python-peps/peps/pep-0519.rst

PEP: 519
Title: Adding a file system path protocol
Version: $Revision$
Last-Modified: $Date$
Author: Brett Cannon <brett@python.org>,
        Koos Zevenhoven <k7hoven@gmail.com>
Status: Final
Type: Standards Track
Content-Type: text/x-rst
Created: 11-May-2016
Python-Version: 3.6
Post-History: 11-May-2016,
              12-May-2016,
              13-May-2016
Resolution: https://mail.python.org/pipermail/python-dev/2016-May/144646.html


Abstract
========

This PEP proposes a protocol for classes which represent a file system
path to be able to provide a ``str`` or ``bytes`` representation.
Changes to Python's standard library are also proposed to utilize this
protocol where appropriate to facilitate the use of path objects where
historically only ``str`` and/or ``bytes`` file system paths are
accepted. The goal is to facilitate the migration of users towards
rich path objects while providing an easy way to work with code
expecting ``str`` or ``bytes``.


Rationale
=========

Historically in Python, file system paths have been represented as
strings or bytes. This choice of representation has stemmed from C's
own decision to represent file system paths as
``const char *`` [#libc-open]_. While that is a totally serviceable
format to use for file system paths, it's not necessarily optimal. At
issue is the fact that while all file system paths can be represented
as strings or bytes, not all strings or bytes represent a file system
path. This can lead to issues where any e.g. string duck-types to a
file system path whether it actually represents a path or not.

To help elevate the representation of file system paths from their
representation as strings and bytes to a richer object representation,
the pathlib module [#pathlib]_ was provisionally introduced in
Python 3.4 through :pep:`428`. While considered by some as an improvement
over strings and bytes for file system paths, it has suffered from a
lack of adoption. Typically the key issue listed for the low adoption
rate has been the lack of support in the standard library. This lack
of support required users of pathlib to manually convert path objects
to strings by calling ``str(path)`` which many found error-prone.

One issue in converting path objects to strings comes from
the fact that the only generic way to get a string representation of
the path was to pass the object to ``str()``. This can pose a
problem when done blindly as nearly all Python objects have some
string representation whether they are a path or not, e.g.
``str(None)`` will give a result that
``builtins.open()`` [#builtins-open]_ will happily use to create a new
file.

Exacerbating this whole situation is the
``DirEntry`` object [#os-direntry]_. While path objects have a
representation that can be extracted using ``str()``, ``DirEntry``
objects expose a ``path`` attribute instead. Having no common
interface between path objects, ``DirEntry``, and any other
third-party path library has become an issue. A solution that allows
any path-representing object to declare that it is a path and a way
to extract a low-level representation that all path objects could
support is desired.

This PEP then proposes to introduce a new protocol to be followed by
objects which represent file system paths. Providing a protocol allows
for explicit signaling of what objects represent file system paths as
well as a way to extract a lower-level representation that can be used
with older APIs which only support strings or bytes.

Discussions regarding path objects that led to this PEP can be found
in multiple threads on the python-ideas mailing list archive
[#python-ideas-archive]_ for the months of March and April 2016 and on
the python-dev mailing list archives [#python-dev-archive]_ during
April 2016.


Proposal
========

This proposal is split into two parts. One part is the proposal of a
protocol for objects to declare and provide support for exposing a
file system path representation. The other part deals with changes to
Python's standard library to support the new protocol. These changes
will also lead to the pathlib module dropping its provisional status.

Protocol
--------

The following abstract base class defines the protocol for an object
to be considered a path object::

    import abc
    import typing as t


    class PathLike(abc.ABC):

        """Abstract base class for implementing the file system path protocol."""

        @abc.abstractmethod
        def __fspath__(self) -> t.Union[str, bytes]:
            """Return the file system path representation of the object."""
            raise NotImplementedError


Objects representing file system paths will implement the
``__fspath__()`` method which will return the ``str`` or ``bytes``
representation of the path. The ``str`` representation is the
preferred low-level path representation as it is human-readable and
what people historically represent paths as.


Standard library changes
------------------------

It is expected that most APIs in Python's standard library that
currently accept a file system path will be updated appropriately to
accept path objects (whether that requires code or simply an update
to documentation will vary). The modules mentioned below, though,
deserve specific details as they have either fundamental changes that
empower the ability to use path objects, or entail additions/removal
of APIs.


builtins
''''''''

``open()`` [#builtins-open]_ will be updated to accept path objects as
well as continue to accept ``str`` and ``bytes``.


os
'''

The ``fspath()`` function will be added with the following semantics::

    import typing as t


    def fspath(path: t.Union[PathLike, str, bytes]) -> t.Union[str, bytes]:
        """Return the string representation of the path.

