black 18.5b1

The Uncompromising Code Formatter

“Any color you like.”

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.

Contents: Installation and usage | The Black code style | Editor integration | Version control integration | Ignoring unmodified files | Testimonials | Show your style | License | Contributing | Change Log | Authors

Installation and usage

Installation

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.

Usage

To get started right away with sensible defaults:

black {source_file_or_directory}

Command line options

Black doesn't provide many options. You can list them by running black --help:

black [OPTIONS] [SRC]...

Options:
  -l, --line-length INTEGER   Where to wrap around.  [default: 88]
  --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]
  -q, --quiet                 Don't emit non-error messages to stderr. Errors
                              are still emitted, silence those with
                              2>/dev/null.
  --pyi                       Consider all input files typing stubs regardless
                              of file extension (useful when piping source on
                              standard input).
  --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.  [default: per-file auto-detection]
  --version                   Show the version and exit.
  --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 --check was used).

NOTE: This is a beta product

Black is already successfully used by several 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 reports.

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 --fast.

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. 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 pycodestyle happy. The coding style used by Black can be viewed as a strict subset of PEP 8.

As for vertical whitespace, Black tries to render one full expression or simple statement per line. If this fits the allotted line length, great.

# in:

l = [1,
     2,
     3,
]

# out:

l = [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:

TracebackException.from_exception(exc, limit, lookup_lines, capture_locals)

# out:

TracebackException.from_exception(
    exc, limit, lookup_lines, capture_locals
)

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 separate lines.

# in:

def very_important_function(template: str, *variables, file: os.PathLike, 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,
    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 example above).

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.

[settings]
multi_line_output=3
include_trailing_comma=True
force_grid_wrap=0
combine_as_imports=True
line_length=88

$ isort --multi-line=3 --trailing-comma --force-grid-wrap=0 --combine-as --line-width=88 [ file.py ]

Line length

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

You'll find Black's own .flake8 config file is configured like this. If you're curious about the reasoning behind B950, Bugbear's documentation 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".

Empty lines

Black avoids spurious vertical whitespace. This is in the spirit of PEP 8 which says that in-function vertical whitespace should only be used sparingly.

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 PEP 257.

Black won't insert empty lines after function docstrings unless that empty line is required due to an inner function starting immediately after.

Trailing commas

Black will add trailing commas to expressions that are split by comma where each element is on its own line. This includes function signatures.

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: numpy_array[3, ].

One exception to adding trailing commas is function signatures containing *, *args, or **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 keep it.

Strings

Black prefers double quotes (" and """) over single quotes (' and '''). 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 the 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.

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.

Slices

PEP 8 recommends to treat : 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 (e.g. 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 omitted (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.

Parentheses

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 interesting cases:

• if (...):
• while (...):
• 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 removed:

return not (this or that)
decision = (maybe.this() and values > 0) or (maybe.that() and values < 0)

Call chains

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 .pyi file 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:

• prefer ... 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 formatting .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;
• prefer ... over pass;
• 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 None, use Optional[] explicitly;
• use float instead of Union[int, float].

Editor integration

Emacs

Use proofit404/blacken.

PyCharm

1. Install black.

$ pip install black

2. Locate your black installation folder.

On macOS / Linux / BSD:

$ which black
/usr/local/bin/black  # possible location

On Windows:

$ where black
%LocalAppData%\Programs\Python\Python36-32\Scripts\black.exe  # possible location

3. Open External tools in PyCharm with File -> Settings -> Tools -> External Tools.

4. 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>
   • Arguments: $FilePath$

5. Format the currently opened file by selecting Tools -> External Tools -> black.

   • Alternatively, you can set a keyboard shortcut by navigating to Preferences -> Keymap.

Vim

Commands and shortcuts:

• ,= or :Black to format the entire file (ranges not supported);
• :BlackUpgrade to upgrade Black inside the virtualenv;
• :BlackVersion to get the current version of Black inside the virtualenv.

Configuration:

• g:black_fast (defaults to 0)
• g:black_linelength (defaults to 88)
• g:black_virtualenv (defaults to ~/.vim/black)

To install with vim-plug:

Plug 'ambv/black',

or with Vundle:

Plugin 'ambv/black'

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 by calling :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 .vimrc or init.vim:

autocmd BufWritePost *.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 joslarson.black-vscode.

SublimeText 3

Use sublack plugin.

IPython Notebook Magic

Use blackcellmagic.

Other editors

Atom/Nuclide integration is planned by the author, others will require external contributions.

