README.rst - platform/external/yapf - Gitiles

 ====
 YAPF
 ====

 .. image:: https://travis-ci.org/google/yapf.svg?branch=master
     :target: https://travis-ci.org/google/yapf
     :alt: Build status

 Introduction
 ============

 Most of the current formatters for Python -- e.g., autopep8, and pep8ify -- are
 made to remove lint errors from code. This has some obvious limitations. For
 instance, code that conforms to the PEP 8 guidelines may not be reformatted.
 But it doesn't mean that the code looks good.

 YAPF takes a different approach. It's based off of 'clang-format', developed by
 Daniel Jasper. In essence, the algorithm takes the code and reformats it to the
 best formatting that conforms to the style guide, even if the original code
 didn't violate the style guide. The idea is also similar to the 'gofmt' tool for
 the Go programming language: end all holy wars about formatting - if the whole
 code base of a project is simply piped through YAPF whenever modifications are
 made, the style remains consistent throughout the project and there's no point
 arguing about style in every code review.

 The ultimate goal is that the code YAPF produces is as good as the code that a
 programmer would write if they were following the style guide.

 .. note::

     YAPF is not an official Google product (experimental or otherwise), it is
     just code that happens to be owned by Google.

 .. contents::

 Installation
 ============

 YAPF supports Python 2.7 and 3.4+.

 To install YAPF from the source directory::

     $ sudo python ./setup.py install

 If you intend to use YAPF as a command-line tool rather than as a library,
 installation is not necessary. YAPF supports being run as a directory by the
 Python interpreter. If you cloned/unzipped yapf into ``DIR``, it's possible to
 run::

     $ PYTHONPATH=DIR python DIR/yapf [options] ...

 Usage
 =====

 Options::

     usage: yapf [-h] [--style STYLE] [-d | -i] [-l START-END | -r] ...

     Formatter for Python code.

     positional arguments:
       files

     optional arguments:
       -h, --help            show this help message and exit
       --style STYLE         specify formatting style: either a style name (for
                             example "pep8" or "google"), or the name of a file
                             with style settings
       -d, --diff            print the diff for the fixed source
       -i, --in-place        make changes to files in place
       -l START-END, --lines START-END
                             range of lines to reformat, one-based
       -r, --recursive       run recursively over directories

 Note: after reformatting a chunk of code, YAPF verifies that it's correct (can
 be parsed by Python itself). This means that if you're reformatting Python 3
 code, it's best to run YAPF itself under Python 3. The same goes for Python 2.

 Formatting style
 ----------------

 The formatting style used by YAPF is configurable and there are many "knobs"
 that can be used to tune how YAPF does formatting. See the ``style.py`` module
 for the full list.

 To control the style, run YAPF with the ``--style`` argument. It accepts either
 one of the predefined styles (currently "pep8" or "google"), or a path to a
 configuration file that specifies the desired style. The file is a simple
 listing of (case-insensitive) ``key = value`` pairs with a ``[style]`` heading.
 For example::

     [style]
     based_on_style = pep8
     spaces_before_comment = 4
     split_before_logical_operator = true

 The ``based_on_style`` setting determines which of the predefined styles this
 custom style is based on (think of it like subclassing).

 Why Not Improve Existing Tools?
 ===============================

 We wanted to use clang-format's reformatting algorithm. It's very powerful and
 designed to come up with the best formatting possible. Existing tools were
 created with different goals in mind, and would require extensive modifications
 to convert to using clang-format's algorithm.

 Can I Use YAPF In My Program?
 =============================

 Please do! YAPF was designed to be used as a library as well as a command line
 tool. This means that a tool or IDE plugin is free to use YAPF.

 Gory Details
 ============

 Algorithm Design
 ----------------

 The main data structure in YAPF is the ``UnwrappedLine`` object. It holds a list
 of ``FormatToken``\s, that we would want to place on a single line if there were
 no column limit. An exception being a comment in the middle of an expression
 statement will force the line to be formatted on more than one line. The
 formatter works on one ``UnwrappedLine`` object at a time.

