efficient text file search.

[noro]

Is there a more efficient method to find a string in a text file then:

f=file('somefile')
for line in f:
    if 'string' in line:
         print 'FOUND'

?

BTW:
does "for line in f: " read a block of line to te memory or is it
simply calls f.readline() many times?

thanks
amit