python：OS模块

 r"""OS routines for NT or Posix depending on what system we're on.

 This exports:
   - all functions from posix or nt, e.g. unlink, stat, etc.
   - os.path is either posixpath or ntpath
   - os.name is either 'posix' or 'nt'
   - os.curdir is a string representing the current directory (always '.')
   - os.pardir is a string representing the parent directory (always '..')
   - os.sep is the (or a most common) pathname separator ('/' or '\\')
   - os.extsep is the extension separator (always '.')
   - os.altsep is the alternate pathname separator (None or '/')
   - os.pathsep is the component separator used in $PATH etc
   - os.linesep is the line separator in text files ('\r' or '\n' or '\r\n')
   - os.defpath is the default search path for executables
   - os.devnull is the file path of the null device ('/dev/null', etc.)

 Programs that import and use 'os' stand a better chance of being
 portable between different platforms.  Of course, they must then
 only use functions that are defined by all platforms (e.g., unlink
 and opendir), and leave all pathname manipulation to os.path
 (e.g., split and join).
 """

 #'
 import abc
 import sys, errno
 import stat as st

 _names = sys.builtin_module_names

 # Note:  more names are added to __all__ later.
 __all__ = ["altsep", "curdir", "pardir", "sep", "pathsep", "linesep",
            "defpath", "name", "path", "devnull", "SEEK_SET", "SEEK_CUR",
            "SEEK_END", "fsencode", "fsdecode", "get_exec_path", "fdopen",
            "popen", "extsep"]

 def _exists(name):
     return name in globals()

 def _get_exports_list(module):
     try:
         return list(module.__all__)
     except AttributeError:
         return [n for n in dir(module) if n[0] != '_']

 # Any new dependencies of the os module and/or changes in path separator
 # requires updating importlib as well.
 if 'posix' in _names:
     name = 'posix'
     linesep = '\n'
     from posix import *
     try:
         from posix import _exit
         __all__.append('_exit')
     except ImportError:
         pass
     import posixpath as path

     try:
         from posix import _have_functions
     except ImportError:
         pass

     import posix
     __all__.extend(_get_exports_list(posix))
     del posix

 elif 'nt' in _names:
     name = 'nt'
     linesep = '\r\n'
     from nt import *
     try:
         from nt import _exit
         __all__.append('_exit')
     except ImportError:
         pass
     import ntpath as path

     import nt
     __all__.extend(_get_exports_list(nt))
     del nt

     try:
         from nt import _have_functions
     except ImportError:
         pass

 else:
     raise ImportError('no os specific module found')

 sys.modules['os.path'] = path
 from os.path import (curdir, pardir, sep, pathsep, defpath, extsep, altsep,
     devnull)

 del _names

 if _exists("_have_functions"):
     _globals = globals()
     def _add(str, fn):
         if (fn in _globals) and (str in _have_functions):
             _set.add(_globals[fn])

     _set = set()
     _add("HAVE_FACCESSAT",  "access")
     _add("HAVE_FCHMODAT",   "chmod")
     _add("HAVE_FCHOWNAT",   "chown")
     _add("HAVE_FSTATAT",    "stat")
     _add("HAVE_FUTIMESAT",  "utime")
     _add("HAVE_LINKAT",     "link")
     _add("HAVE_MKDIRAT",    "mkdir")
     _add("HAVE_MKFIFOAT",   "mkfifo")
     _add("HAVE_MKNODAT",    "mknod")
     _add("HAVE_OPENAT",     "open")
     _add("HAVE_READLINKAT", "readlink")
     _add("HAVE_RENAMEAT",   "rename")
     _add("HAVE_SYMLINKAT",  "symlink")
     _add("HAVE_UNLINKAT",   "unlink")
     _add("HAVE_UNLINKAT",   "rmdir")
     _add("HAVE_UTIMENSAT",  "utime")
     supports_dir_fd = _set

     _set = set()
     _add("HAVE_FACCESSAT",  "access")
     supports_effective_ids = _set

     _set = set()
     _add("HAVE_FCHDIR",     "chdir")
     _add("HAVE_FCHMOD",     "chmod")
     _add("HAVE_FCHOWN",     "chown")
     _add("HAVE_FDOPENDIR",  "listdir")
     _add("HAVE_FEXECVE",    "execve")
     _set.add(stat) # fstat always works
     _add("HAVE_FTRUNCATE",  "truncate")
     _add("HAVE_FUTIMENS",   "utime")
     _add("HAVE_FUTIMES",    "utime")
     _add("HAVE_FPATHCONF",  "pathconf")
     if _exists("statvfs") and _exists("fstatvfs"): # mac os x10.3
         _add("HAVE_FSTATVFS", "statvfs")
     supports_fd = _set

