[Python-Dev] Updates to PEP 471, the os.scandir() proposal

Tue Jul 8 15:52:18 CEST 2014

Hi folks,

After some very good python-dev feedback on my first version of PEP
471, I've updated the PEP to clarify a few things and added various
"Rejected ideas" subsections. Here's a link to the new version (I've
also copied the full text below):

http://legacy.python.org/dev/peps/pep-0471/ -- new PEP as HTML
http://hg.python.org/peps/rev/0da4736c27e8 -- changes

Specifically, I've made these changes (not an exhaustive list):

* Clarified wording in several places, for example "Linux and OS X" ->
"POSIX-based systems"
* Added a new "Notes on exception handling" section
* Added a thorough "Rejected ideas" section with the various ideas
that have been discussed previously and rejected for various reasons
* Added a description of the .full_name attribute, which folks seemed
to generally agree is a good idea
* Removed the "open issues" section, as the three open issues have
either been included (full_name) or rejected (windows_wildcard)

One known error in the PEP is that the "Notes" sections should be
top-level sections, not be subheadings of "Examples". If someone would
like to give me ("benhoyt") commit access to the peps repo, I can fix
this and any other issues that come up.

I'd love to see this finalized! If you're going to comment with
suggestions to change the API, please ensure you've first read the
"rejected ideas" sections in the PEP as well as the relevant
python-dev discussion (linked to in the PEP).

Thanks,
Ben

PEP: 471
Title: os.scandir() function -- a better and faster directory iterator
Version: $Revision$
Last-Modified: $Date$
Author: Ben Hoyt <benhoyt at gmail.com>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 30-May-2014
Python-Version: 3.5
Post-History: 27-Jun-2014, 8-Jul-2014

Abstract
========

This PEP proposes including a new directory iteration function,
``os.scandir()``, in the standard library. This new function adds
useful functionality and increases the speed of ``os.walk()`` by 2-10
times (depending on the platform and file system) by significantly
reducing the number of times ``stat()`` needs to be called.

Rationale
=========

Python's built-in ``os.walk()`` is significantly slower than it needs
to be, because -- in addition to calling ``os.listdir()`` on each
directory -- it executes the ``stat()`` system call or
``GetFileAttributes()`` on each file to determine whether the entry is
a directory or not.

But the underlying system calls -- ``FindFirstFile`` /
``FindNextFile`` on Windows and ``readdir`` on POSIX systems --
already tell you whether the files returned are directories or not, so
no further system calls are needed. Further, the Windows system calls
return all the information for a ``stat_result`` object, such as file
size and last modification time.

In short, you can reduce the number of system calls required for a
tree function like ``os.walk()`` from approximately 2N to N, where N
is the total number of files and directories in the tree. (And because
directory trees are usually wider than they are deep, it's often much
better than this.)

In practice, removing all those extra system calls makes ``os.walk()``
about **8-9 times as fast on Windows**, and about **2-3 times as fast
on POSIX systems**. So we're not talking about micro-
optimizations. See more `benchmarks here`_.

.. _`benchmarks here`: https://github.com/benhoyt/scandir#benchmarks

Somewhat relatedly, many people (see Python `Issue 11406`_) are also
keen on a version of ``os.listdir()`` that yields filenames as it
iterates instead of returning them as one big list. This improves
memory efficiency for iterating very large directories.

So, as well as providing a ``scandir()`` iterator function for calling
directly, Python's existing ``os.walk()`` function could be sped up a
huge amount.

.. _`Issue 11406`: http://bugs.python.org/issue11406

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

The implementation of this proposal was written by Ben Hoyt (initial
version) and Tim Golden (who helped a lot with the C extension
module). It lives on GitHub at `benhoyt/scandir`_.

.. _`benhoyt/scandir`: https://github.com/benhoyt/scandir

Note that this module has been used and tested (see "Use in the wild"
section in this PEP), so it's more than a proof-of-concept. However,
it is marked as beta software and is not extensively battle-tested.
It will need some cleanup and more thorough testing before going into
the standard library, as well as integration into ``posixmodule.c``.

