python-peps/pep-0355.txt

PEP: 355
Title: Path - Object oriented filesystem paths
Version: $Revision$
Last-Modified: $Date$
Author: Björn Lindqvist <bjourne@gmail.com>
Status: Rejected
Type: Standards Track
Content-Type: text/plain
Created: 24-Jan-2006
Python-Version: 2.5
Post-History:


Rejection Notice

    This PEP has been rejected (in this form).  The proposed path class
    is the ultimate kitchen sink; but the notion that it's better to
    implement *all* functionality that uses a path as a method on a single
    class is an anti-pattern.  (E.g.why not open()?  Or execfile()?)
    Subclassing from str is a particularly bad idea; many string
    operations make no sense when applied to a path.  This PEP has
    lingered, and while the discussion flares up from time to time,
    it's time to put this PEP out of its misery.  A less far-fetched
    proposal might be more palatable.


Abstract

    This PEP describes a new class, Path, to be added to the os
    module, for handling paths in an object oriented fashion.  The
    "weak" deprecation of various related functions is also discussed
    and recommended.


Background

    The ideas expressed in this PEP are not recent, but have been
    debated in the Python community for many years.  Many have felt
    that the API for manipulating file paths as offered in the os.path
    module is inadequate.  The first proposal for a Path object was
    raised by Just van Rossum on python-dev in 2001 [2].  In 2003,
    Jason Orendorff released version 1.0 of the "path module" which
    was the first public implementation that used objects to represent
    paths [3].

    The path module quickly became very popular and numerous attempts
    were made to get the path module included in the Python standard
    library; [4], [5], [6], [7].

    This PEP summarizes the ideas and suggestions people have
    expressed about the path module and proposes that a modified
    version should be included in the standard library.


Motivation

    Dealing with filesystem paths is a common task in any programming
    language, and very common in a high-level language like Python.
    Good support for this task is needed, because:

    - Almost every program uses paths to access files.  It makes sense
      that a task, that is so often performed, should be as intuitive
      and as easy to perform as possible.

    - It makes Python an even better replacement language for
      over-complicated shell scripts.

    Currently, Python has a large number of different functions
    scattered over half a dozen modules for handling paths.  This
    makes it hard for newbies and experienced developers to choose
    the right method.

    The Path class provides the following enhancements over the
    current common practice:

    - One "unified" object provides all functionality from previous
      functions.

    - Subclassability - the Path object can be extended to support
      paths other than filesystem paths.  The programmer does not need
      to learn a new API, but can reuse his or her knowledge of Path
      to deal with the extended class.

    - With all related functionality in one place, the right approach
      is easier to learn as one does not have to hunt through many
      different modules for the right functions.

    - Python is an object oriented language.  Just like files,
      datetimes and sockets are objects so are paths, they are not
      merely strings to be passed to functions.  Path objects is
      inherently a pythonic idea.

    - Path takes advantage of properties.  Properties make for more
      readable code.

      if imgpath.ext == 'jpg':
          jpegdecode(imgpath)

      Is better than:

      if os.path.splitexit(imgpath)[1] == 'jpg':
          jpegdecode(imgpath)


Rationale

    The following points summarize the design:

    - Path extends from string, therefore all code which expects
      string pathnames need not be modified and no existing code will
      break.

    - A Path object can be created either by using the classmethod
      Path.cwd, by instantiating the class with a string representing
      a path or by using the default constructor which is equivalent
      to Path(".").

    - Path provides common pathname manipulation, pattern expansion,
      pattern matching and other high-level file operations including
      copying.  Basically Path provides everything path-related except
      the manipulation of file contents, for which file objects are
      better suited.

    - Platform incompatibilities are dealt with by not instantiating
      system specific methods.


Specification

    This class defines the following public interface (docstrings have
    been extracted from the reference implementation, and shortened
    for brevity; see the reference implementation for more detail):

    class Path(str):

        # Special Python methods:
        def __new__(cls, *args) => Path
            """
            Creates a new path object concatenating the *args.  *args
            may only contain Path objects or strings.  If *args is
            empty, Path(os.curdir) is created.
            """
        def __repr__(self): ...
        def __add__(self, more): ...
        def __radd__(self, other): ...