     _set = set()
     _add("HAVE_FACCESSAT",  "access")
     # Some platforms don't support lchmod().  Often the function exists
     # anyway, as a stub that always returns ENOSUP or perhaps EOPNOTSUPP.
     # (No, I don't know why that's a good design.)  ./configure will detect
     # this and reject it--so HAVE_LCHMOD still won't be defined on such
     # platforms.  This is Very Helpful.
     #
     # However, sometimes platforms without a working lchmod() *do* have
     # fchmodat().  (Examples: Linux kernel 3.2 with glibc 2.15,
     # OpenIndiana 3.x.)  And fchmodat() has a flag that theoretically makes
     # it behave like lchmod().  So in theory it would be a suitable
     # replacement for lchmod().  But when lchmod() doesn't work, fchmodat()'s
     # flag doesn't work *either*.  Sadly ./configure isn't sophisticated
     # enough to detect this condition--it only determines whether or not
     # fchmodat() minimally works.
     #
     # Therefore we simply ignore fchmodat() when deciding whether or not
     # os.chmod supports follow_symlinks.  Just checking lchmod() is
     # sufficient.  After all--if you have a working fchmodat(), your
     # lchmod() almost certainly works too.
     #
     # _add("HAVE_FCHMODAT",   "chmod")
     _add("HAVE_FCHOWNAT",   "chown")
     _add("HAVE_FSTATAT",    "stat")
     _add("HAVE_LCHFLAGS",   "chflags")
     _add("HAVE_LCHMOD",     "chmod")
     if _exists("lchown"): # mac os x10.3
         _add("HAVE_LCHOWN", "chown")
     _add("HAVE_LINKAT",     "link")
     _add("HAVE_LUTIMES",    "utime")
     _add("HAVE_LSTAT",      "stat")
     _add("HAVE_FSTATAT",    "stat")
     _add("HAVE_UTIMENSAT",  "utime")
     _add("MS_WINDOWS",      "stat")
     supports_follow_symlinks = _set

     del _set
     del _have_functions
     del _globals
     del _add

 # Python uses fixed values for the SEEK_ constants; they are mapped
 # to native constants if necessary in posixmodule.c
 # Other possible SEEK values are directly imported from posixmodule.c
 SEEK_SET = 0
 SEEK_CUR = 1
 SEEK_END = 2

 # Super directory utilities.
 # (Inspired by Eric Raymond; the doc strings are mostly his)

 def makedirs(name, mode=0o777, exist_ok=False):
     """makedirs(name [, mode=0o777][, exist_ok=False])

     Super-mkdir; create a leaf directory and all intermediate ones.  Works like
     mkdir, except that any intermediate path segment (not just the rightmost)
     will be created if it does not exist. If the target directory already
     exists, raise an OSError if exist_ok is False. Otherwise no exception is
     raised.  This is recursive.

     """
     head, tail = path.split(name)
     if not tail:
         head, tail = path.split(head)
     if head and tail and not path.exists(head):
         try:
             makedirs(head, mode, exist_ok)
         except FileExistsError:
             # Defeats race condition when another thread created the path
             pass
         cdir = curdir
         if isinstance(tail, bytes):
             cdir = bytes(curdir, 'ASCII')
         if tail == cdir:           # xxx/newdir/. exists if xxx/newdir exists
             return
     try:
         mkdir(name, mode)
     except OSError:
         # Cannot rely on checking for EEXIST, since the operating system
         # could give priority to other errors like EACCES or EROFS
         if not exist_ok or not path.isdir(name):
             raise

 def removedirs(name):
     """removedirs(name)

     Super-rmdir; remove a leaf directory and all empty intermediate
     ones.  Works like rmdir except that, if the leaf directory is
     successfully removed, directories corresponding to rightmost path
     segments will be pruned away until either the whole path is
     consumed or an error occurs.  Errors during this latter phase are
     ignored -- they generally mean that a directory was not empty.

