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PEP 355 -- Path - Object oriented filesystem paths

PEP:355
Title:Path - Object oriented filesystem paths
Author:Björn Lindqvist <bjourne at gmail.com>
Status:Rejected
Type:Standards Track
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.

  • 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"))
    
Source: https://github.com/python/peps/blob/master/pep-0355.txt