strange behaviour with remove

[Panard]

Hi!

Can anyone explain this to me :
$ cat test.py
l = [ 1, 2, 3 ]
d = { 'list' : l }

for x in l :
print "rm", x
d[ 'list' ].remove( x )
print "l =", l

print d

$ python test.py
rm 1
l = [2, 3]
rm 3
l = [2]
{'list': [2]}


Why 2 isn't removed ? and why l is changing during the loop ??
Am I missing something ?

My python is 2.3.4

Thanks

--
Panard

[Panard]

Re: strange behaviour with remove

Ok, I reply to myself, it's working when you add, line 3
l = [ i for i in l ]

I think the reason is that when you do a remove on d[ 'list' ] it will also
do a remove on l, because, I think, l in the dictionnary is only a
reference...


Panard wrote:
[color=blue]
> Hi!
>
> Can anyone explain this to me :
> $ cat test.py
> l = [ 1, 2, 3 ]
> d = { 'list' : l }
>
> for x in l :
> print "rm", x
> d[ 'list' ].remove( x )
> print "l =", l
>
> print d
>
> $ python test.py
> rm 1
> l = [2, 3]
> rm 3
> l = [2]
> {'list': [2]}
>
>
> Why 2 isn't removed ? and why l is changing during the loop ??
> Am I missing something ?
>
> My python is 2.3.4
>
> Thanks
>[/color]

--
Panard

[Reinhold Birkenfeld]

Re: strange behaviour with remove

Panard wrote:[color=blue]
> Ok, I reply to myself, it's working when you add, line 3
> l = [ i for i in l ]
>
> I think the reason is that when you do a remove on d[ 'list' ] it will also
> do a remove on l, because, I think, l in the dictionnary is only a
> reference...[/color]

You're partially right. The list referenced to by d['list'] is the same
object as the list referenced to by l (to verify this, do a "print
d['list'] is l"). The main reason that your loop does not work is that
it modifying a list you are iterating over leads to undefined behavior.

In your solution above, you are assigning a new list to the name "l",
iterate over this list, and remove from the original list which is now
only referenced by d['list']. This is the common solution for the
problem, although there is an easier way to copy a list:

l = l[:]

Reinhold

--
Wenn eine Linuxdistributi on so wenig brauchbare Software wie Windows
mitbrächte, wäre das bedauerlich. Was bei Windows der Umfang eines
"kompletten Betriebssystems " ist, nennt man bei Linux eine Rescuedisk.
-- David Kastrup in de.comp.os.unix .linux.misc