Handling a .txt text

[perfringo]

I suggest to use different style for opening files (using 'with'). In order to verify processing all rows one can use enumerate and printing out:

with open('for_net.txt', 'r') as f:
    for i, row in enumerate(f, start=1):
        if 'net' in row.lower():
            print(f'Row #{i} contains "net": {row.strip()}')
        else:
            print(f'Row #{i} does not contain "net"')

if 'for_net.txt' contains following four rows:

This is not
but here is net
and not
and net

Code will output:

Output:
Row #1 does not contain "net"
Row #2 contains "net": but here is net
Row #3 does not contain "net"
Row #4 contains "net": and net