 An ``UnwrappedLine`` typically won't affect the formatting of lines before or
 after it. There is a part of the algorithm that may join two or more
 ``UnwrappedLine``\s into one line. For instance, an if-then statement with a
 short body can be placed on a single line::

     if a == 42: continue

 YAPF's formatting algorithm creates a weighted tree that acts as the solution
 space for the algorithm. Each node in the tree represents the result of a
 formatting decision --- i.e., whether to split or not to split before a token.
 Each formatting decision has a cost associated with it. Therefore, the cost is
 realized on the edge between two nodes. (In reality, the weighted tree doesn't
 have separate edge objects, so the cost resides on the nodes themselves.)

 For example, take the following Python code snippet. For the sake of this
 example, assume that line (1) violates the column limit restriction and needs to
 be reformatted.

 .. code-block:: python

     def xxxxxxxxxxx(aaaaaaaaaaaa, bbbbbbbbb, cccccccc, dddddddd, eeeeee):  # 1
         pass                                                               # 2

 For line (1), the algorithm will build a tree where each node (a
 ``FormattingDecisionState`` object) is the state of the line at that token given
 the decision to split before the token or not. Note: the ``FormatDecisionState``
 objects are copied by value so each node in the graph is unique and a change in
 one doesn't affect other nodes.

 Heuristics are used to determine the costs of splitting or not splitting.
 Because a node holds the state of the tree up to a token's insertion, it can
 easily determine if a splitting decision will violate one of the style
 requirements. For instance, the heuristic is able to apply an extra penalty to
 the edge when not splitting between the previous token and the one being added.

 There are some instances where we will never want to split the line, because
 doing so will always be detrimental (i.e., it will require a backslash-newline,
 which is very rarely desirable). For line (1), we will never want to split the
 first three tokens: ``def``, ``xxxxxxxxxxx``, and ``(``. Nor will we want to
 split between the ``)`` and the ``:`` at the end. These regions are said to be
 "unbreakable." This is reflected in the tree by there not being a "split"
 decision (left hand branch) within the unbreakable region.

 Now that we have the tree, we determine what the "best" formatting is by finding
 the path through the tree with the lowest cost.

 And that's it!
	====
	YAPF
	====

	.. image:: https://travis-ci.org/google/yapf.svg?branch=master
	:target: https://travis-ci.org/google/yapf
	:alt: Build status

	Introduction
	============

	Most of the current formatters for Python -- e.g., autopep8, and pep8ify -- are
	made to remove lint errors from code. This has some obvious limitations. For
	instance, code that conforms to the PEP 8 guidelines may not be reformatted.
	But it doesn't mean that the code looks good.

	YAPF takes a different approach. It's based off of 'clang-format', developed by
	Daniel Jasper. In essence, the algorithm takes the code and reformats it to the
	best formatting that conforms to the style guide, even if the original code
	didn't violate the style guide. The idea is also similar to the 'gofmt' tool for
	the Go programming language: end all holy wars about formatting - if the whole
	code base of a project is simply piped through YAPF whenever modifications are
	made, the style remains consistent throughout the project and there's no point
	arguing about style in every code review.

	The ultimate goal is that the code YAPF produces is as good as the code that a
	programmer would write if they were following the style guide.

	.. note::

	YAPF is not an official Google product (experimental or otherwise), it is
	just code that happens to be owned by Google.

	.. contents::

	Installation
	============

	YAPF supports Python 2.7 and 3.4+.

	To install YAPF from the source directory::

	$ sudo python ./setup.py install

	If you intend to use YAPF as a command-line tool rather than as a library,
	installation is not necessary. YAPF supports being run as a directory by the
	Python interpreter. If you cloned/unzipped yapf into ``DIR``, it's possible to
	run::

	$ PYTHONPATH=DIR python DIR/yapf [options] ...

	Usage
	=====

	Options::

	usage: yapf [-h] [--style STYLE] [-d \| -i] [-l START-END \| -r] ...