     """
     rmdir(name)
     head, tail = path.split(name)
     if not tail:
         head, tail = path.split(head)
     while head and tail:
         try:
             rmdir(head)
         except OSError:
             break
         head, tail = path.split(head)

 def renames(old, new):
     """renames(old, new)

     Super-rename; create directories as necessary and delete any left
     empty.  Works like rename, except creation of any intermediate
     directories needed to make the new pathname good is attempted
     first.  After the rename, directories corresponding to rightmost
     path segments of the old name will be pruned until either the
     whole path is consumed or a nonempty directory is found.

     Note: this function can fail with the new directory structure made
     if you lack permissions needed to unlink the leaf directory or
     file.

     """
     head, tail = path.split(new)
     if head and tail and not path.exists(head):
         makedirs(head)
     rename(old, new)
     head, tail = path.split(old)
     if head and tail:
         try:
             removedirs(head)
         except OSError:
             pass

 __all__.extend(["makedirs", "removedirs", "renames"])

 def walk(top, topdown=True, onerror=None, followlinks=False):
     """Directory tree generator.

     For each directory in the directory tree rooted at top (including top
     itself, but excluding '.' and '..'), yields a 3-tuple

         dirpath, dirnames, filenames

     dirpath is a string, the path to the directory.  dirnames is a list of
     the names of the subdirectories in dirpath (excluding '.' and '..').
     filenames is a list of the names of the non-directory files in dirpath.
     Note that the names in the lists are just names, with no path components.
     To get a full path (which begins with top) to a file or directory in
     dirpath, do os.path.join(dirpath, name).

     If optional arg 'topdown' is true or not specified, the triple for a
     directory is generated before the triples for any of its subdirectories
     (directories are generated top down).  If topdown is false, the triple
     for a directory is generated after the triples for all of its
     subdirectories (directories are generated bottom up).

     When topdown is true, the caller can modify the dirnames list in-place
     (e.g., via del or slice assignment), and walk will only recurse into the
     subdirectories whose names remain in dirnames; this can be used to prune the
     search, or to impose a specific order of visiting.  Modifying dirnames when
     topdown is false is ineffective, since the directories in dirnames have
     already been generated by the time dirnames itself is generated. No matter
     the value of topdown, the list of subdirectories is retrieved before the
     tuples for the directory and its subdirectories are generated.

     By default errors from the os.scandir() call are ignored.  If
     optional arg 'onerror' is specified, it should be a function; it
     will be called with one argument, an OSError instance.  It can
     report the error to continue with the walk, or raise the exception
     to abort the walk.  Note that the filename is available as the
     filename attribute of the exception object.

     By default, os.walk does not follow symbolic links to subdirectories on
     systems that support them.  In order to get this functionality, set the
     optional argument 'followlinks' to true.

     Caution:  if you pass a relative pathname for top, don't change the
     current working directory between resumptions of walk.  walk never
     changes the current directory, and assumes that the client doesn't
     either.

     Example:

     import os
     from os.path import join, getsize
     for root, dirs, files in os.walk('python/Lib/email'):
         print(root, "consumes", end="")
         print(sum([getsize(join(root, name)) for name in files]), end="")
         print("bytes in", len(files), "non-directory files")
         if 'CVS' in dirs:
             dirs.remove('CVS')  # don't visit CVS directories

     """
     top = fspath(top)
     dirs = []
     nondirs = []
     walk_dirs = []

     # We may not have read permission for top, in which case we can't
     # get a list of the files the directory contains.  os.walk
     # always suppressed the exception then, rather than blow up for a
     # minor reason when (say) a thousand readable directories are still
     # left to visit.  That logic is copied here.
     try:
         # Note that scandir is global in this module due
         # to earlier import-*.
         scandir_it = scandir(top)
     except OSError as error:
         if onerror is not None:
             onerror(error)
         return

     with scandir_it:
         while True:
             try:
                 try:
                     entry = next(scandir_it)
                 except StopIteration:
                     break
             except OSError as error:
                 if onerror is not None:
                     onerror(error)
                 return

             try:
                 is_dir = entry.is_dir()
             except OSError:
                 # If is_dir() raises an OSError, consider that the entry is not
                 # a directory, same behaviour than os.path.isdir().
                 is_dir = False

             if is_dir:
                 dirs.append(entry.name)
             else:
                 nondirs.append(entry.name)

             if not topdown and is_dir:
                 # Bottom-up: recurse into sub-directory, but exclude symlinks to
                 # directories if followlinks is False
                 if followlinks:
                     walk_into = True
                 else:
                     try:
                         is_symlink = entry.is_symlink()
                     except OSError:
                         # If is_symlink() raises an OSError, consider that the
                         # entry is not a symbolic link, same behaviour than
                         # os.path.islink().
                         is_symlink = False
                     walk_into = not is_symlink

                 if walk_into:
                     walk_dirs.append(entry.path)

