path: root/Lib/shutil.py

"""Utility functions for copying and archiving files and directory trees.

XXX The functions here don't copy the resource fork or other metadata on Mac.

"""

import os
import sys
import stat
from os.path import abspath
import fnmatch
import collections
import errno
import tarfile

try:
    import bz2
    del bz2
    _BZ2_SUPPORTED = True
except ImportError:
    _BZ2_SUPPORTED = False

try:
    from pwd import getpwnam
except ImportError:
    getpwnam = None

try:
    from grp import getgrnam
except ImportError:
    getgrnam = None

__all__ = ["copyfileobj", "copyfile", "copymode", "copystat", "copy", "copy2",
           "copytree", "move", "rmtree", "Error", "SpecialFileError",
           "ExecError", "make_archive", "get_archive_formats",
           "register_archive_format", "unregister_archive_format",
           "get_unpack_formats", "register_unpack_format",
           "unregister_unpack_format", "unpack_archive",
           "ignore_patterns", "chown", "which"]
           # disk_usage is added later, if available on the platform

class Error(EnvironmentError):
    pass

class SpecialFileError(EnvironmentError):
    """Raised when trying to do a kind of operation (e.g. copying) which is
    not supported on a special file (e.g. a named pipe)"""

class ExecError(EnvironmentError):
    """Raised when a command could not be executed"""

class ReadError(EnvironmentError):
    """Raised when an archive cannot be read"""

class RegistryError(Exception):
    """Raised when a registery operation with the archiving
    and unpacking registeries fails"""


try:
    WindowsError
except NameError:
    WindowsError = None

def copyfileobj(fsrc, fdst, length=16*1024):
    """copy data from file-like object fsrc to file-like object fdst"""
    while 1:
        buf = fsrc.read(length)
        if not buf:
            break
        fdst.write(buf)

def _samefile(src, dst):
    # Macintosh, Unix.
    if hasattr(os.path, 'samefile'):
        try:
            return os.path.samefile(src, dst)
        except OSError:
            return False

    # All other platforms: check for same pathname.
    return (os.path.normcase(os.path.abspath(src)) ==
            os.path.normcase(os.path.abspath(dst)))

def copyfile(src, dst, *, follow_symlinks=True):
    """Copy data from src to dst.

    If follow_symlinks is not set and src is a symbolic link, a new
    symlink will be created instead of copying the file it points to.

    """
    if _samefile(src, dst):
        raise Error("`%s` and `%s` are the same file" % (src, dst))

    for fn in [src, dst]:
        try:
            st = os.stat(fn)
        except OSError:
            # File most likely does not exist
            pass
        else:
            # XXX What about other special files? (sockets, devices...)
            if stat.S_ISFIFO(st.st_mode):
                raise SpecialFileError("`%s` is a named pipe" % fn)

    if not follow_symlinks and os.path.islink(src):
        os.symlink(os.readlink(src), dst)
    else:
        with open(src, 'rb') as fsrc:
            with open(dst, 'wb') as fdst:
                copyfileobj(fsrc, fdst)
    return dst

def copymode(src, dst, *, follow_symlinks=True):
    """Copy mode bits from src to dst.

    If follow_symlinks is not set, symlinks aren't followed if and only
    if both `src` and `dst` are symlinks.  If `lchmod` isn't available
    (e.g. Linux) this method does nothing.

    """
    if not follow_symlinks and os.path.islink(src) and os.path.islink(dst):
        if hasattr(os, 'lchmod'):
            stat_func, chmod_func = os.lstat, os.lchmod
        else:
            return
    elif hasattr(os, 'chmod'):
        stat_func, chmod_func = os.stat, os.chmod
    else:
        return

    st = stat_func(src)
    chmod_func(dst, stat.S_IMODE(st.st_mode))

if hasattr(os, 'listxattr'):
    def _copyxattr(src, dst, *, follow_symlinks=True):
        """Copy extended filesystem attributes from `src` to `dst`.

        Overwrite existing attributes.

        If `follow_symlinks` is false, symlinks won't be followed.

        """

        for name in os.listxattr(src, follow_symlinks=follow_symlinks):
            try:
                value = os.getxattr(src, name, follow_symlinks=follow_symlinks)
                os.setxattr(dst, name, value, follow_symlinks=follow_symlinks)
            except OSError as e:
                if e.errno not in (errno.EPERM, errno.ENOTSUP, errno.ENODATA):
                    raise
else:
    def _copyxattr(*args, **kwargs):
        pass

def copystat(src, dst, *, follow_symlinks=True):
    """Copy all stat info (mode bits, atime, mtime, flags) from src to dst.

