A quick comment about sending me fixes. While I do appreciate them, sending them as a pull request is much easier for me as (a) I don't have to hunt the changes down in the text, and (b) you will see the fixes others have done already to the PEP and I then don't have to figure out what changes have not already been fixed. And honestly, reading the PEP in its rendered format on GitHub is easier IMO than in the text format unless you have something specific to respond to (and even if you do, you can copy and paste the relevant bits into an email reply).
On Wed, 11 May 2016 at 09:43 Brett Cannon <br...@python.org> wrote: > **deep, calming breath** > > Here is the PEP for __fspath__(). The draft lives at > https://github.com/brettcannon/path-pep so feel free to send me PRs for > spelling mistakes, grammatical errors, etc. > > ----- > > PEP: NNN > Title: Adding a file system path protocol > Version: $Revision$ > Last-Modified: $Date$ > Author: Brett Cannon <br...@python.org> > Status: Draft > Type: Standards Track > Content-Type: text/x-rst > Created: 11-May-2016 > Post-History: 11-May-2016 > > > 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 allow users to use the representation of a > file system path that's easiest for them now as they migrate towards > using path objects in the future. > > > 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 more appropriate 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 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 had become an issue. A solution that allowed > any path-representing object to declare that is was a path and a way > to extract a low-level representation that all path objects could > support was 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 clear signalling 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 is changes to Python's > standard library to support the new protocol. These changes will also > have the pathlib module drop 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 > (although this is still an open issue: > `Should os.fspath() return bytes?`_):: > > import typing as t > > > def fspath(path: t.Union[PathLike, str]) -> str: > """Return the string representation of the path. > > If a string is passed in then it is returned unchanged. > """ > if hasattr(path, '__fspath__'): > path = path.__fspath__() > if not isinstance(path, str): > type_name = type(path).__name__ > raise TypeError("expected a str or path object, not " + > type_name) > return path > > 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. If ``str`` is desired and > a ``bytes`` object is considered an error, 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. Finally, 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. Therefore, no function is provided for > the case of wanting a bytes representation but without any automatic > encoding to help discourage the use of multiple bytes encodings on a > single file system. If it is necessary to deal with an existing file > system directory with entries in a non-default encoding, this can be > done with low-level functions using ``str`` and the PEP 383 > ``surrogateescape`` error handler, or by using ``bytes`` directly. > > 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 string, but only if ``__fspath__()`` returns > a string. 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`` and not allow demoting strings. > > 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. > > > 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 > code very much similar to > ``path.__fspath__() if hasattr(path, '__fspath__') else path``. This > will allow for their pre-existing type-checking code to continue to > function. > > 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 the authors 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 > projects. > > > pathlib > ''''''' > > The ``PathLike`` ABC as discussed in the Protocol_ section will be > added to the pathlib module [#pathlib]_. The constructor for > ``pathlib.PurePath`` and ``pathlib.Path`` will be updated to accept > path 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 its > 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()`` that > also allows bytes objects through:: > > /* > Return the file system path of the object. > > If the object is str or bytes, then allow it to pass through with > an incremented refcount. All other types raise a TypeError. > */ > PyObject * > PyOS_RawFSPath(PyObject *path) > { > if (PyObject_HasAttrString(path, "__fspath__")) { > path = PyObject_CallMethodObjArgs(path, "__fspath__", NULL); > if (path == NULL) { > return NULL; > } > } > else { > Py_INCREF(path); > } > > if (!PyUnicode_Check(path) && !PyBytes_Check(path)) { > Py_DECREF(path); > return PyErr_Format(PyExc_TypeError, > "expected a string, bytes, or path object, > not %S", > path->ob_type); > } > > return path; > } > > > Backwards compatibility > ======================= > > There are no explicit backwards-compatibility concerns. Unless an > object incidentally already defines a ``__fspath__()`` method there is > not reason to expect pre-existing code to break or expect to have > their semantics implicitly changed. > > Libraries wishing to support path objects and a version of Python > prior to Python 3.6 can use the idiom of > ``path.__fspath__() if hasattr(path, '__fspath__') else path``. > > > Open Issues > =========== > > Should os.fspath() return bytes? > -------------------------------- > > Some have argued that ``os.fspath()`` should be configurable so that > the user can specify what types are acceptable (e.g. an argument to > say that bytes are acceptable instead of strings, or both types). > Others have suggested that ``os.fspath()`` match the proposed > semantics of ``PyOS_RawFSPath()``. Both camps argue that use of > ``os.fspath()`` will only be for a transitionary period while more > libraries gain acceptance of path objects, and so being more flexible > in what ``os.fspath()`` works with will help with the transition. The > opponents to this -- which support the currently proposed sematnics -- > worry that being so flexible with accepting bytes will lead to people > not properly considering the ramifications of working with bytes, > especially if bytes are transparently appearing in their code due to > ``os.fspath()``. > > > The name and location of the protocol's ABC > ------------------------------------------- > > The name of the ABC being proposed to represent the protocol has not > been discussed very much, not which module it should exist in. > Names other than ``PathLike`` which are viable are ``PathABC`` > and ``FSPathABC``. The name can't be ``Path`` if the ABC is put into > the pathlib module. > > > Type hint for path-like objects > ------------------------------- > > Creating a proper type hint for APIs that accept path objects as well > as strings and bytes will probably be needed. It could be as simple > as defining ``typing.Path``/``typing.FSPath`` to correspond to the ABC > and then having > ``typing.PathLike = typing.Union[typing.Path, str, bytes]``. The type > hint could also potentially be made to be generic to accept the > specific low-level representation, e.g. ``typing.PathLike[str]``. > > In the end the type hinting solution should be properly discussed > with the right type hinting experts if this is the best approach. > > > Rejected Ideas > ============== > > Other names for the protocol's function > --------------------------------------- > > 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 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 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 be promoting 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`` instead of only ``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. For those not wanting the hassle of working with ``bytes``, > ``os.fspath()`` is provided. > > > A generic string encoding mechanism > ----------------------------------- > > At one point there was 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. > > > 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 Koos Zevenhoven and Stephen Turnbull for direct feedback on > early drafts of this PEP. > > > 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()`` documention 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. > > > .. > Local Variables: > mode: indented-text > indent-tabs-mode: nil > sentence-end-double-space: t > fill-column: 70 > coding: utf-8 > End: > >
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