Specifics of proposal
=====================

Specifically, this PEP proposes adding a single function to the ``os``
module in the standard library, ``scandir``, that takes a single,
optional string as its argument::

    scandir(path='.') -> generator of DirEntry objects

Like ``listdir``, ``scandir`` calls the operating system's directory
iteration system calls to get the names of the files in the ``path``
directory, but it's different from ``listdir`` in two ways:

* Instead of returning bare filename strings, it returns lightweight
  ``DirEntry`` objects that hold the filename string and provide
  simple methods that allow access to the additional data the
  operating system returned.

* It returns a generator instead of a list, so that ``scandir`` acts
  as a true iterator instead of returning the full list immediately.

``scandir()`` yields a ``DirEntry`` object for each file and directory
in ``path``. Just like ``listdir``, the ``'.'`` and ``'..'``
pseudo-directories are skipped, and the entries are yielded in
system-dependent order. Each ``DirEntry`` object has the following
attributes and methods:

* ``name``: the entry's filename, relative to the ``path`` argument
  (corresponds to the return values of ``os.listdir``)

* ``full_name``: the entry's full path name -- the equivalent of
  ``os.path.join(path, entry.name)``

* ``is_dir()``: like ``os.path.isdir()``, but much cheaper -- it never
  requires a system call on Windows, and usually doesn't on POSIX
  systems

* ``is_file()``: like ``os.path.isfile()``, but much cheaper -- it
  never requires a system call on Windows, and usually doesn't on
  POSIX systems

* ``is_symlink()``: like ``os.path.islink()``, but much cheaper -- it
  never requires a system call on Windows, and usually doesn't on
  POSIX systems

* ``lstat()``: like ``os.lstat()``, but much cheaper on some systems
  -- it only requires a system call on POSIX systems

The ``is_X`` methods may perform a ``stat()`` call under certain
conditions (for example, on certain file systems on POSIX systems),
and therefore possibly raise ``OSError``. The ``lstat()`` method will
call ``stat()`` on POSIX systems and therefore also possibly raise
``OSError``. See the "Notes on exception handling" section for more
details.

The ``DirEntry`` attribute and method names were chosen to be the same
as those in the new ``pathlib`` module for consistency.

Like the other functions in the ``os`` module, ``scandir()`` accepts
either a bytes or str object for the ``path`` parameter, and returns
the ``DirEntry.name`` and ``DirEntry.full_name`` attributes with the
same type as ``path``. However, it is *strongly recommended* to use
the str type, as this ensures cross-platform support for Unicode
filenames.

Examples
========

Below is a good usage pattern for ``scandir``. This is in fact almost
exactly how the scandir module's faster ``os.walk()`` implementation
uses it::

    dirs = []
    non_dirs = []
    for entry in os.scandir(path):
        if entry.is_dir():
            dirs.append(entry)
        else:
            non_dirs.append(entry)

The above ``os.walk()``-like code will be significantly faster with
scandir than ``os.listdir()`` and ``os.path.isdir()`` on both Windows
and POSIX systems.

Or, for getting the total size of files in a directory tree, showing
use of the ``DirEntry.lstat()`` method and ``DirEntry.full_name``
attribute::

    def get_tree_size(path):
        """Return total size of files in path and subdirs."""
        total = 0
        for entry in os.scandir(path):
            if entry.is_dir():
                total += get_tree_size(entry.full_name)
            else:
                total += entry.lstat().st_size
        return total

Note that ``get_tree_size()`` will get a huge speed boost on Windows,
because no extra stat call are needed, but on POSIX systems the size
information is not returned by the directory iteration functions, so
this function won't gain anything there.

Notes on caching
----------------

The ``DirEntry`` objects are relatively dumb -- the ``name`` and
``full_name`` attributes are obviously always cached, and the ``is_X``
and ``lstat`` methods cache their values (immediately on Windows via
``FindNextFile``, and on first use on POSIX systems via a ``stat``
call) and never refetch from the system.

