Why can I not increment a list this way in Python?

Question

Language is Python. I am trying to add 'a' to all of the values in a list 'sumarr'.

Here is my code:

for b in sumarr:
    b+=a

Why does this not work? I know I can use list comprehensions like this:

sumarr = [b+a for b in sumarr]

But why does the first method not work?

Answer 1

b is a local variable - a reference of the list item, not the actual item in list.

If you do print( b += a ) you will see the expected number, but it won't put that result back into your list.

#! /usr/bin/env python3

sumarr = [1, 2, 3, 4]
a = 3

for b in range( len(sumarr) ):
    sumarr[b] += a

for b in sumarr:
    print( b )

4
5
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7