Go to file
Bill Wendling a1e69adb6d Bump version to 0.27.0 2019-04-07 03:44:52 -07:00
plugins Fix bugs of handling arrays in pre-commit.sh 2019-02-24 09:32:17 +09:00
yapf Bump version to 0.27.0 2019-04-07 03:44:52 -07:00
yapftests Keep comments attached to prev objects. 2019-04-07 03:41:17 -07:00
.coveragerc Ignore some lines for coverage. 2016-06-06 01:40:21 -07:00
.editorconfig add .editorconfig; this allows contributors to automatically follow your style 2016-06-15 10:19:10 -07:00
.flake8 configure flake8 to match your style 2016-07-14 13:31:20 -07:00
.gitignore Add .coverage to .gitignore 2015-04-08 10:21:10 -07:00
.style.yapf Add support for project-local style (in .style.yapf) 2015-04-27 15:05:13 -07:00
.travis.yml Travis CI: Upgrade from Python 3.7.0a4+ to production release 2018-08-28 14:07:22 +02:00
.vimrc Force 2-space indentation for editing yapf in vim 2015-03-21 09:46:39 -07:00
AUTHORS Initial check in of YAPF 0.1. 2015-03-18 13:36:07 -07:00
CHANGELOG Bump version to 0.27.0 2019-04-07 03:44:52 -07:00
CONTRIBUTING.rst Use canonical name for snake case in contributing docs 2018-11-01 23:00:44 -07:00
HACKING.rst Bump version to v0.21.0 2018-03-22 13:16:09 -07:00
LICENSE Initial check in of YAPF 0.1. 2015-03-18 13:36:07 -07:00
MANIFEST.in initial version, include missing files in MANIFEST.in 2018-05-20 21:27:52 +02:00
README.rst Fix SPLIT_BEFORE_BITWISE_OPERATOR in README.rst 2019-02-24 23:43:31 -05:00
pylintrc tweak settings to existing style: score 9.61/10 over non-test code 2016-06-15 10:15:51 -07:00
setup.cfg Look into 'setup.cfg' to see if it has config info 2015-10-25 03:02:41 -07:00
setup.py Update to specify Python 3.6.4 2018-01-26 15:08:07 -08:00
tox.ini Add handling for unpacking with dict expressions 2016-12-29 18:10:34 +01:00



PyPI version

Build status

Coverage status


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 codebase 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. It takes away some of the drudgery of maintaining your code.

Try out YAPF with this online demo.

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


To install YAPF from PyPI:

$ pip install yapf

(optional) If you are using Python 2.7 and want to enable multiprocessing:

$ pip install futures

YAPF is still considered in "alpha" stage, and the released version may change often; therefore, the best way to keep up-to-date with the latest development is to clone this repository.

Note that 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] ...

Python versions

YAPF supports Python 2.7 and 3.6.4+. (Note that some Python 3 features may fail to parse with Python versions before 3.6.4.)

YAPF requires the code it formats to be valid Python for the version YAPF itself runs under. Therefore, if you format Python 3 code with YAPF, run YAPF itself under Python 3 (and similarly for Python 2).



usage: yapf [-h] [-v] [-d | -i] [-r | -l START-END] [-e PATTERN]
            [--style STYLE] [--style-help] [--no-local-style] [-p]
            [files [files ...]]

Formatter for Python code.

positional arguments:

optional arguments:
  -h, --help            show this help message and exit
  -v, --version         show version number and exit
  -d, --diff            print the diff for the fixed source
  -i, --in-place        make changes to files in place
  -r, --recursive       run recursively over directories
  -l START-END, --lines START-END
                        range of lines to reformat, one-based
  -e PATTERN, --exclude PATTERN
                        patterns for files to exclude from formatting
  --style STYLE         specify formatting style: either a style name (for
                        example "pep8" or "google"), or the name of a file
                        with style settings. The default is pep8 unless a
                        .style.yapf or setup.cfg file located in the same
                        directory as the source or one of its parent
                        directories (for stdin, the current directory is
  --style-help          show style settings and exit; this output can be saved
                        to .style.yapf to make your settings permanent
  --no-local-style      don't search for local style definition
  -p, --parallel        Run yapf in parallel when formatting multiple files.
                        Requires concurrent.futures in Python 2.X
  -vv, --verbose        Print out file names while processing

Return Codes

Normally YAPF returns zero on successful program termination and non-zero otherwise.

