited' value of the ARRAY?

Answer 2

It works as expected. (For some interpretation of "expected", at least.)

Re-writing your code to this, to prevent any misinterpretation of what a[-1] is at any point:

a = [a for a in range(0,4)]
for b in a:
    print (b)
    a[-1] = b
    print (a)

shows us

0
[0, 1, 2, 0]
1
[0, 1, 2, 1]
2
[0, 1, 2, 2]
2
[0, 1, 2, 2]

which makes it clear that the b assignment to a[-1] is done immediately, changing the list while iterating.

The four loops do the following:

1. a[-1] gets set to the first value of the list, 0. The result is now [0,1,2,0].
2. a[-1] gets set to the second value, 1. The result is (quite obviously) [0,1,2,1].
3. a[-1] gets set to 2 and so the result is [0,1,2,2] – again, only a[-1] gets changed.
4. Finally, a[-1] gets set to the last value in a, so effectively it does not change and the final result is [0,1,2,2].

Answer 3

a[-1] prints the same value in a[3], the last index in the list. Edit: see comment below for the explanation for this.

The result reads as follows:

[0, 1, 2, 0] -> same as [0, 1, 2, 3], but with a[i] (in this case, a[-1] == a[3]) is replaced with the value at a[0]
[0, 1, 2, 1] -> [0, 1, 2, 3] but a[i] is replaced with the value at a[1]
[0, 1, 2, 2] -> [0, 1, 2, 3] but a[i] is replaced with the value at a[2]
[0, 1, 2, 2] -> [0, 1, 2, 3] but a[i] is replaced with the value at a[3] from the previous iteration.