Python module os.chmod(file, 664) does not change the permission to rw-rw-r— but -w--wx----

Question

Recently I am using Python module os, when I tried to change the permission of a file, I did not get the expected result. For example, I intended to change the permission to

Answer 1

Use permission symbols instead of numbers

Your problem would have been avoided if you had used the more semantically named permission symbols rather than raw magic numbers, e.g. for 664:

#!/usr/bin/env python3

import os
import stat

os.chmod(
    'myfile',
    stat.S_IRUSR |
    stat.S_IWUSR |
    stat.S_IRGRP |
    stat.S_IWGRP |
    stat.S_IROTH
)

This is documented at https://docs.python.org/3/library/os.html#os.chmod and the names are the same as the POSIX C API values documented at man 2 stat.

Another advantage is the greater portability as mentioned in the docs:

Note: Although Windows supports chmod(), you can only set the file’s read-only flag with it (via the stat.S_IWRITE and stat.S_IREAD constants or a corresponding integer value). All other bits are ignored.

chmod +x is demonstrated at: How do you do a simple "chmod +x" from within python?

Tested in Ubuntu 16.04, Python 3.5.2.