Black is the uncompromising Python code formatter. By using it, you agree to cede control over minutiae of hand-formatting. In return, Black gives you speed, determinism, and freedom from pycodestyle nagging about formatting. You will save time and mental energy for more important matters.
Blackened code looks the same regardless of the project you're reading. Formatting becomes transparent after a while and you can focus on the content instead.
Black makes code review faster by producing the smallest diffs possible.
Try it out now using the Black Playground. Watch the PyCon 2019 talk to learn more.
Installation and usage
Black can be installed by running
pip install black. It requires
Python 3.6.0+ to run but you can reformat Python 2 code with it, too.
To get started right away with sensible defaults:
Command line options
Black doesn't provide many options. You can list them by running
black [OPTIONS] [SRC]... Options: -c, --code TEXT Format the code passed in as a string. -l, --line-length INTEGER How many characters per line to allow. [default: 88] -t, --target-version [py27|py33|py34|py35|py36|py37|py38] Python versions that should be supported by Black's output. [default: per-file auto- detection] --py36 Allow using Python 3.6-only syntax on all input files. This will put trailing commas in function signatures and calls also after *args and **kwargs. Deprecated; use --target-version instead. [default: per-file auto-detection] --pyi Format all input files like typing stubs regardless of file extension (useful when piping source on standard input). -S, --skip-string-normalization Don't normalize string quotes or prefixes. --check Don't write the files back, just return the status. Return code 0 means nothing would change. Return code 1 means some files would be reformatted. Return code 123 means there was an internal error. --diff Don't write the files back, just output a diff for each file on stdout. --fast / --safe If --fast given, skip temporary sanity checks. [default: --safe] --include TEXT A regular expression that matches files and directories that should be included on recursive searches. An empty value means all files are included regardless of the name. Use forward slashes for directories on all platforms (Windows, too). Exclusions are calculated first, inclusions later. [default: \.pyi?$] --exclude TEXT A regular expression that matches files and directories that should be excluded on recursive searches. An empty value means no paths are excluded. Use forward slashes for directories on all platforms (Windows, too). Exclusions are calculated first, inclusions later. [default: /(\.eggs|\.git|\.hg|\.mypy _cache|\.nox|\.tox|\.venv|_build|buck- out|build|dist)/] -q, --quiet Don't emit non-error messages to stderr. Errors are still emitted, silence those with 2>/dev/null. -v, --verbose Also emit messages to stderr about files that were not changed or were ignored due to --exclude=. --version Show the version and exit. --config PATH Read configuration from PATH. -h, --help Show this message and exit.
Black is a well-behaved Unix-style command-line tool:
- it does nothing if no sources are passed to it;
- it will read from standard input and write to standard output if
is used as the filename;
- it only outputs messages to users on standard error;
- exits with code 0 unless an internal error occurred (or
NOTE: This is a beta product
Black is already successfully used by many projects, small and big.
It also sports a decent test suite. However, it is still very new.
Things will probably be wonky for a while. This is made explicit by the
"Beta" trove classifier, as well as by the "b" in the version number.
What this means for you is that until the formatter becomes stable,
you should expect some formatting to change in the future. That being
said, no drastic stylistic changes are planned, mostly responses to bug
Also, as a temporary safety measure, Black will check that the
reformatted code still produces a valid AST that is equivalent to the
original. This slows it down. If you're feeling confident, use
The Black code style
Black reformats entire files in place. It is not configurable. It
doesn't take previous formatting into account. It doesn't reformat
blocks that start with
# fmt: off and end with
# fmt: on.
# fmt: on/off
have to be on the same level of indentation. It also
recognizes YAPF's block comments to
the same effect, as a courtesy for straddling code.
How Black wraps lines
Black ignores previous formatting and applies uniform horizontal
and vertical whitespace to your code. The rules for horizontal
whitespace can be summarized as: do whatever makes
The coding style used by Black can be viewed as a strict subset of
As for vertical whitespace, Black tries to render one full expression
or simple statement per line. If this fits the allotted line length,
# in: j = [1, 2, 3, ] # out: j = [1, 2, 3]
If not, Black will look at the contents of the first outer matching
brackets and put that in a separate indented line.