     # Yield before recursion if going top down
     if topdown:
         yield top, dirs, nondirs

         # Recurse into sub-directories
         islink, join = path.islink, path.join
         for dirname in dirs:
             new_path = join(top, dirname)
             # Issue #23605: os.path.islink() is used instead of caching
             # entry.is_symlink() result during the loop on os.scandir() because
             # the caller can replace the directory entry during the "yield"
             # above.
             if followlinks or not islink(new_path):
                 yield from walk(new_path, topdown, onerror, followlinks)
     else:
         # Recurse into sub-directories
         for new_path in walk_dirs:
             yield from walk(new_path, topdown, onerror, followlinks)
         # Yield after recursion if going bottom up
         yield top, dirs, nondirs

 __all__.append("walk")

 if {open, stat} <= supports_dir_fd and {listdir, stat} <= supports_fd:

     def fwalk(top=".", topdown=True, onerror=None, *, follow_symlinks=False, dir_fd=None):
         """Directory tree generator.

         This behaves exactly like walk(), except that it yields a 4-tuple

             dirpath, dirnames, filenames, dirfd

         `dirpath`, `dirnames` and `filenames` are identical to walk() output,
         and `dirfd` is a file descriptor referring to the directory `dirpath`.

         The advantage of fwalk() over walk() is that it's safe against symlink
         races (when follow_symlinks is False).

         If dir_fd is not None, it should be a file descriptor open to a directory,
           and top should be relative; top will then be relative to that directory.
           (dir_fd is always supported for fwalk.)

         Caution:
         Since fwalk() yields file descriptors, those are only valid until the
         next iteration step, so you should dup() them if you want to keep them
         for a longer period.

         Example:

         import os
         for root, dirs, files, rootfd in os.fwalk('python/Lib/email'):
             print(root, "consumes", end="")
             print(sum([os.stat(name, dir_fd=rootfd).st_size for name in files]),
                   end="")
             print("bytes in", len(files), "non-directory files")
             if 'CVS' in dirs:
                 dirs.remove('CVS')  # don't visit CVS directories
         """
         if not isinstance(top, int) or not hasattr(top, '__index__'):
             top = fspath(top)
         # Note: To guard against symlink races, we use the standard
         # lstat()/open()/fstat() trick.
         orig_st = stat(top, follow_symlinks=False, dir_fd=dir_fd)
         topfd = open(top, O_RDONLY, dir_fd=dir_fd)
         try:
             if (follow_symlinks or (st.S_ISDIR(orig_st.st_mode) and
                                     path.samestat(orig_st, stat(topfd)))):
                 yield from _fwalk(topfd, top, topdown, onerror, follow_symlinks)
         finally:
             close(topfd)

     def _fwalk(topfd, toppath, topdown, onerror, follow_symlinks):
         # Note: This uses O(depth of the directory tree) file descriptors: if
         # necessary, it can be adapted to only require O(1) FDs, see issue
         # #13734.

         names = listdir(topfd)
         dirs, nondirs = [], []
         for name in names:
             try:
                 # Here, we don't use AT_SYMLINK_NOFOLLOW to be consistent with
                 # walk() which reports symlinks to directories as directories.
                 # We do however check for symlinks before recursing into
                 # a subdirectory.
                 if st.S_ISDIR(stat(name, dir_fd=topfd).st_mode):
                     dirs.append(name)
                 else:
                     nondirs.append(name)
             except OSError:
                 try:
                     # Add dangling symlinks, ignore disappeared files
                     if st.S_ISLNK(stat(name, dir_fd=topfd, follow_symlinks=False)
                                 .st_mode):
                         nondirs.append(name)
                 except OSError:
                     continue

         if topdown:
             yield toppath, dirs, nondirs, topfd

         for name in dirs:
             try:
                 orig_st = stat(name, dir_fd=topfd, follow_symlinks=follow_symlinks)
                 dirfd = open(name, O_RDONLY, dir_fd=topfd)
             except OSError as err:
                 if onerror is not None:
                     onerror(err)
                 continue
             try:
                 if follow_symlinks or path.samestat(orig_st, stat(dirfd)):
                     dirpath = path.join(toppath, name)
                     yield from _fwalk(dirfd, dirpath, topdown, onerror, follow_symlinks)
             finally:
                 close(dirfd)

         if not topdown:
             yield toppath, dirs, nondirs, topfd

     __all__.append("fwalk")

 # Make sure os.environ exists, at least
 try:
     environ
 except NameError:
     environ = {}

 def execl(file, *args):
     """execl(file, *args)

