Python Cookbook

File Locking Using a Cross-Platform API

Credit: Jonathan Feinberg, John Nielsen

Problem

You need to lock files in a cross-platform way between NT and Posix, but the Python standard library offers only platform-specific ways to lock files.

Solution

When the Python standard library itself doesn’t offer a cross-platform solution, it’s often possible to implement one ourselves:

import os

# needs win32all to work on Windows
if os.name == 'nt':
    import win32con, win32file, pywintypes
    LOCK_EX = win32con.LOCKFILE_EXCLUSIVE_LOCK
    LOCK_SH = 0 # the default
    LOCK_NB = win32con.LOCKFILE_FAIL_IMMEDIATELY
    _ _overlapped = pywintypes.OVERLAPPED(  )

    def lock(file, flags):
        hfile = win32file._get_osfhandle(file.fileno(  ))
        win32file.LockFileEx(hfile, flags, 0, 0xffff0000, _ _overlapped)

    def unlock(file):
        hfile = win32file._get_osfhandle(file.fileno(  ))
        win32file.UnlockFileEx(hfile, 0, 0xffff0000, _ _overlapped)
elif os.name == 'posix':
    from fcntl import LOCK_EX, LOCK_SH, LOCK_NB

    def lock(file, flags):
        fcntl.flock(file.fileno(  ), flags)

    def unlock(file):
        fcntl.flock(file.fileno(  ), fcntl.LOCK_UN)
else:
    raise RuntimeError("PortaLocker only defined for nt and posix platforms")

Discussion

If you have multiple programs or threads that may want to access a shared file, it’s wise to ensure that accesses are synchronized, so that two processes don’t try to modify the file contents at the same time. Failure to do so could corrupt the entire file in some cases.

This recipe supplies two functions, lock and unlock, that ...