# in: ImportantClass.important_method(exc, limit, lookup_lines, capture_locals, extra_argument) # out: ImportantClass.important_method( exc, limit, lookup_lines, capture_locals, extra_argument )
If that still doesn't fit the bill, it will decompose the internal
expression further using the same rule, indenting matching brackets
every time. If the contents of the matching brackets pair are
comma-separated (like an argument list, or a dict literal, and so on)
then Black will first try to keep them on the same line with the
matching brackets. If that doesn't work, it will put all of them in
# in: def very_important_function(template: str, *variables, file: os.PathLike, engine: str, header: bool = True, debug: bool = False): """Applies `variables` to the `template` and writes to `file`.""" with open(file, 'w') as f: ... # out: def very_important_function( template: str, *variables, file: os.PathLike, engine: str, header: bool = True, debug: bool = False, ): """Applies `variables` to the `template` and writes to `file`.""" with open(file, "w") as f: ...
You might have noticed that closing brackets are always dedented and
that a trailing comma is always added. Such formatting produces smaller
diffs; when you add or remove an element, it's always just one line.
Also, having the closing bracket dedented provides a clear delimiter
between two distinct sections of the code that otherwise share the same
indentation level (like the arguments list and the docstring in the
If a data structure literal (tuple, list, set, dict) or a line of "from"
imports cannot fit in the allotted length, it's always split into one
element per line. This minimizes diffs as well as enables readers of
code to find which commit introduced a particular entry. This also
makes Black compatible with isort with
the following configuration.
A compatible `.isort.cfg`
[settings] multi_line_output=3 include_trailing_comma=True force_grid_wrap=0 use_parentheses=True line_length=88
The equivalent command line is:
$ isort --multi-line=3 --trailing-comma --force-grid-wrap=0 --use-parentheses --line-width=88 [ file.py ]
You probably noticed the peculiar default line length. Black defaults
to 88 characters per line, which happens to be 10% over 80. This number
was found to produce significantly shorter files than sticking with 80
(the most popular), or even 79 (used by the standard library). In
general, 90-ish seems like the wise choice.
If you're paid by the line of code you write, you can pass
--line-length with a lower number. Black will try to respect that.
However, sometimes it won't be able to without breaking other rules. In
those rare cases, auto-formatted code will exceed your allotted limit.
You can also increase it, but remember that people with sight disabilities
find it harder to work with line lengths exceeding 100 characters.
It also adversely affects side-by-side diff review on typical screen
resolutions. Long lines also make it harder to present code neatly
in documentation or talk slides.
If you're using Flake8, you can bump
max-line-length to 88 and forget
about it. Alternatively, use Bugbear's
B950 warning instead of E501 and keep the max line length at 80 which
you are probably already using. You'd do it like this:
[flake8] max-line-length = 80 ... select = C,E,F,W,B,B950 ignore = E501,W503,E203
You'll find Black's own .flake8 config file is configured like this.
If you're curious about the reasoning behind B950,
explains it. The tl;dr is "it's like highway speed limits, we won't
bother you if you overdo it by a few km/h".
Black avoids spurious vertical whitespace. This is in the spirit of
PEP 8 which says that in-function vertical whitespace should only be
Black will allow single empty lines inside functions, and single and
double empty lines on module level left by the original editors, except
when they're within parenthesized expressions. Since such expressions
are always reformatted to fit minimal space, this whitespace is lost.
It will also insert proper spacing before and after function definitions.
It's one line before and after inner functions and two lines before and
after module-level functions and classes. Black will not put empty
lines between function/class definitions and standalone comments that
immediately precede the given function/class.
