what is the significance of using list index in for loop of python?

Question

I was trying to understand the following code

a=[0,1,2,3]
for a[-1] in a:
    print(a[-1])

I expected an infinite loop printing 3 but the output is quite incomprehensible as below

output:

can somebody Please explain the working of this code!

Thanks in advance

Mentally insert the line `a[-1] = i` inside that loop, where `i` is the current value in the iteration. `for a[-1] ...` assigns the iteration value to `a[-1]` on every iteration… — deceze, Apr 24 '20 at 07:20
This is an interesting mental exercise. But I fail to see why anyone would write code like this. — Rajarishi Devarajan, Apr 24 '20 at 07:37

Vikas Mulaje · Answer 1 · 2020-04-24T08:39:05.710

1

in keyword in for loop is different than other use cases.

in this for loop list returns an iterator and assign it to the variable in your case last element of list

to break your code this is what it is doing in background

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

edited Apr 24 '20 at 08:39

answered Apr 24 '20 at 07:31

Vikas Mulaje

In that case we should supposed to get 3 as the output in the last iteration but we are getting 2 as the output – Harish Rudroju Apr 24 '20 at 07:55
@HarishRudroju Because in the penultimate iteration, `2` is assigned to `a[-1]`… – deceze Apr 24 '20 at 08:30

1 Answers1