For this reason, ``DirEntry`` objects are intended to be used and
thrown away after iteration, not stored in long-lived data structured
and the methods called again and again.

If developers want "refresh" behaviour (for example, for watching a
file's size change), they can simply use ``pathlib.Path`` objects,
or call the regular ``os.lstat()`` or ``os.path.getsize()`` functions
which get fresh data from the operating system every call.

Notes on exception handling
---------------------------

``DirEntry.is_X()`` and ``DirEntry.lstat()`` are explicitly methods
rather than attributes or properties, to make it clear that they may
not be cheap operations, and they may do a system call. As a result,
these methods may raise ``OSError``.

For example, ``DirEntry.lstat()`` will always make a system call on
POSIX-based systems, and the ``DirEntry.is_X()`` methods will make a
``stat()`` system call on such systems if ``readdir()`` returns a
``d_type`` with a value of ``DT_UNKNOWN``, which can occur under
certain conditions or on certain file systems.

For this reason, when a user requires fine-grained error handling,
it's good to catch ``OSError`` around these method calls and then
handle as appropriate.

For example, below is a version of the ``get_tree_size()`` example
shown above, but with basic error handling added::

    def get_tree_size(path):
        """Return total size of files in path and subdirs. If
        is_dir() or lstat() fails, print an error message to stderr
        and assume zero size (for example, file has been deleted).
        """
        total = 0
        for entry in os.scandir(path):
            try:
                is_dir = entry.is_dir()
            except OSError as error:
                print('Error calling is_dir():', error, file=sys.stderr)
                continue
            if is_dir:
                total += get_tree_size(entry.full_name)
            else:
                try:
                    total += entry.lstat().st_size
                except OSError as error:
                    print('Error calling lstat():', error, file=sys.stderr)
        return total

Support
=======

The scandir module on GitHub has been forked and used quite a bit (see
"Use in the wild" in this PEP), but there's also been a fair bit of
direct support for a scandir-like function from core developers and
others on the python-dev and python-ideas mailing lists. A sampling:

* **python-dev**: a good number of +1's and very few negatives for
  scandir and PEP 471 on `this June 2014 python-dev thread
  <https://mail.python.org/pipermail/python-dev/2014-June/135217.html>`_

* **Nick Coghlan**, a core Python developer: "I've had the local Red
  Hat release engineering team express their displeasure at having to
  stat every file in a network mounted directory tree for info that is
  present in the dirent structure, so a definite +1 to os.scandir from
  me, so long as it makes that info available."
  [`source1 <http://bugs.python.org/issue11406>`_]

* **Tim Golden**, a core Python developer, supports scandir enough to
  have spent time refactoring and significantly improving scandir's C
  extension module.
  [`source2 <https://github.com/tjguk/scandir>`_]

* **Christian Heimes**, a core Python developer: "+1 for something
  like yielddir()"
  [`source3 <https://mail.python.org/pipermail/python-ideas/2012-November/017772.html>`_]
  and "Indeed! I'd like to see the feature in 3.4 so I can remove my
  own hack from our code base."
  [`source4 <http://bugs.python.org/issue11406>`_]

* **Gregory P. Smith**, a core Python developer: "As 3.4beta1 happens
  tonight, this isn't going to make 3.4 so i'm bumping this to 3.5.
  I really like the proposed design outlined above."
  [`source5 <http://bugs.python.org/issue11406>`_]

* **Guido van Rossum** on the possibility of adding scandir to Python
  3.5 (as it was too late for 3.4): "The ship has likewise sailed for
  adding scandir() (whether to os or pathlib). By all means experiment
  and get it ready for consideration for 3.5, but I don't want to add
  it to 3.4."
  [`source6 <https://mail.python.org/pipermail/python-dev/2013-November/130583.html>`_]

Support for this PEP itself (meta-support?) was given by Nick Coghlan
on python-dev: "A PEP reviewing all this for 3.5 and proposing a
specific os.scandir API would be a good thing."
[`source7 <https://mail.python.org/pipermail/python-dev/2013-November/130588.html>`_]