If --diff is supplied, YAPF returns zero when no changes were necessary, non-zero otherwise (including program error). You can use this in a CI workflow to test that code has been YAPF-formatted.

Excluding files from formatting (.yapfignore)

In addition to exclude patterns provided on commandline, YAPF looks for additional patterns specified in a file named .yapfignore located in the working directory from which YAPF is invoked.

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 one of the predefined styles (e.g., pep8 or google), a path to a configuration file that specifies the desired style, or a dictionary of key/value pairs.

The config file is a simple listing of (case-insensitive) key = value pairs with a [yapf] heading. For example:

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).

It's also possible to do the same on the command line with a dictionary. For example:

--style='{based_on_style: chromium, indent_width: 4}'

This will take the chromium base style and modify it to have four space indentations.

YAPF will search for the formatting style in the following manner:

  1. Specified on the command line
  2. In the [style] section of a .style.yapf file in either the current directory or one of its parent directories.
  3. In the [yapf] section of a setup.cfg file in either the current directory or one of its parent directories.
  4. In the ~/.config/yapf/style file in your home directory.

If none of those files are found, the default style is used (PEP8).


An example of the type of formatting that YAPF can do, it will take this ugly code:

x = {  'a':37,'b':42,


y = 'hello ''world'
z = 'hello '+'world'
a = 'hello {}'.format('world')
class foo  (     object  ):
  def f    (self   ):
    return       37*-+2
  def g(self, x,y=42):
      return y
def f  (   a ) :
  return      37+-+a[42-x :  y**3]

and reformat it into:

x = {'a': 37, 'b': 42, 'c': 927}

y = 'hello ' 'world'
z = 'hello ' + 'world'
a = 'hello {}'.format('world')

class foo(object):
    def f(self):
        return 37 * -+2

    def g(self, x, y=42):
        return y

def f(a):
    return 37 + -+a[42 - x:y**3]

Example as a module

The two main APIs for calling yapf are FormatCode and FormatFile, these share several arguments which are described below:

>>> from yapf.yapflib.yapf_api import FormatCode  # reformat a string of code

>>> FormatCode("f ( a = 1, b = 2 )")
'f(a=1, b=2)\n'

A style_config argument: Either a style name or a path to a file that contains formatting style settings. If None is specified, use the default style as set in style.DEFAULT_STYLE_FACTORY.

>>> FormatCode("def g():\n  return True", style_config='pep8')
'def g():\n    return True\n'

A lines argument: A list of tuples of lines (ints), [start, end], that we want to format. The lines are 1-based indexed. It can be used by third-party code (e.g., IDEs) when reformatting a snippet of code rather than a whole file.

>>> FormatCode("def g( ):\n    a=1\n    b = 2\n    return a==b", lines=[(1, 1), (2, 3)])
'def g():\n    a = 1\n    b = 2\n    return a==b\n'

A print_diff (bool): Instead of returning the reformatted source, return a diff that turns the formatted source into reformatter source.

>>> print(FormatCode("a==b", filename="foo.py", print_diff=True))
--- foo.py (original)
+++ foo.py (reformatted)
@@ -1 +1 @@
+a == b

Note: the filename argument for FormatCode is what is inserted into the diff, the default is <unknown>.

FormatFile returns reformatted code from the passed file along with its encoding:

>>> from yapf.yapflib.yapf_api import FormatFile  # reformat a file

>>> print(open("foo.py").read())  # contents of file

>>> FormatFile("foo.py")
('a == b\n', 'utf-8')

The in-place argument saves the reformatted code back to the file:

>>> FormatFile("foo.py", in_place=True)
(None, 'utf-8')

>>> print(open("foo.py").read())  # contents of file (now fixed)
a == b



Align closing bracket with visual indentation.


Allow lambdas to be formatted on more than one line.


Allow dictionary keys to exist on multiple lines. For example:

x = {
    ('this is the first element of a tuple',
     'this is the second element of a tuple'):

Allow splitting before a default / named assignment in an argument list.


Allow splits before the dictionary value.


