Lib/os.py

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

This exports:
  - all functions from posix, nt or ce, e.g. unlink, stat, etc.
  - os.path is either posixpath or ntpath
  - os.name is either 'posix', 'nt' or 'ce'.
  - os.curdir is a string representing the current directory ('.' or ':')
  - os.pardir is a string representing the parent directory ('..' or '::')
  - os.sep is the (or a most common) pathname separator ('/' or ':' 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 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

elif 'ce' in _names:
    name = 'ce'
    linesep = '\r\n'
    from ce import *
    try:
        from ce import _exit
        __all__.append('_exit')
    except ImportError:
        pass
    # We can use the standard Windows path.
    import ntpath as path

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

    try:
        from ce 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

    """

    dirs = []
    nondirs = []

    # 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:
        if name == 'nt' and isinstance(top, bytes):
            scandir_it = _dummy_scandir(top)
        else:
            # Note that scandir is global in this module due
            # to earlier import-*.
            scandir_it = scandir(top)
        entries = list(scandir_it)
    except OSError as error:
        if onerror is not None:
            onerror(error)
        return

    for entry in entries:
        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:
                yield from walk(entry.path, topdown, onerror, followlinks)

    # 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:
        # Yield after recursion if going bottom up
        yield top, dirs, nondirs

class _DummyDirEntry:
    """Dummy implementation of DirEntry

    Only used internally by os.walk(bytes). Since os.walk() doesn't need the
    follow_symlinks parameter: don't implement it, always follow symbolic
    links.
    """

    def __init__(self, dir, name):
        self.name = name
        self.path = path.join(dir, name)
        # Mimick FindFirstFile/FindNextFile: we should get file attributes
        # while iterating on a directory
        self._stat = None
        self._lstat = None
        try:
            self.stat(follow_symlinks=False)
        except OSError:
            pass

    def stat(self, *, follow_symlinks=True):
        if follow_symlinks:
            if self._stat is None:
                self._stat = stat(self.path)
            return self._stat
        else:
            if self._lstat is None:
                self._lstat = stat(self.path, follow_symlinks=False)
            return self._lstat

    def is_dir(self):
        if self._lstat is not None and not self.is_symlink():
            # use the cache lstat
            stat = self.stat(follow_symlinks=False)
            return st.S_ISDIR(stat.st_mode)

        stat = self.stat()
        return st.S_ISDIR(stat.st_mode)

    def is_symlink(self):
        stat = self.stat(follow_symlinks=False)
        return st.S_ISLNK(stat.st_mode)

def _dummy_scandir(dir):
    # listdir-based implementation for bytes patches on Windows
    for name in listdir(dir):
        yield _DummyDirEntry(dir, name)

__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
        """
        # 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):
        for key in self._data:
            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()
    if encoding == 'mbcs':
        errors = 'strict'
    else:
        errors = 'surrogateescape'

    def fsencode(filename):
        """
        Encode filename 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).
        """
        if isinstance(filename, bytes):
            return filename
        elif isinstance(filename, str):
            return filename.encode(encoding, errors)
        else:
            raise TypeError("expect bytes or str, not %s" % type(filename).__name__)

    def fsdecode(filename):
        """
        Decode filename 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).
        """
        if isinstance(filename, str):
            return filename
        elif isinstance(filename, bytes):
            return filename.decode(encoding, errors)
        else:
            raise TypeError("expect bytes or str, not %s" % type(filename).__name__)

    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
        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)