     Execute the executable file with argument list args, replacing the
     current process. """
     execv(file, args)

 def execle(file, *args):
     """execle(file, *args, env)

     Execute the executable file with argument list args and
     environment env, replacing the current process. """
     env = args[-1]
     execve(file, args[:-1], env)

 def execlp(file, *args):
     """execlp(file, *args)

     Execute the executable file (which is searched for along $PATH)
     with argument list args, replacing the current process. """
     execvp(file, args)

 def execlpe(file, *args):
     """execlpe(file, *args, env)

     Execute the executable file (which is searched for along $PATH)
     with argument list args and environment env, replacing the current
     process. """
     env = args[-1]
     execvpe(file, args[:-1], env)

 def execvp(file, args):
     """execvp(file, args)

     Execute the executable file (which is searched for along $PATH)
     with argument list args, replacing the current process.
     args may be a list or tuple of strings. """
     _execvpe(file, args)

 def execvpe(file, args, env):
     """execvpe(file, args, env)

     Execute the executable file (which is searched for along $PATH)
     with argument list args and environment env , replacing the
     current process.
     args may be a list or tuple of strings. """
     _execvpe(file, args, env)

 __all__.extend(["execl","execle","execlp","execlpe","execvp","execvpe"])

 def _execvpe(file, args, env=None):
     if env is not None:
         exec_func = execve
         argrest = (args, env)
     else:
         exec_func = execv
         argrest = (args,)
         env = environ

     head, tail = path.split(file)
     if head:
         exec_func(file, *argrest)
         return
     last_exc = saved_exc = None
     saved_tb = None
     path_list = get_exec_path(env)
     if name != 'nt':
         file = fsencode(file)
         path_list = map(fsencode, path_list)
     for dir in path_list:
         fullname = path.join(dir, file)
         try:
             exec_func(fullname, *argrest)
         except OSError as e:
             last_exc = e
             tb = sys.exc_info()[2]
             if (e.errno != errno.ENOENT and e.errno != errno.ENOTDIR
                 and saved_exc is None):
                 saved_exc = e
                 saved_tb = tb
     if saved_exc:
         raise saved_exc.with_traceback(saved_tb)
     raise last_exc.with_traceback(tb)

 def get_exec_path(env=None):
     """Returns the sequence of directories that will be searched for the
     named executable (similar to a shell) when launching a process.

     *env* must be an environment variable dict or None.  If *env* is None,
     os.environ will be used.
     """
     # Use a local import instead of a global import to limit the number of
     # modules loaded at startup: the os module is always loaded at startup by
     # Python. It may also avoid a bootstrap issue.
     import warnings

     if env is None:
         env = environ

     # {b'PATH': ...}.get('PATH') and {'PATH': ...}.get(b'PATH') emit a
     # BytesWarning when using python -b or python -bb: ignore the warning
     with warnings.catch_warnings():
         warnings.simplefilter("ignore", BytesWarning)

         try:
             path_list = env.get('PATH')
         except TypeError:
             path_list = None

         if supports_bytes_environ:
             try:
                 path_listb = env[b'PATH']
             except (KeyError, TypeError):
                 pass
             else:
                 if path_list is not None:
                     raise ValueError(
                         "env cannot contain 'PATH' and b'PATH' keys")
                 path_list = path_listb

             if path_list is not None and isinstance(path_list, bytes):
                 path_list = fsdecode(path_list)

     if path_list is None:
         path_list = defpath
     return path_list.split(pathsep)

