python 2.7 file overwritten even after using "a" (threads and filelock used)

I've a loop generating threads every 5 seconds and each thread is trying to append same file using filelock module in python but looks like they end up over-writing the file

import threading
import filelock

def loop():

    threading.Timer(5,loop).start()
    lock = filelock.FileLock("PATH", timeout=20)
    with lock.acquire(timeout=0.1, poll_intervall=0.01):
        with open("PATH", "a") as myFile:
            myFile.write("DATA\n\n")
    lock.release()

Edit: Additional Info: After multiple iteration, the data I found in the file was from last thread and not the first.

Edit: as commented by georgexsh below, append is atomic and hence we don't need lock for it.

I suppose you are using this filelock package, as your code use the lock object as a context manager for with statement, the lock would be released when the with block is exited.

write data to lock file is not a wise idea, lock file will be truncated to length 0 when acquiring the lock, as O_TRUNC being used.

if you change inner with block to:

import time

with lock.acquire(timeout=0.1, poll_intervall=0.01):
    with open("PATH", "a") as myFile:
        myFile.write("DATA\n\n")
    time.sleep(1000)

the first thread which acquired the file lock will hold it, other threads would be blocked as you expected.