        If str or bytes is passed in, it is returned unchanged. If __fspath__()
        returns something other than str or bytes then TypeError is raised. If
        this function is given something that is not str, bytes, or os.PathLike
        then TypeError is raised.
        """
        if isinstance(path, (str, bytes)):
            return path

        # Work from the object's type to match method resolution of other magic
        # methods.
        path_type = type(path)
        try:
            path = path_type.__fspath__(path)
        except AttributeError:
            if hasattr(path_type, '__fspath__'):
                raise
        else:
            if isinstance(path, (str, bytes)):
                return path
            else:
                raise TypeError("expected __fspath__() to return str or bytes, "
                                "not " + type(path).__name__)

        raise TypeError("expected str, bytes or os.PathLike object, not "
                        + path_type.__name__)

The ``os.fsencode()`` [#os-fsencode]_ and
``os.fsdecode()`` [#os-fsdecode]_ functions will be updated to accept
path objects. As both functions coerce their arguments to
``bytes`` and ``str``, respectively, they will be updated to call
``__fspath__()`` if present to convert the path object to a ``str`` or
``bytes`` representation, and then perform their appropriate
coercion operations as if the return value from ``__fspath__()`` had
been the original argument to the coercion function in question.

The addition of ``os.fspath()``, the updates to
``os.fsencode()``/``os.fsdecode()``, and the current semantics of
``pathlib.PurePath`` provide the semantics necessary to
get the path representation one prefers. For a path object,
``pathlib.PurePath``/``Path`` can be used. To obtain the ``str`` or
``bytes`` representation without any coercion, then ``os.fspath()``
can be used. If a ``str`` is desired and the encoding of ``bytes``
should be assumed to be the default file system encoding, then
``os.fsdecode()`` should be used. If a ``bytes`` representation is
desired and any strings should be encoded using the default file
system encoding, then ``os.fsencode()`` is used. This PEP recommends
using path objects when possible and falling back to string paths as
necessary and using ``bytes`` as a last resort.

Another way to view this is as a hierarchy of file system path
representations (highest- to lowest-level): path → str → bytes. The
functions and classes under discussion can all accept objects on the
same level of the hierarchy, but they vary in whether they promote or
demote objects to another level. The ``pathlib.PurePath`` class can
promote a ``str`` to a path object. The ``os.fspath()`` function can
demote a path object to a ``str`` or ``bytes`` instance, depending
on what ``__fspath__()`` returns.
The ``os.fsdecode()`` function will demote a path object to
a string or promote a ``bytes`` object to a ``str``. The
``os.fsencode()`` function will demote a path or string object to
``bytes``. There is no function that provides a way to demote a path
object directly to ``bytes`` while bypassing string demotion.

The ``DirEntry`` object [#os-direntry]_ will gain an ``__fspath__()``
method. It will return the same value as currently found on the
``path`` attribute of ``DirEntry`` instances.

The Protocol_ ABC will be added to the ``os`` module under the name
``os.PathLike``.


os.path
'''''''

The various path-manipulation functions of ``os.path`` [#os-path]_
will be updated to accept path objects. For polymorphic functions that
accept both bytes and strings, they will be updated to simply use
``os.fspath()``.

During the discussions leading up to this PEP it was suggested that
``os.path`` not be updated using an "explicit is better than implicit"
argument. The thinking was that since ``__fspath__()`` is polymorphic
itself it may be better to have code working with ``os.path`` extract
the path representation from path objects explicitly. There is also
the consideration that adding support this deep into the low-level OS
APIs will lead to code magically supporting path objects without
requiring any documentation updated, leading to potential complaints
when it doesn't work, unbeknownst to the project author.

But it is the view of this PEP that "practicality beats purity" in
this instance. To help facilitate the transition to supporting path
objects, it is better to make the transition as easy as possible than
to worry about unexpected/undocumented duck typing support for
path objects by projects.

There has also been the suggestion that ``os.path`` functions could be
used in a tight loop and the overhead of checking or calling
``__fspath__()`` would be too costly. In this scenario only
path-consuming APIs would be directly updated and path-manipulating
APIs like the ones in ``os.path`` would go unmodified. This would
require library authors to update their code to support path objects
if they performed any path manipulations, but if the library code
passed the path straight through then the library wouldn't need to be
updated. It is the view of this PEP and Guido, though, that this is an
unnecessary worry and that performance will still be acceptable.


pathlib
'''''''

The constructor for ``pathlib.PurePath`` and ``pathlib.Path`` will be
updated to accept ``PathLike`` objects. Both ``PurePath`` and ``Path``
will continue to not accept ``bytes`` path representations, and so if
``__fspath__()`` returns ``bytes`` it will raise an exception.

The ``path`` attribute will be removed as this PEP makes it
redundant (it has not been included in any released version of Python
and so is not a backwards-compatibility concern).


C API
'''''

The C API will gain an equivalent function to ``os.fspath()``::

    /*
        Return the file system path representation of the object.