 # Change environ to automatically call putenv(), unsetenv if they exist.
 from _collections_abc import MutableMapping

 class _Environ(MutableMapping):
     def __init__(self, data, encodekey, decodekey, encodevalue, decodevalue, putenv, unsetenv):
         self.encodekey = encodekey
         self.decodekey = decodekey
         self.encodevalue = encodevalue
         self.decodevalue = decodevalue
         self.putenv = putenv
         self.unsetenv = unsetenv
         self._data = data

     def __getitem__(self, key):
         try:
             value = self._data[self.encodekey(key)]
         except KeyError:
             # raise KeyError with the original key value
             raise KeyError(key) from None
         return self.decodevalue(value)

     def __setitem__(self, key, value):
         key = self.encodekey(key)
         value = self.encodevalue(value)
         self.putenv(key, value)
         self._data[key] = value

     def __delitem__(self, key):
         encodedkey = self.encodekey(key)
         self.unsetenv(encodedkey)
         try:
             del self._data[encodedkey]
         except KeyError:
             # raise KeyError with the original key value
             raise KeyError(key) from None

     def __iter__(self):
         # list() from dict object is an atomic operation
         keys = list(self._data)
         for key in keys:
             yield self.decodekey(key)

     def __len__(self):
         return len(self._data)

     def __repr__(self):
         return 'environ({{{}}})'.format(', '.join(
             ('{!r}: {!r}'.format(self.decodekey(key), self.decodevalue(value))
             for key, value in self._data.items())))

     def copy(self):
         return dict(self)

     def setdefault(self, key, value):
         if key not in self:
             self[key] = value
         return self[key]

 try:
     _putenv = putenv
 except NameError:
     _putenv = lambda key, value: None
 else:
     if "putenv" not in __all__:
         __all__.append("putenv")

 try:
     _unsetenv = unsetenv
 except NameError:
     _unsetenv = lambda key: _putenv(key, "")
 else:
     if "unsetenv" not in __all__:
         __all__.append("unsetenv")

 def _createenviron():
     if name == 'nt':
         # Where Env Var Names Must Be UPPERCASE
         def check_str(value):
             if not isinstance(value, str):
                 raise TypeError("str expected, not %s" % type(value).__name__)
             return value
         encode = check_str
         decode = str
         def encodekey(key):
             return encode(key).upper()
         data = {}
         for key, value in environ.items():
             data[encodekey(key)] = value
     else:
         # Where Env Var Names Can Be Mixed Case
         encoding = sys.getfilesystemencoding()
         def encode(value):
             if not isinstance(value, str):
                 raise TypeError("str expected, not %s" % type(value).__name__)
             return value.encode(encoding, 'surrogateescape')
         def decode(value):
             return value.decode(encoding, 'surrogateescape')
         encodekey = encode
         data = environ
     return _Environ(data,
         encodekey, decode,
         encode, decode,
         _putenv, _unsetenv)

 # unicode environ
 environ = _createenviron()
 del _createenviron

 def getenv(key, default=None):
     """Get an environment variable, return None if it doesn't exist.
     The optional second argument can specify an alternate default.
     key, default and the result are str."""
     return environ.get(key, default)

 supports_bytes_environ = (name != 'nt')
 __all__.extend(("getenv", "supports_bytes_environ"))

 if supports_bytes_environ:
     def _check_bytes(value):
         if not isinstance(value, bytes):
             raise TypeError("bytes expected, not %s" % type(value).__name__)
         return value

     # bytes environ
     environb = _Environ(environ._data,
         _check_bytes, bytes,
         _check_bytes, bytes,
         _putenv, _unsetenv)
     del _check_bytes

     def getenvb(key, default=None):
         """Get an environment variable, return None if it doesn't exist.
         The optional second argument can specify an alternate default.
         key, default and the result are bytes."""
         return environb.get(key, default)

     __all__.extend(("environb", "getenvb"))

 def _fscodec():
     encoding = sys.getfilesystemencoding()
     errors = sys.getfilesystemencodeerrors()

     def fsencode(filename):
         """Encode filename (an os.PathLike, bytes, or str) to the filesystem
         encoding with 'surrogateescape' error handler, return bytes unchanged.
         On Windows, use 'strict' error handler if the file system encoding is
         'mbcs' (which is the default encoding).
         """
         filename = fspath(filename)  # Does type-checking of `filename`.
         if isinstance(filename, str):
             return filename.encode(encoding, errors)
         else:
             return filename

     def fsdecode(filename):
         """Decode filename (an os.PathLike, bytes, or str) from the filesystem
         encoding with 'surrogateescape' error handler, return str unchanged. On
         Windows, use 'strict' error handler if the file system encoding is
         'mbcs' (which is the default encoding).
         """
         filename = fspath(filename)  # Does type-checking of `filename`.
         if isinstance(filename, bytes):
             return filename.decode(encoding, errors)
         else:
             return filename

