Remove particular permission using os.chmod

Question

How can we remove a particular permission for all using os.chmod ?

In short, how can we write the below using os.chmod

chmod a-x filename

I do know that we can add permission to an existing one and remove also.

In [1]: import os, stat
In [5]: os.chmod(file, os.stat(file).st_mode | stat.S_IRGRP)  # Make file group readable.

But I am not able to figure out the doing the all operation

Answer 1

Cool. So the secret is you first need to get the current permissions. This is a bit of a mess, but it works.

current = stat.S_IMODE(os.lstat("x").st_mode)

The idea is that lstat.st_mode gives you the flags, but you need to crop that to the range that chmod accepts:

help(stat.S_IMODE)
#>>> Help on built-in function S_IMODE in module _stat:
#>>>
#>>> S_IMODE(...)
#>>>     Return the portion of the file's mode that can be set by os.chmod().
#>>>

Then you can remove the stat.S_IEXEC flag with some bit operations, and this gives you the new number to use:

os.chmod("x", current & ~stat.S_IEXEC)

If you're not familiar with bit twiddling, & takes only those bits that both numbers have, and ~ inverts the bits of a number. so x & ~y takes those bits that x has and that y doesn't have.