    If the optional flag `follow_symlinks` is not set, symlinks aren't followed if and
    only if both `src` and `dst` are symlinks.

    """
    def _nop(*args, ns=None, follow_symlinks=None):
        pass

    # follow symlinks (aka don't not follow symlinks)
    follow = follow_symlinks or not (os.path.islink(src) and os.path.islink(dst))
    if follow:
        # use the real function if it exists
        def lookup(name):
            return getattr(os, name, _nop)
    else:
        # use the real function only if it exists
        # *and* it supports follow_symlinks
        def lookup(name):
            fn = getattr(os, name, _nop)
            if fn in os.supports_follow_symlinks:
                return fn
            return _nop

    st = lookup("stat")(src, follow_symlinks=follow)
    mode = stat.S_IMODE(st.st_mode)
    lookup("utime")(dst, ns=(st.st_atime_ns, st.st_mtime_ns),
        follow_symlinks=follow)
    try:
        lookup("chmod")(dst, mode, follow_symlinks=follow)
    except NotImplementedError:
        # if we got a NotImplementedError, it's because
        #   * follow_symlinks=False,
        #   * lchown() is unavailable, and
        #   * either
        #       * fchownat() is unvailable or
        #       * fchownat() doesn't implement AT_SYMLINK_NOFOLLOW.
        #         (it returned ENOSUP.)
        # therefore we're out of options--we simply cannot chown the
        # symlink.  give up, suppress the error.
        # (which is what shutil always did in this circumstance.)
        pass
    if hasattr(st, 'st_flags'):
        try:
            lookup("chflags")(dst, st.st_flags, follow_symlinks=follow)
        except OSError as why:
            for err in 'EOPNOTSUPP', 'ENOTSUP':
                if hasattr(errno, err) and why.errno == getattr(errno, err):
                    break
            else:
                raise
    _copyxattr(src, dst, follow_symlinks=follow)

def copy(src, dst, *, follow_symlinks=True):
    """Copy data and mode bits ("cp src dst"). Return the file's destination.

    The destination may be a directory.

    If follow_symlinks is false, symlinks won't be followed. This
    resembles GNU's "cp -P src dst".

    """
    if os.path.isdir(dst):
        dst = os.path.join(dst, os.path.basename(src))
    copyfile(src, dst, follow_symlinks=follow_symlinks)
    copymode(src, dst, follow_symlinks=follow_symlinks)
    return dst

def copy2(src, dst, *, follow_symlinks=True):
    """Copy data and all stat info ("cp -p src dst"). Return the file's
    destination."

    The destination may be a directory.

    If follow_symlinks is false, symlinks won't be followed. This
    resembles GNU's "cp -P src dst".

    """
    if os.path.isdir(dst):
        dst = os.path.join(dst, os.path.basename(src))
    copyfile(src, dst, follow_symlinks=follow_symlinks)
    copystat(src, dst, follow_symlinks=follow_symlinks)
    return dst

def ignore_patterns(*patterns):
    """Function that can be used as copytree() ignore parameter.

    Patterns is a sequence of glob-style patterns
    that are used to exclude files"""
    def _ignore_patterns(path, names):
        ignored_names = []
        for pattern in patterns:
            ignored_names.extend(fnmatch.filter(names, pattern))
        return set(ignored_names)
    return _ignore_patterns

def copytree(src, dst, symlinks=False, ignore=None, copy_function=copy2,
             ignore_dangling_symlinks=False):
    """Recursively copy a directory tree.

    The destination directory must not already exist.
    If exception(s) occur, an Error is raised with a list of reasons.

    If the optional symlinks flag is true, symbolic links in the
    source tree result in symbolic links in the destination tree; if
    it is false, the contents of the files pointed to by symbolic
    links are copied. If the file pointed by the symlink doesn't
    exist, an exception will be added in the list of errors raised in
    an Error exception at the end of the copy process.

    You can set the optional ignore_dangling_symlinks flag to true if you
    want to silence this exception. Notice that this has no effect on
    platforms that don't support os.symlink.

    The optional ignore argument is a callable. If given, it
    is called with the `src` parameter, which is the directory
    being visited by copytree(), and `names` which is the list of
    `src` contents, as returned by os.listdir():

        callable(src, names) -> ignored_names

    Since copytree() is called recursively, the callable will be
    called once for each directory that is copied. It returns a
    list of names relative to the `src` directory that should
    not be copied.