     return fsencode, fsdecode

 fsencode, fsdecode = _fscodec()
 del _fscodec

 # Supply spawn*() (probably only for Unix)
 if _exists("fork") and not _exists("spawnv") and _exists("execv"):

     P_WAIT = 0
     P_NOWAIT = P_NOWAITO = 1

     __all__.extend(["P_WAIT", "P_NOWAIT", "P_NOWAITO"])

     # XXX Should we support P_DETACH?  I suppose it could fork()**2
     # and close the std I/O streams.  Also, P_OVERLAY is the same
     # as execv*()?

     def _spawnvef(mode, file, args, env, func):
         # Internal helper; func is the exec*() function to use
         if not isinstance(args, (tuple, list)):
             raise TypeError('argv must be a tuple or a list')
         if not args or not args[0]:
             raise ValueError('argv first element cannot be empty')
         pid = fork()
         if not pid:
             # Child
             try:
                 if env is None:
                     func(file, args)
                 else:
                     func(file, args, env)
             except:
                 _exit(127)
         else:
             # Parent
             if mode == P_NOWAIT:
                 return pid # Caller is responsible for waiting!
             while 1:
                 wpid, sts = waitpid(pid, 0)
                 if WIFSTOPPED(sts):
                     continue
                 elif WIFSIGNALED(sts):
                     return -WTERMSIG(sts)
                 elif WIFEXITED(sts):
                     return WEXITSTATUS(sts)
                 else:
                     raise OSError("Not stopped, signaled or exited???")

     def spawnv(mode, file, args):
         """spawnv(mode, file, args) -> integer

 Execute file with arguments from args in a subprocess.
 If mode == P_NOWAIT return the pid of the process.
 If mode == P_WAIT return the process's exit code if it exits normally;
 otherwise return -SIG, where SIG is the signal that killed it. """
         return _spawnvef(mode, file, args, None, execv)

     def spawnve(mode, file, args, env):
         """spawnve(mode, file, args, env) -> integer

 Execute file with arguments from args in a subprocess with the
 specified environment.
 If mode == P_NOWAIT return the pid of the process.
 If mode == P_WAIT return the process's exit code if it exits normally;
 otherwise return -SIG, where SIG is the signal that killed it. """
         return _spawnvef(mode, file, args, env, execve)

     # Note: spawnvp[e] is't currently supported on Windows

     def spawnvp(mode, file, args):
         """spawnvp(mode, file, args) -> integer

 Execute file (which is looked for along $PATH) with arguments from
 args in a subprocess.
 If mode == P_NOWAIT return the pid of the process.
 If mode == P_WAIT return the process's exit code if it exits normally;
 otherwise return -SIG, where SIG is the signal that killed it. """
         return _spawnvef(mode, file, args, None, execvp)

     def spawnvpe(mode, file, args, env):
         """spawnvpe(mode, file, args, env) -> integer

 Execute file (which is looked for along $PATH) with arguments from
 args in a subprocess with the supplied environment.
 If mode == P_NOWAIT return the pid of the process.
 If mode == P_WAIT return the process's exit code if it exits normally;
 otherwise return -SIG, where SIG is the signal that killed it. """
         return _spawnvef(mode, file, args, env, execvpe)

     __all__.extend(["spawnv", "spawnve", "spawnvp", "spawnvpe"])

 if _exists("spawnv"):
     # These aren't supplied by the basic Windows code
     # but can be easily implemented in Python

     def spawnl(mode, file, *args):
         """spawnl(mode, file, *args) -> integer

 Execute file with arguments from args in a subprocess.
 If mode == P_NOWAIT return the pid of the process.
 If mode == P_WAIT return the process's exit code if it exits normally;
 otherwise return -SIG, where SIG is the signal that killed it. """
         return spawnv(mode, file, args)

     def spawnle(mode, file, *args):
         """spawnle(mode, file, *args, env) -> integer

 Execute file with arguments from args in a subprocess with the
 supplied environment.
 If mode == P_NOWAIT return the pid of the process.
 If mode == P_WAIT return the process's exit code if it exits normally;
 otherwise return -SIG, where SIG is the signal that killed it. """
         env = args[-1]
         return spawnve(mode, file, args[:-1], env)

