diff options
Diffstat (limited to 'third_party/python/scandir/README.rst')
| -rw-r--r-- | third_party/python/scandir/README.rst | 211 |
1 files changed, 211 insertions, 0 deletions
diff --git a/third_party/python/scandir/README.rst b/third_party/python/scandir/README.rst new file mode 100644 index 0000000000..a5537517dd --- /dev/null +++ b/third_party/python/scandir/README.rst @@ -0,0 +1,211 @@ + +scandir, a better directory iterator and faster os.walk() +========================================================= + +.. image:: https://img.shields.io/pypi/v/scandir.svg + :target: https://pypi.python.org/pypi/scandir + :alt: scandir on PyPI (Python Package Index) + +.. image:: https://travis-ci.org/benhoyt/scandir.svg?branch=master + :target: https://travis-ci.org/benhoyt/scandir + :alt: Travis CI tests (Linux) + +.. image:: https://ci.appveyor.com/api/projects/status/github/benhoyt/scandir?branch=master&svg=true + :target: https://ci.appveyor.com/project/benhoyt/scandir + :alt: Appveyor tests (Windows) + + +``scandir()`` is a directory iteration function like ``os.listdir()``, +except that instead of returning a list of bare filenames, it yields +``DirEntry`` objects that include file type and stat information along +with the name. Using ``scandir()`` increases the speed of ``os.walk()`` +by 2-20 times (depending on the platform and file system) by avoiding +unnecessary calls to ``os.stat()`` in most cases. + + +Now included in a Python near you! +---------------------------------- + +``scandir`` has been included in the Python 3.5 standard library as +``os.scandir()``, and the related performance improvements to +``os.walk()`` have also been included. So if you're lucky enough to be +using Python 3.5 (release date September 13, 2015) you get the benefit +immediately, otherwise just +`download this module from PyPI <https://pypi.python.org/pypi/scandir>`_, +install it with ``pip install scandir``, and then do something like +this in your code: + +.. code-block:: python + + # Use the built-in version of scandir/walk if possible, otherwise + # use the scandir module version + try: + from os import scandir, walk + except ImportError: + from scandir import scandir, walk + +`PEP 471 <https://www.python.org/dev/peps/pep-0471/>`_, which is the +PEP that proposes including ``scandir`` in the Python standard library, +was `accepted <https://mail.python.org/pipermail/python-dev/2014-July/135561.html>`_ +in July 2014 by Victor Stinner, the BDFL-delegate for the PEP. + +This ``scandir`` module is intended to work on Python 2.6+ and Python +3.2+ (and it has been tested on those versions). + + +Background +---------- + +Python's built-in ``os.walk()`` is significantly slower than it needs to be, +because -- in addition to calling ``listdir()`` on each directory -- it calls +``stat()`` on each file to determine whether the filename is a directory or not. +But both ``FindFirstFile`` / ``FindNextFile`` on Windows and ``readdir`` on Linux/OS +X already tell you whether the files returned are directories or not, so +no further ``stat`` system calls are needed. In short, you can reduce the number +of system calls from about 2N to N, where N is the total number of files and +directories in the tree. + +In practice, removing all those extra system calls makes ``os.walk()`` about +**7-50 times as fast on Windows, and about 3-10 times as fast on Linux and Mac OS +X.** So we're not talking about micro-optimizations. See more benchmarks +in the "Benchmarks" section below. + +Somewhat relatedly, many people have also asked for 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 a faster ``walk()``, scandir adds a new ``scandir()`` function. +They're pretty easy to use, but see "The API" below for the full docs. + + +Benchmarks +---------- + +Below are results showing how many times as fast ``scandir.walk()`` is than +``os.walk()`` on various systems, found by running ``benchmark.py`` with no +arguments: + +==================== ============== ============= +System version Python version Times as fast +==================== ============== ============= +Windows 7 