Black will enforce single empty lines between a class-level docstring
and the first following field or method. This conforms to
Black won't insert empty lines after function docstrings unless that
empty line is required due to an inner function starting immediately
Black will add trailing commas to expressions that are split
by comma where each element is on its own line. This includes function
Unnecessary trailing commas are removed if an expression fits in one
line. This makes it 1% more likely that your line won't exceed the
allotted line length limit. Moreover, in this scenario, if you added
another argument to your call, you'd probably fit it in the same line
anyway. That doesn't make diffs any larger.
One exception to removing trailing commas is tuple expressions with
just one element. In this case Black won't touch the single trailing
comma as this would unexpectedly change the underlying data type. Note
that this is also the case when commas are used while indexing. This is
a tuple in disguise:
One exception to adding trailing commas is function signatures
**kwargs. In this case a trailing comma
is only safe to use on Python 3.6. Black will detect if your file is
already 3.6+ only and use trailing commas in this situation. If you
wonder how it knows, it looks for f-strings and existing use of trailing
commas in function signatures that have stars in them. In other words,
if you'd like a trailing comma in this situation and Black didn't
recognize it was safe to do so, put it there manually and Black will
Black prefers double quotes (
""") over single quotes (
'''). It will replace the latter with the former as long as it
does not result in more backslash escapes than before.
Black also standardizes string prefixes, making them always lowercase.
On top of that, if your code is already Python 3.6+ only or it's using
unicode_literals future import, Black will remove
u from the
string prefix as it is meaningless in those scenarios.
The main reason to standardize on a single form of quotes is aesthetics.
Having one kind of quotes everywhere reduces reader distraction.
It will also enable a future version of Black to merge consecutive
string literals that ended up on the same line (see
#26 for details).
Why settle on double quotes? They anticipate apostrophes in English
text. They match the docstring standard described in PEP 257.
An empty string in double quotes (
"") is impossible to confuse with
a one double-quote regardless of fonts and syntax highlighting used.
On top of this, double quotes for strings are consistent with C which
Python interacts a lot with.
On certain keyboard layouts like US English, typing single quotes is
a bit easier than double quotes. The latter requires use of the Shift
key. My recommendation here is to keep using whatever is faster to type
and let Black handle the transformation.
If you are adopting Black in a large project with pre-existing string
conventions (like the popular "single quotes for data, double quotes for
human-readable strings"), you can
--skip-string-normalization on the command line. This is meant as
an adoption helper, avoid using this for new projects.
Black standardizes most numeric literals to use lowercase letters for the
syntactic parts and uppercase letters for the digits themselves:
1e10 instead of
1E10. Python 2 long literals are
2L instead of
2l to avoid confusion between
Line breaks & binary operators
Black will break a line before a binary operator when splitting a block
of code over multiple lines. This is so that Black is compliant with the
recent changes in the PEP 8
style guide, which emphasizes that this approach improves readability.
This behaviour may raise
W503 line break before binary operator warnings in
style guide enforcement tools like Flake8. Since
W503 is not PEP 8 compliant,
you should tell Flake8 to ignore these warnings.
PEP 8 recommends
: in slices as a binary operator with the lowest priority, and to
leave an equal amount of space on either side, except if a parameter is omitted
ham[1 + 1 :]). It also states that for extended slices, both
operators have to have the same amount of spacing, except if a parameter is
ham[1 + 1 ::]). Black enforces these rules consistently.
This behaviour may raise
E203 whitespace before ':' warnings in style guide
enforcement tools like Flake8. Since
E203 is not PEP 8 compliant, you should
tell Flake8 to ignore these warnings.
Some parentheses are optional in the Python grammar. Any expression can
be wrapped in a pair of parentheses to form an atom. There are a few
for (...) in (...):
assert (...), (...)
from X import (...)
- assignments like:
target = (...)
target: type = (...)
some, *un, packing = (...)
augmented += (...)
In those cases, parentheses are removed when the entire statement fits
in one line, or if the inner expression doesn't have any delimiters to
further split on. If there is only a single delimiter and the expression
starts or ends with a bracket, the parenthesis can also be successfully
omitted since the existing bracket pair will organize the expression
neatly anyway. Otherwise, the parentheses are added.
