Opening already opened file does not raise exception

Question

Consider those two python programs:

script_a.py:

from datetime import datetime
from time import sleep

while True:
    sleep(1)
    with         


        

Answer 1

You are allowed to open a file as many times as you want, so long as the operating system doesn't stop you. This is occasionally useful to get multiple cursors into a file for complex operations.

The reason that script_b.py thinks that the file is empty is that the file is empty:

with open('foo.txt', 'w') as f:

opening a file in w mode immediately erases (i.e. truncates) the file. There's an initial three second gap in script_a where the file is completely 100% empty, and that's what script_b sees.

In the next three second gap after you call f.write, the file is still... probably empty. This is due to buffering - the file on disk is not guaranteed to contain everything that you have written to it with write until you either close (i.e. exit the context manager block) or manually invoke flush on the file handle.

Alternatively, you can open in unbuffered mode, so that writes are always immediately written to disk.

with open('foo.txt','w',0) as f:
   #no buffering, f.write() writes immediately to disk