        # Alternative constructor.
        def cwd(cls): ...

        # Operations on path strings:
        def abspath(self) => Path
            """Returns the absolute path of self as a new Path object."""
        def normcase(self): ...
        def normpath(self): ...
        def realpath(self): ...
        def expanduser(self): ...
        def expandvars(self): ...
        def basename(self): ...
        def expand(self): ...
        def splitpath(self) => (Path, str)
            """p.splitpath() -> Return (p.parent, p.name)."""
        def stripext(self) => Path
            """p.stripext() -> Remove one file extension from the path."""
        def splitunc(self): ...  [1]
        def splitall(self): ...
        def relpath(self): ...
        def relpathto(self, dest): ...

        # Properties about the path:
        parent => Path
            """This Path's parent directory as a new path object."""
        name => str
            """The name of this file or directory without the full path."""
        ext => str
            """
            The file extension or an empty string if Path refers to a
            file without an extension or a directory.
            """
        drive => str
            """
            The drive specifier.  Always empty on systems that don't
            use drive specifiers.
            """
        namebase => str
            """
            The same as path.name, but with one file extension
            stripped off.
            """
        uncshare[1]

        # Operations that return lists of paths:
        def listdir(self, pattern = None): ...
        def dirs(self, pattern = None): ...
        def files(self, pattern = None): ...
        def walk(self, pattern = None): ...
        def walkdirs(self, pattern = None): ...
        def walkfiles(self, pattern = None): ...
        def match(self, pattern) => bool
            """Returns True if self.name matches the given pattern."""

        def matchcase(self, pattern) => bool
            """
            Like match() but is guaranteed to be case sensitive even
            on platforms with case insensitive filesystems.
            """
        def glob(self, pattern):

        # Methods for retrieving information about the filesystem
        # path:
        def exists(self): ...
        def isabs(self): ...
        def isdir(self): ...
        def isfile(self): ...
        def islink(self): ...
        def ismount(self): ...
        def samefile(self, other): ...  [1]
        def atime(self): ...
            """Last access time of the file."""
        def mtime(self): ...
            """Last-modified time of the file."""
        def ctime(self): ...
            """
            Return the system's ctime which, on some systems (like
            Unix) is the time of the last change, and, on others (like
            Windows), is the creation time for path.
            """
        def size(self): ...
        def access(self, mode): ...  [1]
        def stat(self): ...
        def lstat(self): ...
        def statvfs(self): ...  [1]
        def pathconf(self, name): ...  [1]

        # Methods for manipulating information about the filesystem
        # path.
        def utime(self, times) => None
        def chmod(self, mode) => None
        def chown(self, uid, gid) => None [1]
        def rename(self, new) => None
        def renames(self, new) => None

        # Create/delete operations on directories
        def mkdir(self, mode = 0777): ...
        def makedirs(self, mode = 0777): ...
        def rmdir(self): ...
        def removedirs(self): ...

        # Modifying operations on files
        def touch(self): ...
        def remove(self): ...
        def unlink(self): ...

        # Modifying operations on links
        def link(self, newpath): ...
        def symlink(self, newlink): ...
        def readlink(self): ...
        def readlinkabs(self): ...

        # High-level functions from shutil
        def copyfile(self, dst): ...
        def copymode(self, dst): ...
        def copystat(self, dst): ...
        def copy(self, dst): ...
        def copy2(self, dst): ...
        def copytree(self, dst, symlinks = True): ...
        def move(self, dst): ...
        def rmtree(self, ignore_errors = False, onerror = None): ...

        # Special stuff from os
        def chroot(self): ...  [1]
        def startfile(self): ...  [1]


Replacing older functions with the Path class

    In this section, "a ==> b" means that b can be used as a
    replacement for a.

    In the following examples, we assume that the Path class is
    imported with "from path import Path".

