atcontext wrapper

import os
from stat import S_ISDIR
from errno import ENOTDIR
from functools import partial

O_DIRECTORY = getattr(os, "O_DIRECTORY", None)

AT_FUNCTIONS = set()
for name in dir(os):
    if name.endswith("at") and hasattr(getattr(os, name), "__call__"):
        if hasattr(getattr(os, name[:-2], None), "__call__"):
            AT_FUNCTIONS.add(name)
del name


class atcontext:
    def __init__(self, directory, closefd=True, cloexec=False):
        self.cloexec = cloexec
        if isinstance(directory, str):
            self.directory = directory
            self.dirfd = None
            self.closefd = True
        elif isinstance(directory, int):
            self.directory = None
            self.dirfd = directory
            self.closefd = closefd
        else:
            raise TypeError(directory)

    def _check(self):
        if self.directory is None and self.dirfd is None:
            raise ValueError("I/O operation on closed file.")

    def __enter__(self):
        self._check()
        if self.dirfd is None:
            flags = os.O_RDONLY
            if self.cloexec:
                flags |= os.O_CLOEXEC
            if O_DIRECTORY is not None:
                flags |= O_DIRECTORY
            elif not S_ISDIR(os.stat(self.directory).st_mode):
                # must check early as open blocks on fifos (DoS)
                # see Linux's man(2) open, O_DIRECTORY
                raise NotADirectoryError(ENOTDIR, os.strerror(ENOTDIR), self.directory)
            self.dirfd = os.open(self.directory, flags)
        return self

    def __exit__(self, typ, value, tb):
        if self.closefd:
            os.close(self.dirfd)
        self.directory = self.dirfd = None

    def __getattr__(self, name):
        self._check()
        nameat = name + "at"
        if nameat in AT_FUNCTIONS:
            return partial(getattr(os, nameat), self.dirfd)
        raise AttributeError(name)

    def __dir__(self):
        content = super().__dir__() + list(AT_FUNCTIONS)
        return sorted(content)

if __name__ == "__main__":
    with atcontext("/etc") as at:
        print(dir(at))
        f = at.open("fstab", os.O_RDONLY)
        print(os.read(f, 50))
        os.close(f)

    with atcontext("/etc/fstab") as at:
        pass