Why is this happening in python? python if-else statement

The below two if-else statements shows different results.

1

if value in twins:
    tmp = twins[value] + [box]
else:
    tmp = [box]
twins[value] = tmp

2

if value in twins:
    tmp = twins[value] + [box]
    twins[value] = tmp
else:
    tmp = [box]
    twins[value] = tmp

Why is this happening?