        If the object is str or bytes, then allow it to pass through with
        an incremented refcount. If the object defines __fspath__(), then
        return the result of that method. All other types raise a TypeError.
    */
    PyObject *
    PyOS_FSPath(PyObject *path)
    {
        _Py_IDENTIFIER(__fspath__);
        PyObject *func = NULL;
        PyObject *path_repr = NULL;

        if (PyUnicode_Check(path) || PyBytes_Check(path)) {
            Py_INCREF(path);
            return path;
        }

        func = _PyObject_LookupSpecial(path, &PyId___fspath__);
        if (NULL == func) {
            return PyErr_Format(PyExc_TypeError,
                                "expected str, bytes or os.PathLike object, "
                                "not %S",
                                path->ob_type);
        }

        path_repr = PyObject_CallFunctionObjArgs(func, NULL);
        Py_DECREF(func);
        if (!PyUnicode_Check(path_repr) && !PyBytes_Check(path_repr)) {
            Py_DECREF(path_repr);
            return PyErr_Format(PyExc_TypeError,
                                "expected __fspath__() to return str or bytes, "
                                "not %S",
                                path_repr->ob_type);
        }

        return path_repr;
    }




Backwards compatibility
=======================

There are no explicit backwards-compatibility concerns. Unless an
object incidentally already defines a ``__fspath__()`` method there is
no reason to expect the pre-existing code to break or expect to have
its semantics implicitly changed.

Libraries wishing to support path objects and a version of Python
prior to Python 3.6 and the existence of ``os.fspath()`` can use the
idiom of
``path.__fspath__() if hasattr(path, "__fspath__") else path``.


Implementation
==============

This is the task list for what this PEP proposes to be changed in
Python 3.6:

#. Remove the ``path`` attribute from pathlib
   (`done <http://bugs.python.org/issue22570>`__)
#. Remove the provisional status of pathlib
   (`done <https://hg.python.org/lookup/a5a013ca5687>`__)
#. Add ``os.PathLike``
   (`code <https://hg.python.org/lookup/e672cf63d08a>`__ and
   `docs <http://hg.python.org/lookup/6239673d5e1d>`__ done)
#. Add ``PyOS_FSPath()``
   (`code <https://hg.python.org/lookup/780cbe18082e>`__ and
   `docs <http://hg.python.org/lookup/cec1f00c538d>`__ done)
#. Add ``os.fspath()``
   (`done <done <https://hg.python.org/lookup/780cbe18082e>`__)
#. Update ``os.fsencode()``
   (`done <https://hg.python.org/lookup/00991aa5fdb5>`__)
#. Update ``os.fsdecode()``
   (`done <https://hg.python.org/lookup/00991aa5fdb5>`__)
#. Update ``pathlib.PurePath`` and ``pathlib.Path``
   (`done <https://hg.python.org/lookup/a5a013ca5687>`__)

   #. Add ``__fspath__()``
   #. Add ``os.PathLike`` support to the constructors

#. Add ``__fspath__()`` to ``DirEntry``
   (`done <https://hg.python.org/lookup/5a62d682636e>`__)

#. Update ``builtins.open()``
   (`done <https://hg.python.org/lookup/254125a265d2>`__)
#. Update ``os.path``
   (`done <https://hg.python.org/cpython/rev/b64f83d6ff24>`__)
#. Add a `glossary <https://docs.python.org/3.6/glossary.html>`__ entry for "path-like"
   (`done <https://hg.python.org/lookup/9c57178f13dc>`__)
#. Update `"What's New" <https://docs.python.org/3.6/whatsnew/3.6.html>`_
   (`done <https://hg.python.org/cpython/rev/95361959d451>`__)


Rejected Ideas
==============

Other names for the protocol's method
-------------------------------------

Various names were proposed during discussions leading to this PEP,
including ``__path__``, ``__pathname__``, and ``__fspathname__``. In
the end people seemed to gravitate towards ``__fspath__`` for being
unambiguous without being unnecessarily long.


Separate str/bytes methods
--------------------------

At one point it was suggested that ``__fspath__()`` only return
strings and another method named ``__fspathb__()`` be introduced to
return bytes. The thinking is that by making ``__fspath__()`` not be
polymorphic it could make dealing with the potential string or bytes
representations easier. But the general consensus was that returning
bytes will more than likely be rare and that the various functions in
the os module are the better abstraction to promote over direct
calls to ``__fspath__()``.


Providing a ``path`` attribute
------------------------------

To help deal with the issue of ``pathlib.PurePath`` not inheriting
from ``str``, originally it was proposed to introduce a ``path``
attribute to mirror what ``os.DirEntry`` provides. In the end,
though, it was determined that a protocol would provide the same
result while not directly exposing an API that most people will never
need to interact with directly.