	Formatter for Python code.

	positional arguments:
	files

	optional arguments:
	-h, --help show this help message and exit
	--style STYLE specify formatting style: either a style name (for
	example "pep8" or "google"), or the name of a file
	with style settings
	-d, --diff print the diff for the fixed source
	-i, --in-place make changes to files in place
	-l START-END, --lines START-END
	range of lines to reformat, one-based
	-r, --recursive run recursively over directories

	Note: after reformatting a chunk of code, YAPF verifies that it's correct (can
	be parsed by Python itself). This means that if you're reformatting Python 3
	code, it's best to run YAPF itself under Python 3. The same goes for Python 2.

	Formatting style
	----------------

	The formatting style used by YAPF is configurable and there are many "knobs"
	that can be used to tune how YAPF does formatting. See the ``style.py`` module
	for the full list.

	To control the style, run YAPF with the ``--style`` argument. It accepts either
	one of the predefined styles (currently "pep8" or "google"), or a path to a
	configuration file that specifies the desired style. The file is a simple
	listing of (case-insensitive) ``key = value`` pairs with a ``[style]`` heading.
	For example::

	[style]
	based_on_style = pep8
	spaces_before_comment = 4
	split_before_logical_operator = true

	The ``based_on_style`` setting determines which of the predefined styles this
	custom style is based on (think of it like subclassing).

	Why Not Improve Existing Tools?
	===============================

	We wanted to use clang-format's reformatting algorithm. It's very powerful and
	designed to come up with the best formatting possible. Existing tools were
	created with different goals in mind, and would require extensive modifications
	to convert to using clang-format's algorithm.

	Can I Use YAPF In My Program?
	=============================

	Please do! YAPF was designed to be used as a library as well as a command line
	tool. This means that a tool or IDE plugin is free to use YAPF.

	Gory Details
	============

	Algorithm Design
	----------------

	The main data structure in YAPF is the ``UnwrappedLine`` object. It holds a list
	of ``FormatToken``\s, that we would want to place on a single line if there were
	no column limit. An exception being a comment in the middle of an expression
	statement will force the line to be formatted on more than one line. The
	formatter works on one ``UnwrappedLine`` object at a time.

	An ``UnwrappedLine`` typically won't affect the formatting of lines before or
	after it. There is a part of the algorithm that may join two or more
	``UnwrappedLine``\s into one line. For instance, an if-then statement with a
	short body can be placed on a single line::

	if a == 42: continue

	YAPF's formatting algorithm creates a weighted tree that acts as the solution
	space for the algorithm. Each node in the tree represents the result of a
	formatting decision --- i.e., whether to split or not to split before a token.
	Each formatting decision has a cost associated with it. Therefore, the cost is
	realized on the edge between two nodes. (In reality, the weighted tree doesn't
	have separate edge objects, so the cost resides on the nodes themselves.)

	For example, take the following Python code snippet. For the sake of this
	example, assume that line (1) violates the column limit restriction and needs to
	be reformatted.

	.. code-block:: python

	def xxxxxxxxxxx(aaaaaaaaaaaa, bbbbbbbbb, cccccccc, dddddddd, eeeeee): # 1
	pass # 2

	For line (1), the algorithm will build a tree where each node (a
	``FormattingDecisionState`` object) is the state of the line at that token given
	the decision to split before the token or not. Note: the ``FormatDecisionState``
	objects are copied by value so each node in the graph is unique and a change in
	one doesn't affect other nodes.

	Heuristics are used to determine the costs of splitting or not splitting.
	Because a node holds the state of the tree up to a token's insertion, it can
	easily determine if a splitting decision will violate one of the style
	requirements. For instance, the heuristic is able to apply an extra penalty to
	the edge when not splitting between the previous token and the one being added.

	There are some instances where we will never want to split the line, because
	doing so will always be detrimental (i.e., it will require a backslash-newline,
	which is very rarely desirable). For line (1), we will never want to split the
	first three tokens: ``def``, ``xxxxxxxxxxx``, and ``(``. Nor will we want to
	split between the ``)`` and the ``:`` at the end. These regions are said to be
	"unbreakable." This is reflected in the tree by there not being a "split"
	decision (left hand branch) within the unbreakable region.

	Now that we have the tree, we determine what the "best" formatting is by finding
	the path through the tree with the lowest cost.

	And that's it!