Python: Opening a file without creating a lock

Question

I'm trying to create a script in Python to back up some files. But, these files could be renamed or deleted at any time. I don't want my script to prevent that by locking the file; the file should be able to still be deleted at any time during the backup.

How can I do this in Python? And, what happens? Do my objects just become null if the stream cannot be read?

Thank you! I'm somewhat new to Python.

Answer 1

As mentioned by @kindall, this is a Windows-specific issue. Unix OSes allow deleting.

To do this in Windows, I needed to use win32file.CreateFile() to use the Windows-specific dwSharingMode flag (in Python's pywin32, it's just called shareMode).

Rough Example:

import msvcrt
import os
import win32file

py_handle = win32file.CreateFile(
    'filename.txt',
    win32file.GENERIC_READ,
    win32file.FILE_SHARE_DELETE
        | win32file.FILE_SHARE_READ
        | win32file.FILE_SHARE_WRITE,
    None,
    win32file.OPEN_EXISTING,
    win32file.FILE_ATTRIBUTE_NORMAL,
    None
)
try:
    with os.fdopen(
        msvcrt.open_osfhandle(py_handle.handle, os.O_RDONLY)
    ) as file_descriptor:
        ... # read from `file_descriptor`
finally:
    py_handle.Close()

Note: if you need to keep the win32-file open beyond the lifetime of the file-handle object returned, you should invoke PyHandle.detach() on that handle.

Answer 2

On UNIX-like OSs, including Linux, this isn't an issue. Well, some other program could write to the file at the same time you're reading it, which could cause problems (the file you are copying could end up corrupted) but this is solvable with a verification pass.

On Windows, use Volume Snapshot Service (aka Volume Shadow Copy). VSS creates a snapshot of the volume at a moment in time, and you can open files on the snapshot without locking the files on the original volume. A quick Google found a Python module for doing copies using VSS here: http://sourceforge.net/projects/pyvss/