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. It takes away some of the drudgery of maintaining your code.
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To install YAPF from PyPI:
$ pip install yapfYAPF 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] ...YAPF supports Python 2.7 and 3.4.1+.
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).
Options:
usage: yapf [-h] [-v] [-d | -i] [-r | -l START-END] [-e PATTERN]
[--style STYLE] [--style-help] [--no-local-style]
[files [files ...]]
Formatter for Python code.
positional arguments:
files
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 one of the
parent directories of the source file (or current
directory for stdin)
--style-help show style settings and exit
--no-local-style don't search for local style definition (.style.yapf)
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 [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).
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:
- Specified on the command line
- In the [style] section of a .style.yapf file in either the current directory or one of its parent directories.
- In the [yapf] secionf of a setup.cfg file in either the current directory or one of its parent directories.
- 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,
'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]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]The two main APIs for calling yapf are FormatCode and FormatFile, these
share several arguments which are described below:
>>> from yapf.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
+a == bNote: 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.yapf_api import FormatFile # reformat a file
>>> print(open("foo.py").read()) # contents of file
a==b
>>> 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 == bALIGN_CLOSING_BRACKET_WITH_VISUAL_INDENT- Align closing bracket with visual indentation.
ALLOW_MULTILINE_LAMBDAS- Allow lambdas to be formatted on more than one line.
BLANK_LINE_BEFORE_NESTED_CLASS_OR_DEFInsert a blank line before a
deforclassimmediately nested within anotherdeforclass. For example:class Foo: # <------ this blank line def method(): pass
COLUMN_LIMIT- The column limit (or max line-length)
CONTINUATION_INDENT_WIDTH- Indent width used for line continuations.
DEDENT_CLOSING_BRACKETSPut 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( entity='dev3246.region1', key='dns.query_latency_tcp', transform=Transformation.AVERAGE(window=timedelta(seconds=60)), start_ts=now()-timedelta(days=3), end_ts=now(), ) # <--- this bracket is dedented and on a separate line
I18N_COMMENT- 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.
I18N_FUNCTION_CALL- The internationalization function call names. The presence of this function stops reformattting on that line, because the string it has cannot be moved away from the i18n comment.
INDENT_DICTIONARY_VALUEIndent the dictionary value if it cannot fit on the same line as the dictionary key. For example:
config = { 'key1': 'value1', 'key2': value1 + value2, }
INDENT_WIDTH- The number of columns to use for indentation.
JOIN_MULTIPLE_LINES- Join short lines into one line. E.g., single line
ifstatements. SPACES_AROUND_POWER_OPERATOR- Set to
Trueto prefer using spaces around**. SPACES_BEFORE_COMMENT- The number of spaces required before a trailing comment.
SPACE_BETWEEN_ENDING_COMMA_AND_CLOSING_BRACKET- Insert a space between the ending comma and closing bracket of a list, etc.
SPLIT_ARGUMENTS_WHEN_COMMA_TERMINATED- Split before arguments if the argument list is terminated by a comma.
SPLIT_BEFORE_BITWISE_OPERATOR- Set to
Trueto prefer splitting before&,|or^rather than after. SPLIT_BEFORE_FIRST_ARGUMENT- If an argument / parameter list is going to be split, then split before the first argument.
SPLIT_BEFORE_LOGICAL_OPERATOR- Set to
Trueto prefer splitting beforeandororrather than after. SPLIT_BEFORE_NAMED_ASSIGNS- Split named assignments onto individual lines.
SPLIT_PENALTY_AFTER_OPENING_BRACKET- The penalty for splitting right after the opening bracket.
SPLIT_PENALTY_AFTER_UNARY_OPERATOR- The penalty for splitting the line after a unary operator.
SPLIT_PENALTY_BEFORE_IF_EXPR- The penalty for splitting right before an
ifexpression. SPLIT_PENALTY_BITWISE_OPERATOR- The penalty of splitting the line around the
&,|, and^operators. SPLIT_PENALTY_EXCESS_CHARACTER- The penalty for characters over the column limit.
SPLIT_PENALTY_FOR_ADDED_LINE_SPLIT- The penalty incurred by adding a line split to the unwrapped line. The more line splits added the higher the penalty.
SPLIT_PENALTY_IMPORT_NAMESThe penalty of splitting a list of
import asnames. For example:from a_very_long_or_indented_module_name_yada_yad import (long_argument_1, long_argument_2, long_argument_3)
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)
SPLIT_PENALTY_LOGICAL_OPERATOR- The penalty of splitting the line around the
andandoroperators. USE_TABS- Use the Tab character for indentation.
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 reason for this is many-fold. But in essence 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: enableYou can also disable formatting for a single literal like this:
BAZ = {
(1, 2, 3, 4),
(5, 6, 7, 8),
(9, 10, 11, 12),
} # yapf: disableTo 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.
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.
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.
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: continueYAPF'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 # 2For 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 is not an official Google product (experimental or otherwise), it is just code that happens to be owned by Google.