Patches welcome! ✨ 🍰 ✨

Any tool that can pipe code through Black using its stdio mode (just use - as the file name). The formatted code will be returned on stdout (unless --check was 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 File Watchers.

Version control integration

Use pre-commit. Once you have it installed, add this to the .pre-commit-config.yaml in your repository:

repos:
-   repo: https://github.com/ambv/black
    rev: stable
    hooks:
    - id: black
      args: [--line-length=88, --safe]
      python_version: python3.6

Then run pre-commit install and you're ready to go.

args in the above config is optional but shows you how you can change the line length if you really need to. If you're already using Python 3.7, switch the python_version accordingly. Finally, 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 is:

• Windows: C:\\Users\<username>\AppData\Local\black\black\Cache\<version>\cache.<line-length>.pickle
• macOS: /Users/<username>/Library/Caches/black/<version>/cache.<line-length>.pickle
• Linux: /home/<username>/.cache/black/<version>/cache.<line-length>.pickle

Testimonials

Dusty Phillips, writer:

Black is opinionated so you don't have to be.

Hynek Schlawack, creator of attrs, core 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.

Kenneth Reitz, creator of requests and pipenv:

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/ambv/black)

Looks like this:

License

MIT

Contributing to Black

In terms of inspiration, Black is about as configurable as gofmt. This is deliberate.

Bug reports and fixes are always welcome! However, before you suggest a new feature or configuration knob, ask yourself why you want it. If it enables better integration with some workflow, fixes an inconsistency, speeds things up, and so on - go for it! On the other hand, if your answer is "because I don't like a particular formatting" then you're not ready to embrace Black yet. Such changes are unlikely to get accepted. You can still try but prepare to be disappointed.

More details can be found in CONTRIBUTING.

Change Log

18.5b1

• added --pyi (#249)

• added --py36 (#249)

• Python grammar pickle caches are stored with the formatting caches, making Black work in environments where site-packages is not user-writable (#192)

• Black now enforces a PEP 257 empty line after a class-level docstring (and/or fields) and the first method

• fixed invalid code produced when standalone comments were present in a trailer that was omitted from line splitting on a large expression (#237)

• fixed optional parentheses being removed within # fmt: off sections (#224)

• fixed invalid code produced when stars in very long imports were incorrectly wrapped in optional parentheses (#234)

• fixed unstable formatting when inline comments were moved around in a trailer that was omitted from line splitting on a large expression (#238)

• fixed extra empty line between a class declaration and the first method if no class docstring or fields are present (#219)

• fixed extra empty line between a function signature and an inner function or inner class (#196)

18.5b0

• call chains are now formatted according to the fluent interfaces style (#67)

• data structure literals (tuples, lists, dictionaries, and sets) are now also always exploded like imports when they don't fit in a single line (#152)

• slices are now formatted according to PEP 8 (#178)

• parentheses are now also managed automatically on the right-hand side of assignments and return statements (#140)

• math operators now use their respective priorities for delimiting multiline expressions (#148)

• optional parentheses are now omitted on expressions that start or end with a bracket and only contain a single operator (#177)

• empty parentheses in a class definition are now removed (#145, #180)

• string prefixes are now standardized to lowercase and u is removed on Python 3.6+ only code and Python 2.7+ code with the unicode_literals future import (#188, #198, #199)

• typing stub files (.pyi) are now formatted in a style that is consistent with PEP 484 (#207, #210)

• progress when reformatting many files is now reported incrementally

• fixed trailers (content with brackets) being unnecessarily exploded into their own lines after a dedented closing bracket (#119)

• fixed an invalid trailing comma sometimes left in imports (#185)

• fixed non-deterministic formatting when multiple pairs of removable parentheses were used (#183)

• fixed multiline strings being unnecessarily wrapped in optional parentheses in long assignments (#215)

• fixed not splitting long from-imports with only a single name

• fixed Python 3.6+ file discovery by also looking at function calls with unpacking. This fixed non-deterministic formatting if trailing commas where used both in function signatures with stars and function calls with stars but the former would be reformatted to a single line.

