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PEP 519 -- Adding a file system path protocol

PEP:519
Title:Adding a file system path protocol
Author:Brett Cannon <brett at python.org>, Koos Zevenhoven <k7hoven at gmail.com>
Status:Final
Type:Standards Track
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 * [3]. 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 [4] 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() [5] will happily use to create a new file.

Exacerbating this whole situation is the DirEntry object [8]. 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 [1] for the months of March and April 2016 and on the python-dev mailing list archives [2] 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() [5] 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() [6] and os.fsdecode() [7] 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 coersion, 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 [8] 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 [9] 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:

  1. Remove the path attribute from pathlib (done)
  2. Remove the provisional status of pathlib (done)
  3. Add os.PathLike (code and docs done)
  4. Add PyOS_FSPath() (code and docs done)
  5. Add os.fspath() (done <done)
  6. Update os.fsencode() (done)
  7. Update os.fsdecode() (done)
  8. Update pathlib.PurePath and pathlib.Path (done)
    1. Add __fspath__()
    2. Add os.PathLike support to the constructors
  9. Add __fspath__() to DirEntry (done)
  10. Update builtins.open() (done)
  11. Update os.path (done)
  12. Add a glossary entry for "path-like" (done)
  13. Update "What's New" (done)

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 provdinga 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.

Source: https://github.com/python/peps/blob/master/pep-0519.txt