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Number of comments: 8

Note however this fails on Win95, Win98, James Kew, 2003/01/15
Although these platforms have os.name == 'nt', the Win32 API on them does not support the LockFileEx and UnlockFileEx functions -- this recipe will fail with api_error at the win32 function calls. Win32 supports LockFile, UnlockFile on all platforms, but with lower functionality: no shared/exclusive flag, no block-until-locked mode.
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Python 2.3.4, 2004/10/26
This generates warnings under Python 2.3.3, about what will happen in Python 2.3.4:

running py2exe
portalocker.py:62: FutureWarning: hex/oct constants > sys.maxint will return positive values in Python 2.4 and up
  win32file.LockFileEx(hfile, flags, 0, 0xffff0000, __overlapped)
portalocker.py:66: FutureWarning: hex/oct constants > sys.maxint will return positive values in Python 2.4 and up
  win32file.UnlockFileEx(hfile, 0, 0xffff0000, __overlapped)

Is this going to be a problem?
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How to make the recipe work for Python 2.4, Jordan Rastrick, 2005/06/16
The code does cause an error under Python 2.4 Apparently, this is because the hexidecimal constant, 0xffff0000, which evaluated to the integer -65536 in Python 2.3, evaluates to the long integer 4294901760 in Python 2.4 due to the unification of long and short integers. Changing the constant to -0x10000 seems to correct the problem.
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Thanks., Jonathan Feinberg, 2007/12/12
Jordan, I have incorporated your fix into the code, above. Thanks very much.
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home-made file locking, ben w, 2005/07/07

here's an approach to file locking that might be useful to somebody, especially for a consumer-producer type situation. It uses the fact that on windows, renaming a file that is already open will raise an exception.

____Producer___: 

import time, os

#lock readers out by renaming the file 
if os.access("filename", os.F_OK):
     for failure_counter in range(0, 600):
         try:				
	      os.rename("filename", "filename.temp") 
              break #removed the file successfully
	 except:				
	      time.sleep(1) #someone is reading from the file 
    else:
	raise Exception #consumers kept the file open for over 10 min

#file is now locked
file = open("filename.temp", "w") 
file.write("bla bla")
file.close()		
os.rename("filename.temp", "filename") #unlock the file

 
____Consumers___:

import time, os

#wait until file exists (eg. is unlocked
for i in range(0,600):
  if os.acces("filename", os.F_OK):
      break
  else:
      time.sleep(1)
else:
  raise Exception #file still locked after 10 minutes

#access the file
file = open("filename", "r")
contents = file.read()
file.close()

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error in WinXpsp2 , Minsue Hur, 2005/11/02
ActivePython 2.4.1 Build 247 (ActiveState Corp.) based on Python 2.4.1 (#65, Jun 20 2005, 17:01:55) [MSC v.1310 32 bit(Intel)] on win32 Traceback (most recent call last): File "portalocker.py", line 80, in ? portalocker.lock(log, portalocker.LOCK_EX) File "C:\portalocker.py", line 61, in lock win32file.LockFileEx(hfile, flags, 0, 0xffff0000, __overlapped) OverflowError: long int too large to convert to int
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Jonathan Feinberg, 2007/12/12
I have addressed this problem, per the comments above.
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Minor changes: Making portalocker.py slightly more portable..., Lowell Alleman, 2008/05/16
I sent some improvements to Jonathan and he updated portalocker.py on this page to include them. But for the rest of you out there, I wanted to give a quick explanation. In a sentence, all of the changes come down to this: Getting portalocker to deal with more platform-specific-stuff, so that you don't have to.

Changes and notes:
1. I added the LockException class to make error handling more uniform. Previously you had to capture IOError for Unix and pywintypes.error for windows, etc. So I modified both lock() functions to raise just LockException. (BTW. You should only get an exception if your using the LOCK_NB flag, otherwise it just waits (or "blocks") until the lock is available, which means it never raises an exception.)

2. I updated the windows unlock() function to ignore requests to unlock an already unlocked file. This matches the 'posix' behavior, thus making it making it more predictable. (Not that you would normally unlock a file twice, but I somehow manged run into the problem, so perhaps this will help some one else one day.)

3. ***UNSOLVED***: If you request to lock a file multiple time using LOCK_EX on windows, it will block forever! On unix, it lets you run lock() as many times as you want. Perhaps someone with windows API experience can provide a way to determine if we have already locked the file? But in the mean time, just know that locking the same file multiple times will work differently between platforms.


There are number of example on how to use this module, but I haven't seen many (any?) with exception handling. Here is a best-practices example with the new LockException class:

   fp = open("my_lock_file", "w")
   try:
      portalocker.lock(fp, portalocker.LOCK_EX | portalocker.LOCK_NB)
   except portalocker.LockException:
      # File locked by somebody else. Do some other work, then try again later.
      pass
   else:
      # Lock acquired.  Do your thing
      try:
         while chapters in book:
            fp.write(chapters)
      finally:
         unlock(fp)

Ok that's it. Enjoy.
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