Let spacing indicate operator precedence. For example:

a = 1 * 2 + 3 / 4
b = 1 / 2 - 3 * 4
c = (1 + 2) * (3 - 4)
d = (1 - 2) / (3 + 4)
e = 1 * 2 - 3
f = 1 + 2 + 3 + 4

will be formatted as follows to indicate precedence:

a = 1*2 + 3/4
b = 1/2 - 3*4
c = (1+2) * (3-4)
d = (1-2) / (3+4)
e = 1*2 - 3
f = 1 + 2 + 3 + 4

Insert a blank line before a def or class immediately nested within another def or class. For example:

class Foo:
                   # <------ this blank line
    def method():

Insert a blank line before a module docstring.


Insert a blank line before a class-level docstring.


Sets the number of desired blank lines surrounding top-level function and class definitions. For example:

class Foo:
                   # <------ having two blank lines here
                   # <------ is the default setting
class Bar:

Do not split consecutive brackets. Only relevant when DEDENT_CLOSING_BRACKETS is set. For example:

        'key1': 'value1',
        'key2': 'value2',

would reformat to:

    'key1': 'value1',
    'key2': 'value2',

The column limit (or max line-length)


The style for continuation alignment. Possible values are:

  • SPACE: Use spaces for continuation alignment. This is default behavior.

  • FIXED: Use fixed number (CONTINUATION_INDENT_WIDTH) of columns (ie: CONTINUATION_INDENT_WIDTH/INDENT_WIDTH tabs) for continuation alignment.

  • VALIGN-RIGHT: Vertically align continuation lines with indent characters. Slightly right (one more indent character) if cannot vertically align continuation lines with indent characters.

    For options FIXED, and VALIGN-RIGHT are only available when USE_TABS is enabled.


Indent width used for line continuations.


Put closing brackets on a separate line, dedented, if the bracketed expression can't fit in a single line. Applies to all kinds of brackets, including function definitions and calls. For example:

config = {
    'key1': 'value1',
    'key2': 'value2',
}  # <--- this bracket is dedented and on a separate line

time_series = self.remote_client.query_entity_counters(
)  # <--- this bracket is dedented and on a separate line

Disable the heuristic which places each list element on a separate line if the list is comma-terminated.


Place each dictionary entry onto its own line.


The regex for an internationalization comment. The presence of this comment stops reformatting of that line, because the comments are required to be next to the string they translate.


The internationalization function call names. The presence of this function stops reformatting on that line, because the string it has cannot be moved away from the i18n comment.


Indent the dictionary value if it cannot fit on the same line as the dictionary key. For example:

config = {
    'key2': value1 +

The number of columns to use for indentation.


Set to True to prefer indented blank lines rather than empty


Join short lines into one line. E.g., single line if statements.


Do not include spaces around selected binary operators. For example:

1 + 2 * 3 - 4 / 5

will be formatted as follows when configured with *,/:

1 + 2*3 - 4/5

Set to True to prefer using spaces around **.


Set to True to prefer spaces around the assignment operator for default or keyword arguments.


The number of spaces required before a trailing comment. This can be a single value (representing the number of spaces before each trailing comment) or list of of values (representing alignment column values; trailing comments within a block will be aligned to the first column value that is greater than the maximum line length within the block). For example:

With spaces_before_comment=5:

1 + 1 # Adding values

will be formatted as:

1 + 1     # Adding values <-- 5 spaces between the end of the statement and comment

With spaces_before_comment=15, 20:

1 + 1 # Adding values
two + two # More adding

longer_statement # This is a longer statement
short # This is a shorter statement

a_very_long_statement_that_extends_beyond_the_final_column # Comment
short # This is a shorter statement

will be formatted as:

1 + 1          # Adding values <-- end of line comments in block aligned to col 15
two + two      # More adding

longer_statement    # This is a longer statement <-- end of line comments in block aligned to col 20
short               # This is a shorter statement

a_very_long_statement_that_extends_beyond_the_final_column  # Comment <-- the end of line comments are aligned based on the line length
short                                                       # This is a shorter statement

Insert a space between the ending comma and closing bracket of a list, etc.


Split before arguments if the argument list is terminated by a comma.


If a comma separated list (dict, list, tuple, or function def) is on a line that is too long, split such that all elements are on a single line.


