Alter list items within loop

[Tim Harig]

On 2009-06-11, Brendan <brendandetracey at yahoo.com> wrote:
> Can someone please explain what is happening in the output below? The
> number 3 never gets printed. Does Python make a copy of a list before
> it iterates through it?:

You can see what is happening by printing the list as you work through the
loop:

>>> e = range(1,5)
>>> for i in e:
...         print e
...         print i
...         if i == 2 :
...                 e.remove(i)
...
[1, 2, 3, 4]
1
[1, 2, 3, 4]
2
[1, 3, 4]
4

first loop:
i = 0
e[i] = e[0] = 1

second loop
i = 1
e[i] = e[1] = 2

third loop
i = 2
e[i] = e[2] = 4

> number 3 never gets printed. Does Python make a copy of a list before
> it iterates through it?:

No, complex types are passed by reference unless explicity copied.  You can
do what you want by making an explicit copy before entering the loop:

>>> e = range(1,5)
>>> for i in e[:]:
...         print e
...         print i
...         if i == 2 :
...                 e.remove(i)
...
[1, 2, 3, 4]
1
[1, 2, 3, 4]
2
[1, 3, 4]
3
[1, 3, 4]
4