    The optional copy_function argument is a callable that will be used
    to copy each file. It will be called with the source path and the
    destination path as arguments. By default, copy2() is used, but any
    function that supports the same signature (like copy()) can be used.

    """
    names = os.listdir(src)
    if ignore is not None:
        ignored_names = ignore(src, names)
    else:
        ignored_names = set()

    os.makedirs(dst)
    errors = []
    for name in names:
        if name in ignored_names:
            continue
        srcname = os.path.join(src, name)
        dstname = os.path.join(dst, name)
        try:
            if os.path.islink(srcname):
                linkto = os.readlink(srcname)
                if symlinks:
                    # We can't just leave it to `copy_function` because legacy
                    # code with a custom `copy_function` may rely on copytree
                    # doing the right thing.
                    os.symlink(linkto, dstname)
                    copystat(srcname, dstname, follow_symlinks=not symlinks)
                else:
                    # ignore dangling symlink if the flag is on
                    if not os.path.exists(linkto) and ignore_dangling_symlinks:
                        continue
                    # otherwise let the copy occurs. copy2 will raise an error
                    copy_function(srcname, dstname)
            elif os.path.isdir(srcname):
                copytree(srcname, dstname, symlinks, ignore, copy_function)
            else:
                # Will raise a SpecialFileError for unsupported file types
                copy_function(srcname, dstname)
        # catch the Error from the recursive copytree so that we can
        # continue with other files
        except Error as err:
            errors.extend(err.args[0])
        except EnvironmentError as why:
            errors.append((srcname, dstname, str(why)))
    try:
        copystat(src, dst)
    except OSError as why:
        if WindowsError is not None and isinstance(why, WindowsError):
            # Copying file access times may fail on Windows
            pass
        else:
            errors.append((src, dst, str(why)))
    if errors:
        raise Error(errors)
    return dst

# version vulnerable to race conditions
def _rmtree_unsafe(path, onerror):
    try:
        if os.path.islink(path):
            # symlinks to directories are forbidden, see bug #1669
            raise OSError("Cannot call rmtree on a symbolic link")
    except OSError:
        onerror(os.path.islink, path, sys.exc_info())
        # can't continue even if onerror hook returns
        return
    names = []
    try:
        names = os.listdir(path)
    except os.error:
        onerror(os.listdir, path, sys.exc_info())
    for name in names:
        fullname = os.path.join(path, name)
        try:
            mode = os.lstat(fullname).st_mode
        except os.error:
            mode = 0
        if stat.S_ISDIR(mode):
            _rmtree_unsafe(fullname, onerror)
        else:
            try:
                os.unlink(fullname)
            except os.error:
                onerror(os.unlink, fullname, sys.exc_info())
    try:
        os.rmdir(path)
    except os.error:
        onerror(os.rmdir, path, sys.exc_info())

# Version using fd-based APIs to protect against races
def _rmtree_safe_fd(topfd, path, onerror):
    names = []
    try:
        names = os.listdir(topfd)
    except OSError as err:
        err.filename = path
        onerror(os.listdir, path, sys.exc_info())
    for name in names:
        fullname = os.path.join(path, name)
        try:
            orig_st = os.stat(name, dir_fd=topfd, follow_symlinks=False)
            mode = orig_st.st_mode
        except OSError:
            mode = 0
        if stat.S_ISDIR(mode):
            try:
                dirfd = os.open(name, os.O_RDONLY, dir_fd=topfd)
            except OSError:
                onerror(os.open, fullname, sys.exc_info())
            else:
                try:
                    if os.path.samestat(orig_st, os.fstat(dirfd)):
                        _rmtree_safe_fd(dirfd, fullname, onerror)
                        try:
                            os.rmdir(name, dir_fd=topfd)
                        except OSError:
                            onerror(os.rmdir, fullname, sys.exc_info())
                    else:
                        try:
                            # This can only happen if someone replaces
                            # a directory with a symlink after the call to
                            # stat.S_ISDIR above.
                            raise OSError("Cannot call rmtree on a symbolic "
                                          "link")
                        except OSError:
                            onerror(os.path.islink, fullname, sys.exc_info())
                finally:
                    os.close(dirfd)
        else:
            try:
                os.unlink(name, dir_fd=topfd)
            except OSError:
                onerror(os.unlink, fullname, sys.exc_info())

_use_fd_functions = ({os.open, os.stat, os.unlink, os.rmdir} <=
                     os.supports_dir_fd and
                     os.listdir in os.supports_fd and
                     os.stat in os.supports_follow_symlinks)

def rmtree(path, ignore_errors=False, onerror=None):
    """Recursively delete a directory tree.