Please note that Black does not add or remove any additional nested
parentheses that you might want to have for clarity or further
code organization. For example those parentheses are not going to be
return not (this or that) decision = (maybe.this() and values > 0) or (maybe.that() and values < 0)
Some popular APIs, like ORMs, use call chaining. This API style is known
as a fluent interface.
Black formats those by treating dots that follow a call or an indexing
operation like a very low priority delimiter. It's easier to show the
behavior than to explain it. Look at the example:
def example(session): result = ( session.query(models.Customer.id) .filter( models.Customer.account_id == account_id, models.Customer.email == email_address, ) .order_by(models.Customer.id.asc()) .all() )
Typing stub files
PEP 484 describes the syntax for type hints in Python. One of the
use cases for typing is providing type annotations for modules which
cannot contain them directly (they might be written in C, or they might
be third-party, or their implementation may be overly dynamic, and so on).
To solve this, stub files with the
extension can be
used to describe typing information for an external module. Those stub
files omit the implementation of classes and functions they
describe, instead they only contain the structure of the file (listing
globals, functions, and classes with their members). The recommended
code style for those files is more terse than PEP 8:
...on the same line as the class/function signature;
- avoid vertical whitespace between consecutive module-level functions,
names, or methods and fields within a single class;
- use a single blank line between top-level class definitions, or none
if the classes are very small.
Black enforces the above rules. There are additional guidelines for
.pyi file that are not enforced yet but might be in
a future version of the formatter:
- all function bodies should be empty (contain
...instead of the body);
- do not use docstrings;
- for arguments with a default, use
...instead of the actual default;
- avoid using string literals in type annotations, stub files support
forward references natively (like Python 3.7 code with
from __future__ import annotations);
- use variable annotations instead of type comments, even for stubs that
target older versions of Python;
- for arguments that default to
Black is able to read project-specific default values for its
command line options from a
pyproject.toml file. This is
especially useful for specifying custom
patterns for your project.
Pro-tip: If you're asking yourself "Do I need to configure anything?"
the answer is "No". Black is all about sensible defaults.
What on Earth is a
PEP 518 defines
pyproject.toml as a configuration file to store build system
requirements for Python projects. With the help of tools
like Poetry or
Flit it can fully replace the
Where Black looks for the file
By default Black looks for
pyproject.toml starting from the common
base directory of all files and directories passed on the command line.
If it's not there, it looks in parent directories. It stops looking
when it finds the file, or a
.git directory, or a
or the root of the file system, whichever comes first.
If you're formatting standard input, Black will look for configuration
starting from the current working directory.
You can also explicitly specify the path to a particular file that you
--config. In this situation Black will not look for any
If you're running with
--verbose, you will see a blue message if
a file was found and used.
blackd will not use
As the file extension suggests,
pyproject.toml is a TOML file. It contains separate
sections for different tools. Black is using the
section. The option keys are the same as long names of options on
the command line.
Note that you have to use single-quoted strings in TOML for regular
expressions. It's the equivalent of r-strings in Python. Multiline
strings are treated as verbose regular expressions by Black. Use
to denote a significant space character.
[tool.black] line-length = 88 target-version = ['py37'] include = '\.pyi?$' exclude = ''' ( /( \.eggs # exclude a few common directories in the | \.git # root of the project | \.hg | \.mypy_cache | \.tox | \.venv | _build | buck-out | build | dist )/ | foo.py # also separately exclude a file named foo.py in # the root of the project ) '''
Command-line options have defaults that you can see in
pyproject.toml can override those defaults. Finally, options
provided by the user on the command line override both.
Black will only ever use one
pyproject.toml file during an entire
run. It doesn't look for multiple files, and doesn't compose
configuration from different levels of the file hierarchy.
