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

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].