Use in the wild
===============

To date, the ``scandir`` implementation is definitely useful, but has
been clearly marked "beta", so it's uncertain how much use of it there
is in the wild. Ben Hoyt has had several reports from people using it.
For example:

* Chris F: "I am processing some pretty large directories and was half
  expecting to have to modify getdents. So thanks for saving me the
  effort." [via personal email]

* bschollnick: "I wanted to let you know about this, since I am using
  Scandir as a building block for this code. Here's a good example of
  scandir making a radical performance improvement over os.listdir."
  [`source8 <https://github.com/benhoyt/scandir/issues/19>`_]

* Avram L: "I'm testing our scandir for a project I'm working on.
  Seems pretty solid, so first thing, just want to say nice work!"
  [via personal email]

Others have `requested a PyPI package`_ for it, which has been
created. See `PyPI package`_.

.. _`requested a PyPI package`: https://github.com/benhoyt/scandir/issues/12
.. _`PyPI package`: https://pypi.python.org/pypi/scandir

GitHub stats don't mean too much, but scandir does have several
watchers, issues, forks, etc. Here's the run-down as of the stats as
of July 7, 2014:

* Watchers: 17
* Stars: 57
* Forks: 20
* Issues: 4 open, 26 closed

**However, the much larger point is this:**, if this PEP is accepted,
``os.walk()`` can easily be reimplemented using ``scandir`` rather
than ``listdir`` and ``stat``, increasing the speed of ``os.walk()``
very significantly. There are thousands of developers, scripts, and
production code that would benefit from this large speedup of
``os.walk()``. For example, on GitHub, there are almost as many uses
of ``os.walk`` (194,000) as there are of ``os.mkdir`` (230,000).

Rejected ideas
==============

Naming
------

The only other real contender for this function's name was
``iterdir()``. However, ``iterX()`` functions in Python (mostly found
in Python 2) tend to be simple iterator equivalents of their
non-iterator counterparts. For example, ``dict.iterkeys()`` is just an
iterator version of ``dict.keys()``, but the objects returned are
identical. In ``scandir()``'s case, however, the return values are
quite different objects (``DirEntry`` objects vs filename strings), so
this should probably be reflected by a difference in name -- hence
``scandir()``.

See some `relevant discussion on python-dev
<https://mail.python.org/pipermail/python-dev/2014-June/135228.html>`_.

Wildcard support
----------------

``FindFirstFile``/``FindNextFile`` on Windows support passing a
"wildcard" like ``*.jpg``, so at first folks (this PEP's author
included) felt it would be a good idea to include a
``windows_wildcard`` keyword argument to the ``scandir`` function so
users could pass this in.

However, on further thought and discussion it was decided that this
would be bad idea, *unless it could be made cross-platform* (a
``pattern`` keyword argument or similar). This seems easy enough at
first -- just use the OS wildcard support on Windows, and something
like ``fnmatch`` or ``re`` afterwards on POSIX-based systems.

Unfortunately the exact Windows wildcard matching rules aren't really
documented anywhere by Microsoft, and they're quite quirky (see this
`blog post
<http://blogs.msdn.com/b/oldnewthing/archive/2007/12/17/6785519.aspx>`_),
meaning it's very problematic to emulate using ``fnmatch`` or regexes.

So the consensus was that Windows wildcard support was a bad idea.
It would be possible to add at a later date if there's a
cross-platform way to achieve it, but not for the initial version.

Read more on the `this Nov 2012 python-ideas thread
<https://mail.python.org/pipermail/python-ideas/2012-November/017770.html>`_
and this `June 2014 python-dev thread on PEP 471
<https://mail.python.org/pipermail/python-dev/2014-June/135217.html>`_.

DirEntry attributes being properties
------------------------------------

In some ways it would be nicer for the ``DirEntry`` ``is_X()`` and
``lstat()`` to be properties instead of methods, to indicate they're
very cheap or free. However, this isn't quite the case, as ``lstat()``
will require an OS call on POSIX-based systems but not on Windows.
Even ``is_dir()`` and friends may perform an OS call on POSIX-based
systems if the ``dirent.d_type`` value is ``DT_UNKNOWN`` (on certain
file systems).

