My colleague showed me a trick on how to do this earlier. What I was trying to do is poll a folder for a bunch of files and if they’re not there keep polling until they arrive, then process them. The problem is before they can be processed we need to be sure that the file transfer is complete.

The trick on how to do this is to try and open the file once it arrives in ‘append’ mode. This will fail if another process is using the file. So what we can do in this case is then keep trying to open it in append mode until it’s successful and at that point we know that there is no longer a lock on the file.

Sounds a bit hacky but we couldn’t think of a more elegant way to do it.

import os, time

def is_locked(filepath):
    """Checks if a file is locked by opening it in append mode.
    If no exception thrown, then the file is not locked.
    """
    locked = None
    file_object = None
    if os.path.exists(filepath):
        try:
            print "Trying to open %s." % filepath
            buffer_size = 8
            # Opening file in append mode and read the first 8 characters.
            file_object = open(filepath, 'a', buffer_size)
            if file_object:
                print "%s is not locked." % filepath
                locked = False
        except IOError, message:
            print "File is locked (unable to open in append mode). %s." % \
                  message
            locked = True
        finally:
            if file_object:
                file_object.close()
                print "%s closed." % filepath
    else:
        print "%s not found." % filepath
    return locked

def wait_for_files(filepaths):
    """Checks if the files are ready.

    For a file to be ready it must exist and can be opened in append
    mode.
    """
    wait_time = 5
    for filepath in filepaths:
        # If the file doesn't exist, wait wait_time seconds and try again
        # until it's found.
        while not os.path.exists(filepath):
            print "%s hasn't arrived. Waiting %s seconds." % \
                  (filepath, wait_time)
            time.sleep(wait_time)
        # If the file exists but locked, wait wait_time seconds and check
        # again until it's no longer locked by another process.
        while is_locked(filepath):
            print "%s is currently in use. Waiting %s seconds." % \
                  (filepath, wait_time)
            time.sleep(wait_time)

# Test
if __name__ == '__main__':
    files = [r"C:\testfolder\testfile1.txt",
             r"C:\testfolder\testfile2.txt"]
    print wait_for_files(files)<br>

Output:

Trying to open C:\testfolder\testfile1.txt.
File is locked (unable to open in append mode). [Errno 13] Permission denied: ‘C:\\testfolder\\testfile1.txt’.
C:\testfolder\testfile1.txt is currently in use. Waiting 5 seconds.
Trying to open C:\testfolder\testfile1.txt.
C:\testfolder\testfile1.txt is not locked.
C:\testfolder\testfile1.txt closed.
Trying to open C:\testfolder\testfile2.txt.
C:\testfolder\testfile2.txt is not locked.
C:\testfolder\testfile2.txt closed.