Better AssertEqual() For Os.stat(myfile).st_mode

- 1 answer

I have a code that checks the st_mode of a file:

self.assertEqual(16877, os.stat(my_directory).st_mode)

Only old school unix experts are able to decipher the integer value 16877 fluently.

Is there more readable way to check for exactly this value?



If I may extend the question a bit and understand it as “Is there more readable way to check file modes?”, then I'll suggest adding a custom assertion. The target:

self.assertFileMode(my_directory, user="rwx", group="rx", others="rx")

How to do it.

Let's put that assertion in a mixin:

import os
import stat

class FileAssertions(object):
    FILE_PERMS = {
        'user': {'r': stat.S_IRUSR, 'w': stat.S_IWUSR, 'x': stat.S_IXUSR, 's': stat.S_ISUID},
        'group': {'r': stat.S_IRGRP, 'w': stat.S_IWGRP, 'x': stat.S_IXGRP, 's': stat.S_ISGID},
        'others': {'r': stat.S_IROTH, 'w': stat.S_IWOTH, 'x': stat.S_IXOTH},

    def assertFileMode(self, path, **kwargs):
        mode = os.stat(path).st_mode
        for key, perm_defs in self.FILE_PERMS.items():
            expected = kwargs.pop(key, None)
            if expected is not None:
                actual_perms = mode & sum(perm_defs.values())
                expected_perms = sum(perm_defs[flag] for flag in expected)

                if actual_perms != expected_perms:
                    msg = '{key} permissions: {expected} != {actual} for {path}'.format(
                        key=key, path=path,
                        actual=''.join(sorted(flag for flag, value in perm_defs.items()
                                              if value & mode != 0))
                    raise self.failureException(msg)
        if kwargs:
            raise TypeError('assertFileMode: unknown arguments %s' % ', '.join(kwargs))

Using it

Now, how about we test some file modes?

# We use our mixin
class MyTestCase(FileAssertions, TestCase):
    def test_some_paths(self):
        # Test all permissions
        self.assertFileMode('/foo/bar', user='rwx', group='rx', others='')

        # Only test user permissions
        self.assertFileMode('/foo/bar', user='rwx')

        # We support the suid/sgid bits as well
        self.assertFileMode('/foo/bar', user='rwxs', group='rxs', others='rx')

Example output:

AssertionError: user permissions: rw != rwx for /foo/bar


  • Only permissions given to the method are tested. To test that no permissions exist, pass an empty string.
  • Most of the complexity comes from generating a user-friendly message.
  • Permissions are sorted alphabetically in the error messages, so they are easier to eyeball-compare.
  • To keep it simple, I did not handle testing the sticky bit.