     __all__.extend(["spawnl", "spawnle"])

 if _exists("spawnvp"):
     # At the moment, Windows doesn't implement spawnvp[e],
     # so it won't have spawnlp[e] either.
     def spawnlp(mode, file, *args):
         """spawnlp(mode, file, *args) -> integer

 Execute file (which is looked for along $PATH) with arguments from
 args in a subprocess with the supplied environment.
 If mode == P_NOWAIT return the pid of the process.
 If mode == P_WAIT return the process's exit code if it exits normally;
 otherwise return -SIG, where SIG is the signal that killed it. """
         return spawnvp(mode, file, args)

     def spawnlpe(mode, file, *args):
         """spawnlpe(mode, file, *args, env) -> integer

 Execute file (which is looked for along $PATH) with arguments from
 args in a subprocess with the supplied environment.
 If mode == P_NOWAIT return the pid of the process.
 If mode == P_WAIT return the process's exit code if it exits normally;
 otherwise return -SIG, where SIG is the signal that killed it. """
         env = args[-1]
         return spawnvpe(mode, file, args[:-1], env)

     __all__.extend(["spawnlp", "spawnlpe"])

 # Supply os.popen()
 def popen(cmd, mode="r", buffering=-1):
     if not isinstance(cmd, str):
         raise TypeError("invalid cmd type (%s, expected string)" % type(cmd))
     if mode not in ("r", "w"):
         raise ValueError("invalid mode %r" % mode)
     if buffering == 0 or buffering is None:
         raise ValueError("popen() does not support unbuffered streams")
     import subprocess, io
     if mode == "r":
         proc = subprocess.Popen(cmd,
                                 shell=True,
                                 stdout=subprocess.PIPE,
                                 bufsize=buffering)
         return _wrap_close(io.TextIOWrapper(proc.stdout), proc)
     else:
         proc = subprocess.Popen(cmd,
                                 shell=True,
                                 stdin=subprocess.PIPE,
                                 bufsize=buffering)
         return _wrap_close(io.TextIOWrapper(proc.stdin), proc)

 # Helper for popen() -- a proxy for a file whose close waits for the process
 class _wrap_close:
     def __init__(self, stream, proc):
         self._stream = stream
         self._proc = proc
     def close(self):
         self._stream.close()
         returncode = self._proc.wait()
         if returncode == 0:
             return None
         if name == 'nt':
             return returncode
         else:
             return returncode << 8  # Shift left to match old behavior
     def __enter__(self):
         return self
     def __exit__(self, *args):
         self.close()
     def __getattr__(self, name):
         return getattr(self._stream, name)
     def __iter__(self):
         return iter(self._stream)

 # Supply os.fdopen()
 def fdopen(fd, *args, **kwargs):
     if not isinstance(fd, int):
         raise TypeError("invalid fd type (%s, expected integer)" % type(fd))
     import io
     return io.open(fd, *args, **kwargs)

 # For testing purposes, make sure the function is available when the C
 # implementation exists.
 def _fspath(path):
     """Return the path representation of a path-like object.

     If str or bytes is passed in, it is returned unchanged. Otherwise the
     os.PathLike interface is used to get the path representation. If the
     path representation is not str or bytes, TypeError is raised. If the
     provided path is not str, bytes, or os.PathLike, TypeError is raised.
     """
     if isinstance(path, (str, bytes)):
         return path

     # Work from the object's type to match method resolution of other magic
     # methods.
     path_type = type(path)
     try:
         path_repr = path_type.__fspath__(path)
     except AttributeError:
         if hasattr(path_type, '__fspath__'):
             raise
         else:
             raise TypeError("expected str, bytes or os.PathLike object, "
                             "not " + path_type.__name__)
     if isinstance(path_repr, (str, bytes)):
         return path_repr
     else:
         raise TypeError("expected {}.__fspath__() to return str or bytes, "
                         "not {}".format(path_type.__name__,
                                         type(path_repr).__name__))

 # If there is no C implementation, make the pure Python version the
 # implementation as transparently as possible.
 if not _exists('fspath'):
     fspath = _fspath
     fspath.__name__ = "fspath"

 class PathLike(abc.ABC):

     """Abstract base class for implementing the file system path protocol."""

     @abc.abstractmethod
     def __fspath__(self):
         """Return the file system path representation of the object."""
         raise NotImplementedError

     @classmethod
     def __subclasshook__(cls, subclass):
         return hasattr(subclass, '__fspath__')

python:os