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[Python-Dev] PEP 471 -- os.scandir() function -- a better andfaster directory iterator

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Chris Angelico <rosuav at gmail.com> writes:> On Sat, Jun 28, 2014 at 11:05 PM, Akira Li <4kir4.1i at gmail.com> wrote:>> Have you considered adding support for paths relative to directory>> descriptors [1] via keyword only dir_fd=None parameter if it may lead to>> more efficient implementations on some platforms?>>>> [1]:https://docs.python.org/3.4/library/os.html#dir-fd>> Potentially more efficient and also potentially safer (see 'man> openat')... but an enhancement that can wait, if necessary.>Introducing the feature later creates unnecessary incompatibilities.Either it should be explicitly rejected in the PEP 471 andsomething-like `os.scandir(os.open(relative_path, dir_fd=fd))` recommendedinstead (assuming `os.scandir in os.supports_fd` like `os.listdir()`).At C level it could be implemented using fdopendir/openat or scandirat.Here's the function description using Argument Clinic DSL:/*[clinic input]os.scandir    path : path_t(allow_fd=True, nullable=True) = '.'        *path* can be specified as either str or bytes. On some        platforms, *path* may also be specified as an open file        descriptor; the file descriptor must refer to a directory.  If        this functionality is unavailable, using it raises        NotImplementedError.    *    dir_fd : dir_fd = None        If not None, it should be a file descriptor open to a        directory, and *path* should be a relative string; path will        then be relative to that directory.  if *dir_fd* is        unavailable, using it raises NotImplementedError.Yield a DirEntry object for each file and directory in *path*.Just like os.listdir, the '.' and '..' pseudo-directories are skipped,and the entries are yielded in system-dependent order.{parameters}It's an error to use *dir_fd* when specifying *path* as an open filedescriptor.[clinic start generated code]*/And corresponding tests (from test_posix:PosixTester), to show thecompatibility with os.listdir argument parsing in detail:    def test_scandir_default(self):        # When scandir is called without argument,        # it's the same as scandir(os.curdir).        self.assertIn(support.TESTFN, [e.name for e in posix.scandir()])    def _test_scandir(self, curdir):        filenames = sorted(e.name for e in posix.scandir(curdir))        self.assertIn(support.TESTFN, filenames)        #NOTE: assume listdir, scandir accept the same types on the platform        self.assertEqual(sorted(posix.listdir(curdir)), filenames)    def test_scandir(self):        self._test_scandir(os.curdir)    def test_scandir_none(self):        # it's the same as scandir(os.curdir).        self._test_scandir(None)    def test_scandir_bytes(self):        # When scandir is called with a bytes object,        # the returned entries names are still of type str.        # Call `os.fsencode(entry.name)` to get bytes        self.assertIn('a', {'a'})        self.assertNotIn(b'a', {'a'})        self._test_scandir(b'.')    @unittest.skipUnless(posix.scandir in os.supports_fd,                         "test needs fd support for posix.scandir()")    def test_scandir_fd_minus_one(self):        # it's the same as scandir(os.curdir).        self._test_scandir(-1)    def test_scandir_float(self):        # invalid args        self.assertRaises(TypeError, posix.scandir, -1.0)    @unittest.skipUnless(posix.scandir in os.supports_fd,                         "test needs fd support for posix.scandir()")    def test_scandir_fd(self):        fd = posix.open(posix.getcwd(), posix.O_RDONLY)        self.addCleanup(posix.close, fd)        self._test_scandir(fd)        self.assertEqual(            sorted(posix.scandir('.')),            sorted(posix.scandir(fd)))        # call 2nd time to test rewind        self.assertEqual(            sorted(posix.scandir('.')),            sorted(posix.scandir(fd)))    @unittest.skipUnless(posix.scandir in os.supports_dir_fd,                         "test needs dir_fd support for os.scandir()")    def test_scandir_dir_fd(self):        relpath = 'relative_path'        with support.temp_dir() as parent:            fullpath = os.path.join(parent, relpath)            with support.temp_dir(path=fullpath):                support.create_empty_file(os.path.join(parent, 'a'))                support.create_empty_file(os.path.join(fullpath, 'b'))                fd = posix.open(parent, posix.O_RDONLY)                self.addCleanup(posix.close, fd)                self.assertEqual(                    sorted(posix.scandir(relpath, dir_fd=fd)),                    sorted(posix.scandir(fullpath)))                # check that fd is still useful                self.assertEqual(                    sorted(posix.scandir(relpath, dir_fd=fd)),                    sorted(posix.scandir(fullpath)))--Akira

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