$ pip install black
- Locate your
On macOS / Linux / BSD:
$ which black /usr/local/bin/black # possible location
$ where black %LocalAppData%\Programs\Python\Python36-32\Scripts\black.exe # possible location
- Open External tools in PyCharm/IntelliJ IDEA
PyCharm -> Preferences -> Tools -> External Tools
On Windows / Linux / BSD:
File -> Settings -> Tools -> External Tools
Click the + icon to add a new external tool with the following values:
- Name: Black
- Description: Black is the uncompromising Python code formatter.
- Program: <install_location_from_step_2>
Format the currently opened file by selecting
Tools -> External Tools -> black.
- Alternatively, you can set a keyboard shortcut by navigating to
Preferences or Settings -> Keymap -> External Tools -> External Tools - Black.
- Alternatively, you can set a keyboard shortcut by navigating to
Optionally, run Black on every file save:
- Make sure you have the File Watcher plugin installed.
- Go to
Preferences or Settings -> Tools -> File Watchersand click
+to add a new watcher:
- Name: Black
- File type: Python
- Scope: Project Files
- Program: <install_location_from_step_2>
- Output paths to refresh:
- Working directory:
- Uncheck "Auto-save edited files to trigger the watcher"
Wing supports black via the OS Commands tool, as explained in the Wing documentation on pep8 formatting. The detailed procedure is:
$ pip install black
- Make sure it runs from the command line, e.g.
$ black --help
- In Wing IDE, activate the OS Commands panel and define the command black to execute black on the currently selected file:
- Use the Tools -> OS Commands menu selection
- click on + in OS Commands -> New: Command line..
- Title: black
- Command Line: black %s
- I/O Encoding: Use Default
- Key Binding: F1
- [x] Raise OS Commands when executed
- [x] Auto-save files before execution
- [x] Line mode
- Select a file in the editor and press F1 , or whatever key binding you selected in step 3, to reformat the file.
Commands and shortcuts:
:Blackto format the entire file (ranges not supported);
:BlackUpgradeto upgrade Black inside the virtualenv;
:BlackVersionto get the current version of Black inside the
To install with vim-plug:
or with Vundle:
or you can copy the plugin from plugin/black.vim.
Let me know if this requires any changes to work with Vim 8's builtin
packadd, or Pathogen, and so on.
This plugin requires Vim 7.0+ built with Python 3.6+ support. It
needs Python 3.6 to be able to run Black inside the Vim process which
is much faster than calling an external command.
On first run, the plugin creates its own virtualenv using the right
Python version and automatically installs Black. You can upgrade it later
:BlackUpgrade and restarting Vim.
If you need to do anything special to make your virtualenv work and
install Black (for example you want to run a version from master),
create a virtualenv manually and point
g:black_virtualenv to it.
The plugin will use it.
To run Black on save, add the following line to
autocmd BufWritePre *.py execute ':Black'
How to get Vim with Python 3.6?
On Ubuntu 17.10 Vim comes with Python 3.6 by default.
On macOS with Homebrew run:
brew install vim --with-python3.
When building Vim from source, use:
./configure --enable-python3interp=yes. There's many guides online how
to do this.
Visual Studio Code
Use sublack plugin.
Jupyter Notebook Magic
Python Language Server
Other editors will require external contributions.
Patches welcome! ✨ 🍰 ✨
Any tool that can pipe code through Black using its stdio mode (just
- as the file name).
The formatted code will be returned on stdout (unless
passed). Black will still emit messages on stderr but that shouldn't
affect your use case.
This can be used for example with PyCharm's or IntelliJ's File Watchers.
blackd is a small HTTP server that exposes Black's functionality over
a simple protocol. The main benefit of using it is to avoid paying the
cost of starting up a new Black process every time you want to blacken
blackd is not packaged alongside Black by default because it has additional
dependencies. You will need to do
pip install black[d] to install it.
