a bug in list.remove?

[Tim Chase]

> I have 2 lists. What Im doing is check the first list and remove all 
> occurances of the elements in the second list from the first list, like so:
>  >>> ps = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]
>  >>> qs = [6,7,8,9,10,11,12,1,2]
>  >>> for p in ps:
>     if p in qs:
>         ps.remove(p)
> 
> The problem Im having is that when I do
>  >>> print ps
>  it gives me
> [2, 3, 4, 5, 7, 9, 11, 13, 14, 15]
> which is incorrect since 2,7,9,11 shouldnt be in that list. Is this a 
> bug in .remove? or is my algorithm somewhat flawed?


I'd go with the "somewhat flawed" answer.

I'd just use

ps = [x for x in ps if x not in qs]

which will remove *all* instances of x from ps if it exists in 
qs.  There's a subtle difference from the remove() method, which 
will only remove the first instance:

 >>> help([].remove)
Help on built-in function remove:

remove(...)
     L.remove(value) -- remove first occurrence of value



If qs is large, you'll get improved performance by converting it 
to a set first:

 >>> s = set(qs)
 >>> ps = [x for x in ps if x not in s]


As for your algorithm, you're modifying the list over which 
you're iterating--at best, often considered bad form...at worst, 
I've had Python throw exceptions at me for attempting it.

-tkc