    If ignore_errors is set, errors are ignored; otherwise, if onerror
    is set, it is called to handle the error with arguments (func,
    path, exc_info) where func is platform and implementation dependent;
    path is the argument to that function that caused it to fail; and
    exc_info is a tuple returned by sys.exc_info().  If ignore_errors
    is false and onerror is None, an exception is raised.

    """
    if ignore_errors:
        def onerror(*args):
            pass
    elif onerror is None:
        def onerror(*args):
            raise
    if _use_fd_functions:
        # While the unsafe rmtree works fine on bytes, the fd based does not.
        if isinstance(path, bytes):
            path = os.fsdecode(path)
        # Note: To guard against symlink races, we use the standard
        # lstat()/open()/fstat() trick.
        try:
            orig_st = os.lstat(path)
        except Exception:
            onerror(os.lstat, path, sys.exc_info())
            return
        try:
            fd = os.open(path, os.O_RDONLY)
        except Exception:
            onerror(os.lstat, path, sys.exc_info())
            return
        try:
            if os.path.samestat(orig_st, os.fstat(fd)):
                _rmtree_safe_fd(fd, path, onerror)
                try:
                    os.rmdir(path)
                except os.error:
                    onerror(os.rmdir, path, sys.exc_info())
            else:
                try:
                    # symlinks to directories are forbidden, see bug #1669
                    raise OSError("Cannot call rmtree on a symbolic link")
                except OSError:
                    onerror(os.path.islink, path, sys.exc_info())
        finally:
            os.close(fd)
    else:
        return _rmtree_unsafe(path, onerror)

# Allow introspection of whether or not the hardening against symlink
# attacks is supported on the current platform
rmtree.avoids_symlink_attacks = _use_fd_functions

def _basename(path):
    # A basename() variant which first strips the trailing slash, if present.
    # Thus we always get the last component of the path, even for directories.
    return os.path.basename(path.rstrip(os.path.sep))

def move(src, dst):
    """Recursively move a file or directory to another location. This is
    similar to the Unix "mv" command. Return the file or directory's
    destination.

    If the destination is a directory or a symlink to a directory, the source
    is moved inside the directory. The destination path must not already
    exist.

    If the destination already exists but is not a directory, it may be
    overwritten depending on os.rename() semantics.

    If the destination is on our current filesystem, then rename() is used.
    Otherwise, src is copied to the destination and then removed. Symlinks are
    recreated under the new name if os.rename() fails because of cross
    filesystem renames.

    A lot more could be done here...  A look at a mv.c shows a lot of
    the issues this implementation glosses over.

    """
    real_dst = dst
    if os.path.isdir(dst):
        if _samefile(src, dst):
            # We might be on a case insensitive filesystem,
            # perform the rename anyway.
            os.rename(src, dst)
            return

        real_dst = os.path.join(dst, _basename(src))
        if os.path.exists(real_dst):
            raise Error("Destination path '%s' already exists" % real_dst)
    try:
        os.rename(src, real_dst)
    except OSError:
        if os.path.islink(src):
            linkto = os.readlink(src)
            os.symlink(linkto, real_dst)
            os.unlink(src)
        elif os.path.isdir(src):
            if _destinsrc(src, dst):
                raise Error("Cannot move a directory '%s' into itself '%s'." % (src, dst))
            copytree(src, real_dst, symlinks=True)
            rmtree(src)
        else:
            copy2(src, real_dst)
            os.unlink(src)
    return real_dst

def _destinsrc(src, dst):
    src = abspath(src)
    dst = abspath(dst)
    if not src.endswith(os.path.sep):
        src += os.path.sep
    if not dst.endswith(os.path.sep):
        dst += os.path.sep
    return dst.startswith(src)

def _get_gid(name):
    """Returns a gid, given a group name."""
    if getgrnam is None or name is None:
        return None
    try:
        result = getgrnam(name)
    except KeyError:
        result = None
    if result is not None:
        return result[2]
    return None

def _get_uid(name):
    """Returns an uid, given a user name."""
    if getpwnam is None or name is None:
        return None
    try:
        result = getpwnam(name)
    except KeyError:
        result = None
    if result is not None:
        return result[2]
    return None

def _make_tarball(base_name, base_dir, compress="gzip", verbose=0, dry_run=0,
                  owner=None, group=None, logger=None):
    """Create a (possibly compressed) tar file from all the files under
    'base_dir'.