Also, people would expect the attribute access ``entry.is_dir`` to
only ever raise ``AttributeError``, not ``OSError`` in the case it
makes a system call under the covers. Calling code would have to have
a ``try``/``except`` around what looks like a simple attribute access,
and so it's much better to make them *methods*.

See `this May 2013 python-dev thread
<https://mail.python.org/pipermail/python-dev/2013-May/126184.html>`_
where this PEP author makes this case and there's agreement from a
core developers.

DirEntry fields being "static" attribute-only objects
-----------------------------------------------------

In `this July 2014 python-dev message
<https://mail.python.org/pipermail/python-dev/2014-July/135303.html>`_,
Paul Moore suggested a solution that was a "thin wrapper round the OS
feature", where the ``DirEntry`` object had only static attributes:
``name``, ``full_name``, and ``is_X``, with the ``st_X`` attributes
only present on Windows. The idea was to use this simpler, lower-level
function as a building block for higher-level functions.

At first there was general agreement that simplifying in this way was
a good thing. However, there were two problems with this approach.
First, the assumption is the ``is_dir`` and similar attributes are
always present on POSIX, which isn't the case (if ``d_type`` is not
present or is ``DT_UNKNOWN``). Second, it's a much harder-to-use API
in practice, as even the ``is_dir`` attributes aren't always present
on POSIX, and would need to be tested with ``hasattr()`` and then
``os.stat()`` called if they weren't present.

See `this July 2014 python-dev response
<https://mail.python.org/pipermail/python-dev/2014-July/135312.html>`_
from this PEP's author detailing why this option is a non-ideal
solution, and the subsequent reply from Paul Moore voicing agreement.

DirEntry fields being static with an ensure_lstat option
--------------------------------------------------------

Another seemingly simpler and attractive option was suggested by
Nick Coghlan in this `June 2014 python-dev message
<https://mail.python.org/pipermail/python-dev/2014-June/135261.html>`_:
make ``DirEntry.is_X`` and ``DirEntry.lstat_result`` properties, and
populate ``DirEntry.lstat_result`` at iteration time, but only if
the new argument ``ensure_lstat=True`` was specified on the
``scandir()`` call.

This does have the advantage over the above in that you can easily get
the stat result from ``scandir()`` if you need it. However, it has the
serious disadvantage that fine-grained error handling is messy,
because ``stat()`` will be called (and hence potentially raise
``OSError``) during iteration, leading to a rather ugly, hand-made
iteration loop::

    it = os.scandir(path)
    while True:
        try:
            entry = next(it)
        except OSError as error:
            handle_error(path, error)
        except StopIteration:
            break

Or it means that ``scandir()`` would have to accept an ``onerror``
argument -- a function to call when ``stat()`` errors occur during
iteration. This seems to this PEP's author neither as direct nor as
Pythonic as ``try``/``except`` around a ``DirEntry.lstat()`` call.

See `Ben Hoyt's July 2014 reply
<https://mail.python.org/pipermail/python-dev/2014-July/135312.html>`_
to the discussion summarizing this and detailing why he thinks the
original PEP 471 proposal is "the right one" after all.

Return values being (name, stat_result) two-tuples
--------------------------------------------------

Initially this PEP's author proposed this concept as a function called
``iterdir_stat()`` which yielded two-tuples of (name, stat_result).
This does have the advantage that there are no new types introduced.
However, the ``stat_result`` is only partially filled on POSIX-based
systems (most fields set to ``None`` and other quirks), so they're not
really ``stat_result`` objects at all, and this would have to be
thoroughly documented as different from ``os.stat()``.

Also, Python has good support for proper objects with attributes and
methods, which makes for a saner and simpler API than two-tuples. It
also makes the ``DirEntry`` objects more extensible and future-proof
as operating systems add functionality and we want to include this in
``DirEntry``.