>
> Ok, perhaps I'm being a little dim but if I do
>
> s = 'Spam is not ham'
> s.replace('Spam', '')
> and write it to a new file, should I not get 'is not ham'?
>
> Instead I am getting an empty file or in the case of multiple lines in
> the file, the whole of the line 'Spam is not ham' is gone but 'I'm fond
> of ham' remains.
>
s should be left completely alone as your code is right now.