You can start the server on the default port, binding only to the local interface
blackd. You will see a single line mentioning the server's version,
and the host and port it's listening on.
blackd will then print an access log
similar to most web servers on standard output, merged with any exception traces
caused by invalid formatting requests.
blackd provides even less options than Black. You can see them by running
Usage: blackd [OPTIONS] Options: --bind-host TEXT Address to bind the server to. --bind-port INTEGER Port to listen on --version Show the version and exit. -h, --help Show this message and exit.
There is no official blackd client tool (yet!). You can test that blackd is
blackd --bind-port 9090 & # or let blackd choose a port curl -s -XPOST "localhost:9090" -d "print('valid')"
blackd only accepts
POST requests at the
/ path. The body of the request
should contain the python source code to be formatted, encoded
according to the
charset field in the
Content-Type request header. If no
charset is specified,
There are a few HTTP headers that control how the source is formatted. These
correspond to command line flags for Black. There is one exception to this:
X-Protocol-Version which if present, should have the value
1, otherwise the
request is rejected with
HTTP 501 (Not Implemented).
The headers controlling how code is formatted are:
X-Line-Length: corresponds to the
--line-lengthcommand line flag.
X-Skip-String-Normalization: corresponds to the
command line flag. If present and its value is not the empty string, no string
normalization will be performed.
X-Fast-Or-Safe: if set to
blackdwill act as Black does when
--fastcommand line flag.
X-Python-Variant: if set to
blackdwill act as Black does when
--pyicommand line flag. Otherwise, its value must correspond to
a Python version or a set of comma-separated Python versions, optionally
py. For example, to request code that is compatible
with Python 3.5 and 3.6, set the header to
If any of these headers are set to invalid values,
blackd returns a
error response, mentioning the name of the problematic header in the message body.
Apart from the above,
blackd can produce the following response codes:
HTTP 204: If the input is already well-formatted. The response body is
HTTP 200: If formatting was needed on the input. The response body
contains the blackened Python code, and the
Content-Typeheader is set
HTTP 400: If the input contains a syntax error. Details of the error are
returned in the response body.
HTTP 500: If there was any kind of error while trying to format the input.
The response body contains a textual representation of the error.
Version control integration
repos: - repo: https://github.com/psf/black rev: stable hooks: - id: black language_version: python3.6
pre-commit install and you're ready to go.
args in the hook. Instead, store necessary configuration
pyproject.toml so that editors and command-line usage of Black all
behave consistently for your project. See Black's own pyproject.toml
for an example.
If you're already using Python 3.7, switch the
stable is a tag that is pinned to the latest
release on PyPI. If you'd rather run on master, this is also an option.
Ignoring unmodified files
Black remembers files it has already formatted, unless the
--diff flag is used or
code is passed via standard input. This information is stored per-user. The exact
location of the file depends on the Black version and the system on which Black
is run. The file is non-portable. The standard location on common operating systems
file-mode is an int flag that determines whether the file was formatted as 3.6+ only,
as .pyi, and whether string normalization was omitted.
To override the location of these files on macOS or Linux, set the environment variable
XDG_CACHE_HOME to your preferred location. For example, if you want to put the cache in
the directory you're running Black from, set
XDG_CACHE_HOME=.cache. Black will then
write the above files to
The following notable open-source projects trust Black with enforcing
a consistent code style: pytest, tox, Pyramid, Django Channels, Hypothesis,
attrs, SQLAlchemy, Poetry, PyPA applications (Warehouse, Pipenv, virtualenv),
every Datadog Agent Integration.
Are we missing anyone? Let us know.
Dusty Phillips, writer:
Black is opinionated so you don't have to be.
Hynek Schlawack, creator of
developer of Twisted and CPython:
An auto-formatter that doesn't suck is all I want for Xmas!
Carl Meyer, Django core developer:
At least the name is good.
This vastly improves the formatting of our code. Thanks a ton!
Show your style
Use the badge in your project's README.md:
[![Code style: black](https://img.shields.io/badge/code%20style-black-000000.svg)](https://github.com/psf/black)
Using the badge in README.rst:
.. image:: https://img.shields.io/badge/code%20style-black-000000.svg :target: https://github.com/psf/black