CODEWITHSUNDEEP
pythonpython-3.xoperating-systemfile-handlingfile-copying
Yash
Yash

Reputation: 220

Read a file while some other program is writing the file in python

I am reading a file continously and another program modifies it. When i try to read the it only prints blank spaces.

File that reads

   import os
   import time
   f=open("file.py","r",os.O_NONBLOCK)
   while 1:
     x=f.read()
     if x.find("bye")!=-1:
        break
     else:
        time.sleep(1)
   f.close()

File that writes

  import os
  f=open("file.py","w",os.O_NONBLOCK)
  f.write("bye")
  f.flush()
  f.close()

file.py

  hello

The program only prints blank spaces

Upvotes: 1

Views: 2319

Answers (1)

pb360
pb360

Reputation: 324

What you are trying to do should be fairly easy. I am pretty sure your code will technically work, but you really should handle files using a context manager. I also restructured your code to do what I think you intended to do a little better.

File that reads

    import os
    import time
   
    we_have_written_bye = False 

    while we_have_written_bye = False:
        with open("file.py", "r") as f         
            x = f.read()
            if x.find("bye")!=-1:
                we_have_written_bye = True

     # file closes automatically with a context manager so 
     # this is removed. Note, if bye was not written yet, we
     # close the file, then wait for a second by sleeping below
     
     time.sleep(1)

File that writes

  import os
  
  with open("file.py", "w", os.O_NONBLOCK) as f 
    f.write("bye")
    f.flush() # not sure why you want to flush the file contents here
    f.close()

file.py

  hello

The two programs should work seamlessly. This is because the file object can not be opened if another program is writing to it. You may run into an issue with this, but if the writes are small I believe the standard library will wait long enough for the file lock to be given up.

For a tutorial on context managers please see:

https://www.youtube.com/watch?v=Lv1treHIckI

this is part of a great series of semi-advanced python tutorials that will certainly up your game. Helped me tonnes

Upvotes: 3

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