64-bit 2.7.7 64-bit 10.4 +Windows 7 64-bit SSD 2.7.7 64-bit 10.3 +Windows 7 64-bit NFS 2.7.6 64-bit 36.8 +Windows 7 64-bit SSD 3.4.1 64-bit 9.9 +Windows 7 64-bit SSD 3.5.0 64-bit 9.5 +CentOS 6.2 64-bit 2.6.6 64-bit 3.9 +Ubuntu 14.04 64-bit 2.7.6 64-bit 5.8 +Mac OS X 10.9.3 2.7.5 64-bit 3.8 +==================== ============== ============= + +All of the above tests were done using the fast C version of scandir +(source code in ``_scandir.c``). + +Note that the gains are less than the above on smaller directories and greater +on larger directories. This is why ``benchmark.py`` creates a test directory +tree with a standardized size. + + +The API +------- + +walk() +~~~~~~ + +The API for ``scandir.walk()`` is exactly the same as ``os.walk()``, so just +`read the Python docs <https://docs.python.org/3.5/library/os.html#os.walk>`_. + +scandir() +~~~~~~~~~ + +The full docs for ``scandir()`` and the ``DirEntry`` objects it yields are +available in the `Python documentation here <https://docs.python.org/3.5/library/os.html#os.scandir>`_. +But below is a brief summary as well. + + scandir(path='.') -> iterator of DirEntry objects for given path + +Like ``listdir``, ``scandir`` calls the operating system's directory +iteration system calls to get the names of the files in the given +``path``, 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 may have 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 +sub-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 scandir ``path`` + argument (corresponds to the return values of ``os.listdir``) + +* ``path``: the entry's full path name (not necessarily an absolute + path) -- the equivalent of ``os.path.join(scandir_path, entry.name)`` + +* ``is_dir(*, follow_symlinks=True)``: similar to + ``pathlib.Path.is_dir()``, but the return value is cached on the + ``DirEntry`` object; doesn't require a system call in most cases; + don't follow symbolic links if ``follow_symlinks`` is False + +* ``is_file(*, follow_symlinks=True)``: similar to + ``pathlib.Path.is_file()``, but the return value is cached on the + ``DirEntry`` object; doesn't require a system call in most cases; + don't follow symbolic links if ``follow_symlinks`` is False + +* ``is_symlink()``: similar to ``pathlib.Path.is_symlink()``, but the + return value is cached on the ``DirEntry`` object; doesn't require a + system call in most cases + +* ``stat(*, follow_symlinks=True)``: like ``os.stat()``, but the + return value is cached on the ``DirEntry`` object; does not require a + system call on Windows (except for symlinks); don't follow symbolic links + (like ``os.lstat()``) if ``follow_symlinks`` is False + +* ``inode()``: return the inode number of the entry; the return value + is cached on the ``DirEntry`` object + +Here's a very simple example of ``scandir()`` showing use of the +``DirEntry.name`` attribute and the ``DirEntry.is_dir()`` method: + +.. code-block:: python + + def subdirs(path): + """Yield directory names not starting with '.' under given path.""" + for entry in os.scandir(path): + if not entry.name.startswith('.') and entry.is_dir(): + yield entry.name + +This ``subdirs()`` function will be significantly faster with scandir +than ``os.listdir()`` and ``os.path.isdir()`` on both Windows and POSIX +systems, especially on medium-sized or large directories. + + +Further reading +--------------- + +* `The Python docs for scandir <https://docs.python.org/3.5/library/os.html#os.scandir>`_ +* `PEP 471 <https://www.python.org/dev/peps/pep-0471/>`_, the + (now-accepted) Python Enhancement Proposal that proposed adding + ``scandir`` to the standard library -- a lot of details here, + including rejected ideas and previous discussion + + +Flames, comments, bug reports +----------------------------- + +Please send flames, comments, and questions about scandir to Ben Hoyt: + +http://benhoyt.com/ + +File bug reports for the version in the Python 3.5 standard library +`here <https://docs.python.org/3.5/bugs.html>`_, or file bug reports +or feature requests for this module at the GitHub project page: + +https://github.com/benhoyt/scandir |