    'compress' must be "gzip" (the default), "bzip2", or None.

    'owner' and 'group' can be used to define an owner and a group for the
    archive that is being built. If not provided, the current owner and group
    will be used.

    The output tar file will be named 'base_name' +  ".tar", possibly plus
    the appropriate compression extension (".gz", or ".bz2").

    Returns the output filename.
    """
    tar_compression = {'gzip': 'gz', None: ''}
    compress_ext = {'gzip': '.gz'}

    if _BZ2_SUPPORTED:
        tar_compression['bzip2'] = 'bz2'
        compress_ext['bzip2'] = '.bz2'

    # flags for compression program, each element of list will be an argument
    if compress is not None and compress not in compress_ext:
        raise ValueError("bad value for 'compress', or compression format not "
                         "supported : {0}".format(compress))

    archive_name = base_name + '.tar' + compress_ext.get(compress, '')
    archive_dir = os.path.dirname(archive_name)

    if not os.path.exists(archive_dir):
        if logger is not None:
            logger.info("creating %s", archive_dir)
        if not dry_run:
            os.makedirs(archive_dir)

    # creating the tarball
    if logger is not None:
        logger.info('Creating tar archive')

    uid = _get_uid(owner)
    gid = _get_gid(group)

    def _set_uid_gid(tarinfo):
        if gid is not None:
            tarinfo.gid = gid
            tarinfo.gname = group
        if uid is not None:
            tarinfo.uid = uid
            tarinfo.uname = owner
        return tarinfo

    if not dry_run:
        tar = tarfile.open(archive_name, 'w|%s' % tar_compression[compress])
        try:
            tar.add(base_dir, filter=_set_uid_gid)
        finally:
            tar.close()

    return archive_name

def _call_external_zip(base_dir, zip_filename, verbose=False, dry_run=False):
    # XXX see if we want to keep an external call here
    if verbose:
        zipoptions = "-r"
    else:
        zipoptions = "-rq"
    from distutils.errors import DistutilsExecError
    from distutils.spawn import spawn
    try:
        spawn(["zip", zipoptions, zip_filename, base_dir], dry_run=dry_run)
    except DistutilsExecError:
        # XXX really should distinguish between "couldn't find
        # external 'zip' command" and "zip failed".
        raise ExecError("unable to create zip file '%s': "
            "could neither import the 'zipfile' module nor "
            "find a standalone zip utility") % zip_filename

def _make_zipfile(base_name, base_dir, verbose=0, dry_run=0, logger=None):
    """Create a zip file from all the files under 'base_dir'.

    The output zip file will be named 'base_name' + ".zip".  Uses either the
    "zipfile" Python module (if available) or the InfoZIP "zip" utility
    (if installed and found on the default search path).  If neither tool is
    available, raises ExecError.  Returns the name of the output zip
    file.
    """
    zip_filename = base_name + ".zip"
    archive_dir = os.path.dirname(base_name)

    if not os.path.exists(archive_dir):
        if logger is not None:
            logger.info("creating %s", archive_dir)
        if not dry_run:
            os.makedirs(archive_dir)

    # If zipfile module is not available, try spawning an external 'zip'
    # command.
    try:
        import zipfile
    except ImportError:
        zipfile = None

    if zipfile is None:
        _call_external_zip(base_dir, zip_filename, verbose, dry_run)
    else:
        if logger is not None:
            logger.info("creating '%s' and adding '%s' to it",
                        zip_filename, base_dir)

        if not dry_run:
            zip = zipfile.ZipFile(zip_filename, "w",
                                  compression=zipfile.ZIP_DEFLATED)

            for dirpath, dirnames, filenames in os.walk(base_dir):
                for name in filenames:
                    path = os.path.normpath(os.path.join(dirpath, name))
                    if os.path.isfile(path):
                        zip.write(path, path)
                        if logger is not None:
                            logger.info("adding '%s'", path)
            zip.close()

    return zip_filename

_ARCHIVE_FORMATS = {
    'gztar': (_make_tarball, [('compress', 'gzip')], "gzip'ed tar-file"),
    'tar':   (_make_tarball, [('compress', None)], "uncompressed tar file"),
    'zip':   (_make_zipfile, [], "ZIP file")
    }

if _BZ2_SUPPORTED:
    _ARCHIVE_FORMATS['bztar'] = (_make_tarball, [('compress', 'bzip2')],
                                "bzip2'ed tar-file")

def get_archive_formats():
    """Returns a list of supported formats for archiving and unarchiving.

