python-peps/peps/pep-0355.rst

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/x-rst
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 their 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): ...  # See footnote [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): ...  # See footnote [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): ...  # See footnote [1]
        def stat(self): ...
        def lstat(self): ...
        def statvfs(self): ...  # See footnote [1]
        def pathconf(self, name): ...  # See footnote [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 # See footnote [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): ...  # See footnote [1]
        def startfile(self): ...  # See footnote [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``.

* Replacing ``os.path.join``::

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

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


* 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


* 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.

* 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)

* 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()

* Finding the relative path to a file::

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

* Splitting a path into directory and filename::

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

* 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.