[Python-ideas] chdir context manager

[Chris Jerdonek]

On Sat, Jan 19, 2013 at 2:10 AM, Daniel Shahaf <d.s at daniel.shahaf.name> wrote:
> The following is a common pattern (used by, for example,
> shutil.make_archive):
>
>     save_cwd = os.getcwd()
>     try:
>         foo()
>     finally:
>         os.chdir(save_cwd)

FWIW, test.support has such a context manager (though test.support is
not for public consumption and test.support's implementation does more
than one thing, though see issue 15415):

http://hg.python.org/cpython/file/48cddcb9c841/Lib/test/support.py#l738

--Chris


>
> I suggest this deserves a context manager:
>
>     with saved_cwd():
>         foo()
>
> Initial feedback on IRC suggests shutil as where this functionality
> should live (other suggestions were made, such as pathlib).  Hence,
> attached patch implements this as shutil.saved_cwd, based on os.fchdir.
>
> The patch also adds os.chdir to os.supports_dir_fd and documents the
> context manager abilities of builtins.open() in its reference.
>
> Thoughts?
>
> Thanks,
>
> Daniel
>
>
> diff -r 74b0461346f0 Doc/library/functions.rst
> --- a/Doc/library/functions.rst Fri Jan 18 17:53:18 2013 -0800
> +++ b/Doc/library/functions.rst Sat Jan 19 09:39:27 2013 +0000
> @@ -828,6 +828,9 @@ are always available.  They are listed h
>     Open *file* and return a corresponding :term:`file object`.  If the file
>     cannot be opened, an :exc:`OSError` is raised.
>
> +   This function can be used as a :term:`context manager` that closes the
> +   file when it exits.
> +
>     *file* is either a string or bytes object giving the pathname (absolute or
>     relative to the current working directory) of the file to be opened or
>     an integer file descriptor of the file to be wrapped.  (If a file descriptor
> diff -r 74b0461346f0 Doc/library/os.rst
> --- a/Doc/library/os.rst        Fri Jan 18 17:53:18 2013 -0800
> +++ b/Doc/library/os.rst        Sat Jan 19 09:39:27 2013 +0000
> @@ -1315,6 +1315,9 @@ features:
>     This function can support :ref:`specifying a file descriptor <path_fd>`.  The
>     descriptor must refer to an opened directory, not an open file.
>
> +   See also :func:`shutil.saved_cwd` for a context manager that restores the
> +   current working directory.
> +
>     Availability: Unix, Windows.
>
>     .. versionadded:: 3.3
> diff -r 74b0461346f0 Doc/library/shutil.rst
> --- a/Doc/library/shutil.rst    Fri Jan 18 17:53:18 2013 -0800
> +++ b/Doc/library/shutil.rst    Sat Jan 19 09:39:27 2013 +0000
> @@ -36,6 +36,19 @@ copying and removal. For operations on i
>  Directory and files operations
>  ------------------------------
>
> +.. function:: saved_cwd()
> +
> +   Return a :term:`context manager` that restores the current working directory
> +   when it exits.  See :func:`os.chdir` for changing the current working
> +   directory.
> +
> +   The context manager returns an open file descriptor for the saved directory.
> +
> +   Only available when :func:`os.chdir` supports file descriptor arguments.
> +
> +   .. versionadded:: 3.4
> +
> +
>  .. function:: copyfileobj(fsrc, fdst[, length])
>
>     Copy the contents of the file-like object *fsrc* to the file-like object *fdst*.
> diff -r 74b0461346f0 Lib/os.py
> --- a/Lib/os.py Fri Jan 18 17:53:18 2013 -0800
> +++ b/Lib/os.py Sat Jan 19 09:39:27 2013 +0000
> @@ -120,6 +120,7 @@ if _exists("_have_functions"):
>
>      _set = set()
>      _add("HAVE_FACCESSAT",  "access")
> +    _add("HAVE_FCHDIR",     "chdir")
>      _add("HAVE_FCHMODAT",   "chmod")
>      _add("HAVE_FCHOWNAT",   "chown")
>      _add("HAVE_FSTATAT",    "stat")
> diff -r 74b0461346f0 Lib/shutil.py
> --- a/Lib/shutil.py     Fri Jan 18 17:53:18 2013 -0800
> +++ b/Lib/shutil.py     Sat Jan 19 09:39:27 2013 +0000
> @@ -38,6 +38,7 @@ __all__ = ["copyfileobj", "copyfile", "c
>             "unregister_unpack_format", "unpack_archive",
>             "ignore_patterns", "chown", "which"]
>             # disk_usage is added later, if available on the platform
> +           # saved_cwd is added later, if available on the platform
>
>  class Error(OSError):
>      pass
> @@ -1111,3 +1112,20 @@ def which(cmd, mode=os.F_OK | os.X_OK, p
>                  if _access_check(name, mode):
>                      return name
>      return None
> +
> +# Define the chdir context manager.
> +if os.chdir in os.supports_dir_fd:
> +    class saved_cwd:
> +        def __init__(self):
> +            pass
> +        def __enter__(self):
> +            self.dh = os.open(os.curdir,
> +                              os.O_RDONLY | getattr(os, 'O_DIRECTORY', 0))
> +            return self.dh
> +        def __exit__(self, exc_type, exc_value, traceback):
> +            try:
> +                os.chdir(self.dh)
> +            finally:
> +                os.close(self.dh)
> +            return False
> +    __all__.append('saved_cwd')
> diff -r 74b0461346f0 Lib/test/test_shutil.py
> --- a/Lib/test/test_shutil.py   Fri Jan 18 17:53:18 2013 -0800
> +++ b/Lib/test/test_shutil.py   Sat Jan 19 09:39:27 2013 +0000
> @@ -1276,6 +1276,20 @@ class TestShutil(unittest.TestCase):
>          rv = shutil.copytree(src_dir, dst_dir)
>          self.assertEqual(['foo'], os.listdir(rv))
>
> +    def test_saved_cwd(self):
> +        if hasattr(os, 'fchdir'):
> +            temp_dir = self.mkdtemp()
> +            orig_dir = os.getcwd()
> +            with shutil.saved_cwd() as dir_fd:
> +                os.chdir(temp_dir)
> +                new_dir = os.getcwd()
> +                self.assertIsInstance(dir_fd, int)
> +            final_dir = os.getcwd()
> +            self.assertEqual(orig_dir, final_dir)
> +            self.assertEqual(temp_dir, new_dir)
> +        else:
> +            self.assertFalse(hasattr(shutil, 'saved_cwd'))
> +
>
>  class TestWhich(unittest.TestCase):
>
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