    Each element of the returned sequence is a tuple (name, description)
    """
    formats = [(name, registry[2]) for name, registry in
               _ARCHIVE_FORMATS.items()]
    formats.sort()
    return formats

def register_archive_format(name, function, extra_args=None, description=''):
    """Registers an archive format.

    name is the name of the format. function is the callable that will be
    used to create archives. If provided, extra_args is a sequence of
    (name, value) tuples that will be passed as arguments to the callable.
    description can be provided to describe the format, and will be returned
    by the get_archive_formats() function.
    """
    if extra_args is None:
        extra_args = []
    if not callable(function):
        raise TypeError('The %s object is not callable' % function)
    if not isinstance(extra_args, (tuple, list)):
        raise TypeError('extra_args needs to be a sequence')
    for element in extra_args:
        if not isinstance(element, (tuple, list)) or len(element) !=2:
            raise TypeError('extra_args elements are : (arg_name, value)')

    _ARCHIVE_FORMATS[name] = (function, extra_args, description)

def unregister_archive_format(name):
    del _ARCHIVE_FORMATS[name]

def make_archive(base_name, format, root_dir=None, base_dir=None, verbose=0,
                 dry_run=0, owner=None, group=None, logger=None):
    """Create an archive file (eg. zip or tar).

    'base_name' is the name of the file to create, minus any format-specific
    extension; 'format' is the archive format: one of "zip", "tar", "bztar"
    or "gztar".

    'root_dir' is a directory that will be the root directory of the
    archive; ie. we typically chdir into 'root_dir' before creating the
    archive.  'base_dir' is the directory where we start archiving from;
    ie. 'base_dir' will be the common prefix of all files and
    directories in the archive.  'root_dir' and 'base_dir' both default
    to the current directory.  Returns the name of the archive file.

    'owner' and 'group' are used when creating a tar archive. By default,
    uses the current owner and group.
    """
    save_cwd = os.getcwd()
    if root_dir is not None:
        if logger is not None:
            logger.debug("changing into '%s'", root_dir)
        base_name = os.path.abspath(base_name)
        if not dry_run:
            os.chdir(root_dir)

    if base_dir is None:
        base_dir = os.curdir

    kwargs = {'dry_run': dry_run, 'logger': logger}

    try:
        format_info = _ARCHIVE_FORMATS[format]
    except KeyError:
        raise ValueError("unknown archive format '%s'" % format)

    func = format_info[0]
    for arg, val in format_info[1]:
        kwargs[arg] = val

    if format != 'zip':
        kwargs['owner'] = owner
        kwargs['group'] = group

    try:
        filename = func(base_name, base_dir, **kwargs)
    finally:
        if root_dir is not None:
            if logger is not None:
                logger.debug("changing back to '%s'", save_cwd)
            os.chdir(save_cwd)

    return filename


def get_unpack_formats():
    """Returns a list of supported formats for unpacking.

    Each element of the returned sequence is a tuple
    (name, extensions, description)
    """
    formats = [(name, info[0], info[3]) for name, info in
               _UNPACK_FORMATS.items()]
    formats.sort()
    return formats

def _check_unpack_options(extensions, function, extra_args):
    """Checks what gets registered as an unpacker."""
    # first make sure no other unpacker is registered for this extension
    existing_extensions = {}
    for name, info in _UNPACK_FORMATS.items():
        for ext in info[0]:
            existing_extensions[ext] = name

    for extension in extensions:
        if extension in existing_extensions:
            msg = '%s is already registered for "%s"'
            raise RegistryError(msg % (extension,
                                       existing_extensions[extension]))

    if not callable(function):
        raise TypeError('The registered function must be a callable')


def register_unpack_format(name, extensions, function, extra_args=None,
                           description=''):
    """Registers an unpack format.

    `name` is the name of the format. `extensions` is a list of extensions
    corresponding to the format.

    `function` is the callable that will be
    used to unpack archives. The callable will receive archives to unpack.
    If it's unable to handle an archive, it needs to raise a ReadError
    exception.