• fixed crash on dealing with optional parentheses (#193)

• fixed "is", "is not", "in", and "not in" not considered operators for splitting purposes

• fixed crash when dead symlinks where encountered

18.4a4

• don't populate the cache on --check (#175)

18.4a3

• added a "cache"; files already reformatted that haven't changed on disk won't be reformatted again (#109)

• --check and --diff are no longer mutually exclusive (#149)

• generalized star expression handling, including double stars; this fixes multiplication making expressions "unsafe" for trailing commas (#132)

• Black no longer enforces putting empty lines behind control flow statements (#90)

• Black now splits imports like "Mode 3 + trailing comma" of isort (#127)

• fixed comment indentation when a standalone comment closes a block (#16, #32)

• fixed standalone comments receiving extra empty lines if immediately preceding a class, def, or decorator (#56, #154)

• fixed --diff not showing entire path (#130)

• fixed parsing of complex expressions after star and double stars in function calls (#2)

• fixed invalid splitting on comma in lambda arguments (#133)

• fixed missing splits of ternary expressions (#141)

18.4a2

• fixed parsing of unaligned standalone comments (#99, #112)

• fixed placement of dictionary unpacking inside dictionary literals (#111)

• Vim plugin now works on Windows, too

• fixed unstable formatting when encountering unnecessarily escaped quotes in a string (#120)

18.4a1

• added --quiet (#78)

• added automatic parentheses management (#4)

• added pre-commit integration (#103, #104)

• fixed reporting on --check with multiple files (#101, #102)

• fixed removing backslash escapes from raw strings (#100, #105)

18.4a0

• added --diff (#87)

• add line breaks before all delimiters, except in cases like commas, to better comply with PEP 8 (#73)

• standardize string literals to use double quotes (almost) everywhere (#75)

• fixed handling of standalone comments within nested bracketed expressions; Black will no longer produce super long lines or put all standalone comments at the end of the expression (#22)

• fixed 18.3a4 regression: don't crash and burn on empty lines with trailing whitespace (#80)

• fixed 18.3a4 regression: # yapf: disable usage as trailing comment would cause Black to not emit the rest of the file (#95)

• when CTRL+C is pressed while formatting many files, Black no longer freaks out with a flurry of asyncio-related exceptions

• only allow up to two empty lines on module level and only single empty lines within functions (#74)

18.3a4

• # fmt: off and # fmt: on are implemented (#5)

• automatic detection of deprecated Python 2 forms of print statements and exec statements in the formatted file (#49)

• use proper spaces for complex expressions in default values of typed function arguments (#60)

• only return exit code 1 when --check is used (#50)

• don't remove single trailing commas from square bracket indexing (#59)

• don't omit whitespace if the previous factor leaf wasn't a math operator (#55)

• omit extra space in kwarg unpacking if it's the first argument (#46)

• omit extra space in Sphinx auto-attribute comments (#68)

18.3a3

• don't remove single empty lines outside of bracketed expressions (#19)

• added ability to pipe formatting from stdin to stdin (#25)

• restored ability to format code with legacy usage of async as a name (#20, #42)

• even better handling of numpy-style array indexing (#33, again)

18.3a2

• changed positioning of binary operators to occur at beginning of lines instead of at the end, following a recent change to PEP8 (#21)

• ignore empty bracket pairs while splitting. This avoids very weirdly looking formattings (#34, #35)

• remove a trailing comma if there is a single argument to a call

• if top level functions were separated by a comment, don't put four empty lines after the upper function

• fixed unstable formatting of newlines with imports

• fixed unintentional folding of post scriptum standalone comments into last statement if it was a simple statement (#18, #28)

• fixed missing space in numpy-style array indexing (#33)

• fixed spurious space after star-based unary expressions (#31)

18.3a1

• added --check

• only put trailing commas in function signatures and calls if it's safe to do so. If the file is Python 3.6+ it's always safe, otherwise only safe if there are no *args or **kwargs used in the signature or call. (#8)

• fixed invalid spacing of dots in relative imports (#6, #13)

• fixed invalid splitting after comma on unpacked variables in for-loops (#23)

• fixed spurious space in parenthesized set expressions (#7)

• fixed spurious space after opening parentheses and in default arguments (#14, #17)

• fixed spurious space after unary operators when the operand was a complex expression (#15)

18.3a0

• first published version, Happy 🍰 Day 2018!

• alpha quality

• date-versioned (see: https://calver.org/)

Authors

Glued together by Łukasz Langa.

Maintained with Carol Willing, Carl Meyer, Mika Naylor, and Zsolt Dollenstein.

Multiple contributions by:

• Anthony Sottile
• Artem Malyshev
• Christian Heimes
• Daniel M. Capella
• Eli Treuherz
• Hugo van Kemenade
• Ivan Katanić
• Jelle Zijlstra
• Jonas Obrist
• Luka Sterbic
• Miguel Gaiowski
• Osaetin Daniel
• Sunil Kapil
• Vishwas B Sharma

Contents: Installation and Usage | The Black code style | Editor integration | Version control integration | Ignoring unmodified files | Testimonials | Show your style | License | Contributing | Change Log | Authors