Set to True to prefer splitting before '&', '|' or '^' rather than after.


Set to True to prefer splitting before '+', '-', '*', '/', '//', or '@' rather than after.


Split before the closing bracket if a list or dict literal doesn't fit on a single line.


Split before a dictionary or set generator (comp_for). For example, note the split before the for:

foo = {
    variable: 'Hello world, have a nice day!'
    for variable in bar if variable != 42

Split before the '.' if we need to split a longer expression:

foo = ('This is a really long string: {}, {}, {}, {}'.format(a, b, c, d))

would reformat to something like:

foo = ('This is a really long string: {}, {}, {}, {}'
       .format(a, b, c, d))

Split after the opening paren which surrounds an expression if it doesn't fit on a single line.


If an argument / parameter list is going to be split, then split before the first argument.


Set to True to prefer splitting before and or or rather than after.


Split named assignments onto individual lines.


For list comprehensions and generator expressions with multiple clauses (e.g multiple "for" calls, "if" filter expressions) and which need to be reflowed, split each clause onto its own line. For example:

result = [
    a_var + b_var for a_var in xrange(1000) for b_var in xrange(1000)
    if a_var % b_var]

would reformat to something like:

result = [
    a_var + b_var
    for a_var in xrange(1000)
    for b_var in xrange(1000)
    if a_var % b_var]

The penalty for splitting right after the opening bracket.


The penalty for splitting the line after a unary operator.


The penalty of splitting the line around the +, -, *, /, //, %, and @ operators.


The penalty for splitting right before an if expression.


The penalty of splitting the line around the &, |, and ^ operators.


The penalty for splitting a list comprehension or generator expression.


The penalty for characters over the column limit.


The penalty incurred by adding a line split to the unwrapped line. The more line splits added the higher the penalty.


The penalty of splitting a list of import as names. For example:

from a_very_long_or_indented_module_name_yada_yad import (long_argument_1,

would reformat to something like:

from a_very_long_or_indented_module_name_yada_yad import (
    long_argument_1, long_argument_2, long_argument_3)

The penalty of splitting the line around the and and or operators.


Use the Tab character for indentation.

(Potentially) Frequently Asked Questions

Why does YAPF destroy my awesome formatting?

YAPF tries very hard to get the formatting correct. But for some code, it won't be as good as hand-formatting. In particular, large data literals may become horribly disfigured under YAPF.

The reasons for this are manyfold. In short, YAPF is simply a tool to help with development. It will format things to coincide with the style guide, but that may not equate with readability.

What can be done to alleviate this situation is to indicate regions YAPF should ignore when reformatting something:

# yapf: disable
FOO = {
    # ... some very large, complex data literal.

BAR = [
    # ... another large data literal.
# yapf: enable

You can also disable formatting for a single literal like this:

BAZ = {
    (1, 2, 3, 4),
    (5, 6, 7, 8),
    (9, 10, 11, 12),
}  # yapf: disable

To preserve the nice dedented closing brackets, use the dedent_closing_brackets in your style. Note that in this case all brackets, including function definitions and calls, are going to use that style. This provides consistency across the formatted codebase.

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.

I still get non Pep8 compliant code! Why?

YAPF tries very hard to be fully PEP 8 compliant. However, it is paramount to not risk altering the semantics of your code. Thus, YAPF tries to be as safe as possible and does not change the token stream (e.g., by adding parenthesis). All these cases however, can be easily fixed manually. For instance,

from my_package import my_function_1, my_function_2, my_function_3, my_function_4, my_function_5

FOO = my_variable_1 + my_variable_2 + my_variable_3 + my_variable_4 + my_variable_5 + my_variable_6 + my_variable_7 + my_variable_8

won't be split, but you can easily get it right by just adding parenthesis:

from my_package import (my_function_1, my_function_2, my_function_3,
                        my_function_4, my_function_5)

FOO = (my_variable_1 + my_variable_2 + my_variable_3 + my_variable_4 +
       my_variable_5 + my_variable_6 + my_variable_7 + my_variable_8)

Gory Details

Algorithm Design

The main data structure in YAPF is the UnwrappedLine object. It holds a list of FormatTokens, 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 UnwrappedLines 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.

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!