    If provided, `extra_args` is a sequence of
    (name, value) tuples that will be passed as arguments to the callable.
    description can be provided to describe the format, and will be returned
    by the get_unpack_formats() function.
    """
    if extra_args is None:
        extra_args = []
    _check_unpack_options(extensions, function, extra_args)
    _UNPACK_FORMATS[name] = extensions, function, extra_args, description

def unregister_unpack_format(name):
    """Removes the pack format from the registery."""
    del _UNPACK_FORMATS[name]

def _ensure_directory(path):
    """Ensure that the parent directory of `path` exists"""
    dirname = os.path.dirname(path)
    if not os.path.isdir(dirname):
        os.makedirs(dirname)

def _unpack_zipfile(filename, extract_dir):
    """Unpack zip `filename` to `extract_dir`
    """
    try:
        import zipfile
    except ImportError:
        raise ReadError('zlib not supported, cannot unpack this archive.')

    if not zipfile.is_zipfile(filename):
        raise ReadError("%s is not a zip file" % filename)

    zip = zipfile.ZipFile(filename)
    try:
        for info in zip.infolist():
            name = info.filename

            # don't extract absolute paths or ones with .. in them
            if name.startswith('/') or '..' in name:
                continue

            target = os.path.join(extract_dir, *name.split('/'))
            if not target:
                continue

            _ensure_directory(target)
            if not name.endswith('/'):
                # file
                data = zip.read(info.filename)
                f = open(target, 'wb')
                try:
                    f.write(data)
                finally:
                    f.close()
                    del data
    finally:
        zip.close()

def _unpack_tarfile(filename, extract_dir):
    """Unpack tar/tar.gz/tar.bz2 `filename` to `extract_dir`
    """
    try:
        tarobj = tarfile.open(filename)
    except tarfile.TarError:
        raise ReadError(
            "%s is not a compressed or uncompressed tar file" % filename)
    try:
        tarobj.extractall(extract_dir)
    finally:
        tarobj.close()

_UNPACK_FORMATS = {
    'gztar': (['.tar.gz', '.tgz'], _unpack_tarfile, [], "gzip'ed tar-file"),
    'tar':   (['.tar'], _unpack_tarfile, [], "uncompressed tar file"),
    'zip':   (['.zip'], _unpack_zipfile, [], "ZIP file")
    }

if _BZ2_SUPPORTED:
    _UNPACK_FORMATS['bztar'] = (['.bz2'], _unpack_tarfile, [],
                                "bzip2'ed tar-file")

def _find_unpack_format(filename):
    for name, info in _UNPACK_FORMATS.items():
        for extension in info[0]:
            if filename.endswith(extension):
                return name
    return None

def unpack_archive(filename, extract_dir=None, format=None):
    """Unpack an archive.

    `filename` is the name of the archive.

    `extract_dir` is the name of the target directory, where the archive
    is unpacked. If not provided, the current working directory is used.

    `format` is the archive format: one of "zip", "tar", or "gztar". Or any
    other registered format. If not provided, unpack_archive will use the
    filename extension and see if an unpacker was registered for that
    extension.

    In case none is found, a ValueError is raised.
    """
    if extract_dir is None:
        extract_dir = os.getcwd()

    if format is not None:
        try:
            format_info = _UNPACK_FORMATS[format]
        except KeyError:
            raise ValueError("Unknown unpack format '{0}'".format(format))

        func = format_info[1]
        func(filename, extract_dir, **dict(format_info[2]))
    else:
        # we need to look at the registered unpackers supported extensions
        format = _find_unpack_format(filename)
        if format is None:
            raise ReadError("Unknown archive format '{0}'".format(filename))

        func = _UNPACK_FORMATS[format][1]
        kwargs = dict(_UNPACK_FORMATS[format][2])
        func(filename, extract_dir, **kwargs)


if hasattr(os, 'statvfs'):

    __all__.append('disk_usage')
    _ntuple_diskusage = collections.namedtuple('usage', 'total used free')

    def disk_usage(path):
        """Return disk usage statistics about the given path.

        Returned value is a named tuple with attributes 'total', 'used' and
        'free', which are the amount of total, used and free space, in bytes.
        """
        st = os.statvfs(path)
        free = st.f_bavail * st.f_frsize
        total = st.f_blocks * st.f_frsize
        used = (st.f_blocks - st.f_bfree) * st.f_frsize
        return _ntuple_diskusage(total, used, free)

elif os.name == 'nt':

    import nt
    __all__.append('disk_usage')
    _ntuple_diskusage = collections.namedtuple('usage', 'total used free')

    def disk_usage(path):
        """Return disk usage statistics about the given path.