    1. Replacing os.path.join
    --------------------------

    os.path.join(os.getcwd(), "foobar")
    ==>
    Path(Path.cwd(), "foobar")

    os.path.join("foo", "bar", "baz")
    ==>
    Path("foo", "bar", "baz")


    2. Replacing os.path.splitext
    ------------------------------

    fname = "Python2.4.tar.gz"
    os.path.splitext(fname)[1]
    ==>
    fname = Path("Python2.4.tar.gz")
    fname.ext

    Or if you want both parts:

    fname = "Python2.4.tar.gz"
    base, ext = os.path.splitext(fname)
    ==>
    fname = Path("Python2.4.tar.gz")
    base, ext = fname.namebase, fname.extx


    3. Replacing glob.glob
    -----------------------

    lib_dir = "/lib"
    libs = glob.glob(os.path.join(lib_dir, "*s.o"))
    ==>
    lib_dir = Path("/lib")
    libs = lib_dir.files("*.so")


Deprecations

    Introducing this module to the standard library introduces a need
    for the "weak" deprecation of a number of existing modules and
    functions.  These modules and functions are so widely used that
    they cannot be truly deprecated, as in generating
    DeprecationWarning.  Here "weak deprecation" means notes in the
    documentation only.

    The table below lists the existing functionality that should be
    deprecated.

        Path method/property    Deprecates function
        --------------------    -------------------
        normcase()              os.path.normcase()
        normpath()              os.path.normpath()
        realpath()              os.path.realpath()
        expanduser()            os.path.expanduser()
        expandvars()            os.path.expandvars()
        parent                  os.path.dirname()
        name                    os.path.basename()
        splitpath()             os.path.split()
        drive                   os.path.splitdrive()
        ext                     os.path.splitext()
        splitunc()              os.path.splitunc()
        __new__()               os.path.join(), os.curdir
        listdir()               os.listdir() [fnmatch.filter()]
        match()                 fnmatch.fnmatch()
        matchcase()             fnmatch.fnmatchcase()
        glob()                  glob.glob()
        exists()                os.path.exists()
        isabs()                 os.path.isabs()
        isdir()                 os.path.isdir()
        isfile()                os.path.isfile()
        islink()                os.path.islink()
        ismount()               os.path.ismount()
        samefile()              os.path.samefile()
        atime()                 os.path.getatime()
        ctime()                 os.path.getctime()
        mtime()                 os.path.getmtime()
        size()                  os.path.getsize()
        cwd()                   os.getcwd()
        access()                os.access()
        stat()                  os.stat()
        lstat()                 os.lstat()
        statvfs()               os.statvfs()
        pathconf()              os.pathconf()
        utime()                 os.utime()
        chmod()                 os.chmod()
        chown()                 os.chown()
        rename()                os.rename()
        renames()               os.renames()
        mkdir()                 os.mkdir()
        makedirs()              os.makedirs()
        rmdir()                 os.rmdir()
        removedirs()            os.removedirs()
        remove()                os.remove()
        unlink()                os.unlink()
        link()                  os.link()
        symlink()               os.symlink()
        readlink()              os.readlink()
        chroot()                os.chroot()
        startfile()             os.startfile()
        copyfile()              shutil.copyfile()
        copymode()              shutil.copymode()
        copystat()              shutil.copystat()
        copy()                  shutil.copy()
        copy2()                 shutil.copy2()
        copytree()              shutil.copytree()
        move()                  shutil.move()
        rmtree()                shutil.rmtree()

    The Path class deprecates the whole of os.path, shutil, fnmatch
    and glob.  A big chunk of os is also deprecated.


Closed Issues

    A number contentious issues have been resolved since this PEP
    first appeared on python-dev:

    * The __div__() method was removed.  Overloading the / (division)
      operator may be "too much magic" and make path concatenation
      appear to be division.  The method can always be re-added later
      if the BDFL so desires.  In its place, __new__() got an *args
      argument that accepts both Path and string objects.  The *args
      are concatenated with os.path.join() which is used to construct
      the Path object.  These changes obsoleted the problematic
      joinpath() method which was removed.

    * The methods and the properties getatime()/atime,
      getctime()/ctime, getmtime()/mtime and getsize()/size duplicated
      each other.  These methods and properties have been merged to
      atime(), ctime(), mtime() and size().  The reason they are not
      properties instead, is because there is a possibility that they
      may change unexpectedly.  The following example is not
      guaranteed to always pass the assertion:

        p = Path("foobar")
        s = p.size()
        assert p.size() == s


Open Issues

    Some functionality of Jason Orendorff's path module have been
    omitted:

    * Function for opening a path - better handled by the builtin
      open().