Have ``__fspath__()`` only return strings
------------------------------------------

Much of the discussion that led to this PEP revolved around whether
``__fspath__()`` should be polymorphic and return ``bytes`` as well as
``str`` or only return ``str``. The general sentiment for this view
was that ``bytes`` are difficult to work with due to their
inherent lack of information about their encoding and :pep:`383` makes
it possible to represent all file system paths using ``str`` with the
``surrogateescape`` handler. Thus, it would be better to forcibly
promote the use of ``str`` as the low-level path representation for
high-level path objects.

In the end, it was decided that using ``bytes`` to represent paths is
simply not going to go away and thus they should be supported to some
degree. The hope is that people will gravitate towards path objects
like pathlib and that will move people away from operating directly
with ``bytes``.


A generic string encoding mechanism
-----------------------------------

At one point there was a discussion of developing a generic mechanism
to extract a string representation of an object that had semantic
meaning (``__str__()`` does not necessarily return anything of
semantic significance beyond what may be helpful for debugging). In
the end, it was deemed to lack a motivating need beyond the one this
PEP is trying to solve in a specific fashion.


Have __fspath__ be an attribute
-------------------------------

It was briefly considered to have ``__fspath__`` be an attribute
instead of a method. This was rejected for two reasons. One,
historically protocols have been implemented as "magic methods" and
not "magic methods and attributes". Two, there is no guarantee that
the lower-level representation of a path object will be pre-computed,
potentially misleading users that there was no expensive computation
behind the scenes in case the attribute was implemented as a property.

This also indirectly ties into the idea of introducing a ``path``
attribute to accomplish the same thing. This idea has an added issue,
though, of accidentally having any object with a ``path`` attribute
meet the protocol's duck typing. Introducing a new magic method for
the protocol helpfully avoids any accidental opting into the protocol.


Provide specific type hinting support
-------------------------------------

There was some consideration to providing a generic ``typing.PathLike``
class which would allow for e.g. ``typing.PathLike[str]`` to specify
a type hint for a path object which returned a string representation.
While potentially beneficial, the usefulness was deemed too small to
bother adding the type hint class.

This also removed any desire to have a class in the ``typing`` module
which represented the union of all acceptable path-representing types
as that can be represented with
``typing.Union[str, bytes, os.PathLike]`` easily enough and the hope
is users will slowly gravitate to path objects only.


Provide ``os.fspathb()``
------------------------

It was suggested that to mirror the structure of e.g.
``os.getcwd()``/``os.getcwdb()``, that ``os.fspath()`` only return
``str`` and that another function named ``os.fspathb()`` be
introduced that only returned ``bytes``. This was rejected as the
purposes of the ``*b()`` functions are tied to querying the file
system where there is a need to get the raw bytes back. As this PEP
does not work directly with data on a file system (but which *may*
be), the view was taken this distinction is unnecessary. It's also
believed that the need for only bytes will not be common enough to
need to support in such a specific manner as ``os.fsencode()`` will
provide similar functionality.


Call ``__fspath__()`` off of the instance
-----------------------------------------

An earlier draft of this PEP had ``os.fspath()`` calling
``path.__fspath__()`` instead of ``type(path).__fspath__(path)``. The
changed to be consistent with how other magic methods in Python are
resolved.


Acknowledgements
================

Thanks to everyone who participated in the various discussions related
to this PEP that spanned both python-ideas and python-dev. Special
thanks to Stephen Turnbull for direct feedback on early drafts of this
PEP. More special thanks to Koos Zevenhoven and Ethan Furman for not
only feedback on early drafts of this PEP but also helping to drive
the overall discussion on this topic across the two mailing lists.


References
==========

.. [#python-ideas-archive] The python-ideas mailing list archive
   (https://mail.python.org/pipermail/python-ideas/)

.. [#python-dev-archive] The python-dev mailing list archive
   (https://mail.python.org/pipermail/python-dev/)

.. [#libc-open] ``open()`` documentation for the C standard library
   (http://www.gnu.org/software/libc/manual/html_node/Opening-and-Closing-Files.html)

.. [#pathlib] The ``pathlib`` module
   (https://docs.python.org/3/library/pathlib.html#module-pathlib)

.. [#builtins-open] The ``builtins.open()`` function
   (https://docs.python.org/3/library/functions.html#open)

.. [#os-fsencode] The ``os.fsencode()`` function
   (https://docs.python.org/3/library/os.html#os.fsencode)

.. [#os-fsdecode] The ``os.fsdecode()`` function
   (https://docs.python.org/3/library/os.html#os.fsdecode)

.. [#os-direntry] The ``os.DirEntry`` class
   (https://docs.python.org/3/library/os.html#os.DirEntry)

.. [#os-path] The ``os.path`` module
   (https://docs.python.org/3/library/os.path.html#module-os.path)


Copyright
=========

This document has been placed in the public domain.



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