        Returned valus is a named tuple with attributes 'total', 'used' and
        'free', which are the amount of total, used and free space, in bytes.
        """
        total, free = nt._getdiskusage(path)
        used = total - free
        return _ntuple_diskusage(total, used, free)


def chown(path, user=None, group=None):
    """Change owner user and group of the given path.

    user and group can be the uid/gid or the user/group names, and in that case,
    they are converted to their respective uid/gid.
    """

    if user is None and group is None:
        raise ValueError("user and/or group must be set")

    _user = user
    _group = group

    # -1 means don't change it
    if user is None:
        _user = -1
    # user can either be an int (the uid) or a string (the system username)
    elif isinstance(user, str):
        _user = _get_uid(user)
        if _user is None:
            raise LookupError("no such user: {!r}".format(user))

    if group is None:
        _group = -1
    elif not isinstance(group, int):
        _group = _get_gid(group)
        if _group is None:
            raise LookupError("no such group: {!r}".format(group))

    os.chown(path, _user, _group)

def get_terminal_size(fallback=(80, 24)):
    """Get the size of the terminal window.

    For each of the two dimensions, the environment variable, COLUMNS
    and LINES respectively, is checked. If the variable is defined and
    the value is a positive integer, it is used.

    When COLUMNS or LINES is not defined, which is the common case,
    the terminal connected to sys.__stdout__ is queried
    by invoking os.get_terminal_size.

    If the terminal size cannot be successfully queried, either because
    the system doesn't support querying, or because we are not
    connected to a terminal, the value given in fallback parameter
    is used. Fallback defaults to (80, 24) which is the default
    size used by many terminal emulators.

    The value returned is a named tuple of type os.terminal_size.
    """
    # columns, lines are the working values
    try:
        columns = int(os.environ['COLUMNS'])
    except (KeyError, ValueError):
        columns = 0

    try:
        lines = int(os.environ['LINES'])
    except (KeyError, ValueError):
        lines = 0

    # only query if necessary
    if columns <= 0 or lines <= 0:
        try:
            size = os.get_terminal_size(sys.__stdout__.fileno())
        except (NameError, OSError):
            size = os.terminal_size(fallback)
        if columns <= 0:
            columns = size.columns
        if lines <= 0:
            lines = size.lines

    return os.terminal_size((columns, lines))

def which(cmd, mode=os.F_OK | os.X_OK, path=None):
    """Given a command, mode, and a PATH string, return the path which
    conforms to the given mode on the PATH, or None if there is no such
    file.

    `mode` defaults to os.F_OK | os.X_OK. `path` defaults to the result
    of os.environ.get("PATH"), or can be overridden with a custom search
    path.

    """
    # Check that a given file can be accessed with the correct mode.
    # Additionally check that `file` is not a directory, as on Windows
    # directories pass the os.access check.
    def _access_check(fn, mode):
        return (os.path.exists(fn) and os.access(fn, mode)
                and not os.path.isdir(fn))

    # Short circuit. If we're given a full path which matches the mode
    # and it exists, we're done here.
    if _access_check(cmd, mode):
        return cmd

    path = (path or os.environ.get("PATH", os.defpath)).split(os.pathsep)

    if sys.platform == "win32":
        # The current directory takes precedence on Windows.
        if not os.curdir in path:
            path.insert(0, os.curdir)

        # PATHEXT is necessary to check on Windows.
        pathext = os.environ.get("PATHEXT", "").split(os.pathsep)
        # See if the given file matches any of the expected path extensions.
        # This will allow us to short circuit when given "python.exe".
        matches = [cmd for ext in pathext if cmd.lower().endswith(ext.lower())]
        # If it does match, only test that one, otherwise we have to try
        # others.
        files = [cmd] if matches else [cmd + ext.lower() for ext in pathext]
    else:
        # On other platforms you don't have things like PATHEXT to tell you
        # what file suffixes are executable, so just pass on cmd as-is.
        files = [cmd]

    seen = set()
    for dir in path:
        dir = os.path.normcase(dir)
        if not dir in seen:
            seen.add(dir)
            for thefile in files:
                name = os.path.join(dir, thefile)
                if _access_check(name, mode):
                    return name
    return None