Using python to check write permission in a ntfs file system

Question

I need to write into several files that usually needs higher level permission than a regular user. Before doing anything, I would like the check if the current user can write in all of them. If so, proceed to writing. Otherwise warns the user and asks to check first the user level permission.

In nix systems, like Linux and Mac OS, I can rely on os.access, but it seems that simply does not work on Windows. It tells that the file always have write permission, but proceeding to the writing I stumble in an unexpected exception of PermissionError - Permission denied. I saw the StackOverflow link pointing to the win32security (Checking folder/file ntfs permissions using python). But the topic is 10 years old, and checking for library today, it doesn't seems that library is currently supported or mantained.

Snippet to check permission:

import os

if os.name == 'posix':
    file_to_write = '/etc/hosts'
else:
    file_to_write = 'C:\\Windows\\System32\\drivers\\etc\\hosts'

if os.access(file_to_write, os.W_OK):
    print("I can write in!")
else:
    print("Can't write. No permission.")

Then, the code to write in the file:

import os

if os.name == 'posix':
    file_to_write = '/etc/hosts'
else:
    file_to_write = 'C:\\Windows\\System32\\drivers\\etc\\hosts'

file_resource = open(file_to_write, 'a')
file_resource.write("\ntest")
file_resource.close()

Testing my snippets, using python3, in any operational system, or even testing any file in the system (not necessarily the hosts file in my example), python shall tells previously if the file can be written by the current user. If so, then the second snippet must not return an exception of PermissionError - Permission denied, and the target file shall have a word "test" in the file's last line. Otherwise, I won't even try the second snippet. In Windows environment, python dumbly tells that any existing file have write permission.

You could try to open the file in append mode and see if that throws an exception. — Aran-Fey, Sep 15 '19 at 18:53
`os.stat` (i.e. `st_mode`) and `os.access` in Windows do not support file security, just the readonly file attribute. However, unless a script is administrative or it's searching for the first match that's accessible (e.g. `shutil.which`), it's an anti-pattern to manually check `access` before opening a file. It's open to race conditions, since the permissions may have changed or the file may have been unlinked when the program tries to open it. So, in general, don't check `access`; just handle the exception if opening fails. — Eryk Sun, Sep 16 '19 at 00:18

Barb · Answer 1 · 2019-09-16T13:01:41.543

Maybe use a with statement that automatically closes. 'a+' will create the file if it does not already exist and append the file. For reading and writing you can use 'r+':

import os

if os.name == 'posix':
    file_to_write = '/etc/hosts'
else:
    file_to_write = 'C:\\Windows\\System32\\drivers\\etc\\hosts'

with open('file_to_write', 'a+') as file_resource: 
    file_resource.write("\ntest") # .close() is automatically called

If this does not work the permission error will be because you are trying to write to a path that needs a user privilege. If that is the case consider using an alternative of the chmod on a Linux machine. In windows you can change the settings by right clicking the file. Navigate to the properties settings, then the security settings to allow 'write'

Using python to check write permission in a ntfs file system

1 Answers1