Slow comparison between two lists

[Hrvoje Niksic]

Jani Tiainen <redetin at gmail.com> writes:

> for addr in list_external:
>     if addr not in list_internal:
>         addr.status = 1 # New address
>
> But in my case running that loop takes about 10 minutes. What I am
> doing wrong?

The nested loop takes time proportional to the product of the number
of elements in the loops.  To avoid it, convert the inner loop to a
set, which can be looked up in constant time:

internal = set(list_internal)
for addr in list_external:
    if addr in internal:
        addr.status = 1