save data in .txt after certain interval

[Gribouillis]

Use the flush() method. Also your use of writelines is not correct

import time, datetime
 
ctr=100
 
opn = open("paillasse/ecrlog.txt","a+")
 
while(ctr):
    opn.write(str(ctr) + '\n')
    opn.flush()
    ctr=ctr-1
    time.sleep(1)
    print(ctr)
 
opn.close() #close file