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def some_condition(line):
    return "ddd" in line

with open('testin.csv') as infile:
    for line in infile:
        if some_condition(line):
            donotline=line
        else:
            f = open('testout.csv','a+')
            f.write(line)

I have been trying to read from testin and write to testout.csv with the above code. The code runs but take approimatley 5 mins to read through 160000 lines. Is there any means to speed it up.
thanks
nuncio

Don't open a new output file every time through the loop. Just open it once before the loop starts.

Try this perhaps

import itertools as itt

def some_condition(line):
    return "ddd" in line

open('testout.csv', 'w').writelines(itt.filterfalse(condition, open('testin.csv')))