How to avoid duplicates when writing chat.id to a file?

Question

Python

There is a code:

global sub
sub = [line.rstrip('_\n') for line in open(subfile,'rt')]
if str(m.chat.id) not in sub:
  with open(subfile, 'a') as f:
    f.write(str(m.chat.id) + " \n")
else:
        bot.send_message(m.chat.id, 'hi')

It writes m.chat.id to the sub file. In theory, it should only do it once, but it does it every time, regardless of if else, and because of this, duplicates appear. How to fix it?

Answer 1

this is exactly how it should work

f = open('file.txt', 'r')
for line in f:
    if str(m.chat.id) in line:
        f.close()
        break
    else:
        f = open('file.txt', 'a')
        f.write(str(m.chat.id) + '\n')
        f.close()
        bot.send_message(m.chat.id, 'hi')