How to remove subset from a file efficiently?

[AJL]

On 12 Jan 2006 09:04:21 -0800
"fynali" <iladijas at gmail.com> wrote:

> Hi all,
> 
> I have two files:
> 
>   - PSP0000320.dat (quite a large list of mobile numbers),
>   - CBR0000319.dat (a subset of the above, a list of barred bumbers)
> 
...

> Objective: to remove the numbers present in barred-list from the
> PSPfile.

How fast does this run?

a = set(file('PSP0000320.dat'))
b = set(file('CBR0000319.dat'))
file('PSP-CBR.dat', 'w').writelines(a.difference(b))

AJL