    * Functions for reading and writing whole files - better handled
      by file objects' own read() and write() methods.

    * A chdir() function may be a worthy inclusion.

    * A deprecation schedule needs to be set up.  How much
      functionality should Path implement?  How much of existing
      functionality should it deprecate and when?

    * The name obviously has to be either "path" or "Path," but where
      should it live?  In its own module or in os?

    * Due to Path subclassing either str or unicode, the following
      non-magic, public methods are available on Path objects:

        capitalize(), center(), count(), decode(), encode(),
        endswith(), expandtabs(), find(), index(), isalnum(),
        isalpha(), isdigit(), islower(), isspace(), istitle(),
        isupper(), join(), ljust(), lower(), lstrip(), replace(),
        rfind(), rindex(), rjust(), rsplit(), rstrip(), split(),
        splitlines(), startswith(), strip(), swapcase(), title(),
        translate(), upper(), zfill()

      On python-dev it has been argued whether this inheritance is
      sane or not.  Most persons debating said that most string
      methods doesn't make sense in the context of filesystem paths --
      they are just dead weight.  The other position, also argued on
      python-dev, is that inheriting from string is very convenient
      because it allows code to "just work" with Path objects without
      having to be adapted for them.

      One of the problems is that at the Python level, there is no way
      to make an object "string-like enough," so that it can be passed
      to the builtin function open() (and other builtins expecting a
      string or buffer), unless the object inherits from either str or
      unicode.  Therefore, to not inherit from string requires changes
      in CPython's core.

    The functions and modules that this new module is trying to
    replace (os.path, shutil, fnmatch, glob and parts of os) are
    expected to be available in future Python versions for a long
    time, to preserve backwards compatibility.


Reference Implementation

    Currently, the Path class is implemented as a thin wrapper around
    the standard library modules fnmatch, glob, os, os.path and
    shutil.  The intention of this PEP is to move functionality from
    the aforementioned modules to Path while they are being
    deprecated.

    For more detail and an implementation see:

        http://wiki.python.org/moin/PathModule


Examples

    In this section, "a ==> b" means that b can be used as a
    replacement for a.

    1. Make all python files in the a directory executable
    ------------------------------------------------------

        DIR = '/usr/home/guido/bin'
        for f in os.listdir(DIR):
            if f.endswith('.py'):
                path = os.path.join(DIR, f)
                os.chmod(path, 0755)
        ==>
        for f in Path('/usr/home/guido/bin').files("*.py"):
            f.chmod(0755)

    2. Delete emacs backup files
    ----------------------------

        def delete_backups(arg, dirname, names):
            for name in names:
                if name.endswith('~'):
                    os.remove(os.path.join(dirname, name))
        os.path.walk(os.environ['HOME'], delete_backups, None)
        ==>
        d = Path(os.environ['HOME'])
        for f in d.walkfiles('*~'):
            f.remove()

    3. Finding the relative path to a file
    --------------------------------------

        b = Path('/users/peter/')
        a = Path('/users/peter/synergy/tiki.txt')
        a.relpathto(b)

    4. Splitting a path into directory and filename
    -----------------------------------------------

        os.path.split("/path/to/foo/bar.txt")
        ==>
        Path("/path/to/foo/bar.txt").splitpath()

    5. List all Python scripts in the current directory tree
    --------------------------------------------------------

        list(Path().walkfiles("*.py"))


References and Footnotes

    [1] Method is not guaranteed to be available on all platforms.

    [2] "(idea) subclassable string: path object?", van Rossum, 2001
        https://mail.python.org/pipermail/python-dev/2001-August/016663.html

    [3] "path module v1.0 released", Orendorff, 2003
        https://mail.python.org/pipermail/python-announce-list/2003-January/001984.html

    [4] "Some RFE for review", Birkenfeld, 2005
        https://mail.python.org/pipermail/python-dev/2005-June/054438.html

    [5] "path module", Orendorff, 2003
        https://mail.python.org/pipermail/python-list/2003-July/174289.html

    [6] "PRE-PEP: new Path class", Roth, 2004
        https://mail.python.org/pipermail/python-list/2004-January/201672.html

    [7] http://wiki.python.org/moin/PathClass


Copyright

    This document has been placed in the public domain.


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