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The uncompromising Python code formatter

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Black Logo

The Uncompromising Code Formatter

Build Status Actions Status Actions Status Documentation Status Coverage Status License: MIT PyPI Downloads Code style: black

“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.

Try it out now using the Black Playground. Watch the PyCon 2019 talk to learn more.

Contents: Installation and usage | Code style | Pragmatism | pyproject.toml | Editor integration | blackd | black-primer | Version control integration | GitHub Actions | Ignoring unmodified files | Used by | Testimonials | Show your style | 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.

Install from GitHub

If you can't wait for the latest hotness and want to install from GitHub, use:

pip install git+git://github.com/psf/black

Usage

To get started right away with sensible defaults:

black {source_file_or_directory}

You can run Black as a package if running it as a script doesn't work:

python -m black {source_file_or_directory}

Command line options

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

Usage: black [OPTIONS] [SRC]...

  The uncompromising code formatter.

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]

  --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.

  --color / --no-color            Show colored diff. Only applies when
                                  `--diff` is given.

  --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|\.svn|_build|buck-
                                  out|build|dist)/]

  --force-exclude TEXT            Like --exclude, but files and directories
                                  matching this regex will be excluded even
                                  when they are passed explicitly as arguments

  -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 FILE                   Read configuration from FILE 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 --check was used).

Using Black with other tools

While Black enforces formatting that conforms to PEP 8, other tools may raise warnings about Black's changes or will overwrite Black's changes. A good example of this is isort. Since Black is barely configurable, these tools should be configured to neither warn about nor overwrite Black's changes.

Actual details on Black compatible configurations for various tools can be found in compatible_configs.

Migrating your code style without ruining git blame

A long-standing argument against moving to automated code formatters like Black is that the migration will clutter up the output of git blame. This was a valid argument, but since Git version 2.23, Git natively supports ignoring revisions in blame with the --ignore-rev option. You can also pass a file listing the revisions to ignore using the --ignore-revs-file option. The changes made by the revision will be ignored when assigning blame. Lines modified by an ignored revision will be blamed on the previous revision that modified those lines.

So when migrating your project's code style to Black, reformat everything and commit the changes (preferably in one massive commit). Then put the full 40 characters commit identifier(s) into a file.

# Migrate code style to Black
5b4ab991dede475d393e9d69ec388fd6bd949699

Afterwards, you can pass that file to git blame and see clean and meaningful blame information.

$ git blame important.py --ignore-revs-file .git-blame-ignore-revs
7a1ae265 (John Smith 2019-04-15 15:55:13 -0400 1) def very_important_function(text, file):
abdfd8b0 (Alice Doe  2019-09-23 11:39:32 -0400 2)     text = text.lstrip()
7a1ae265 (John Smith 2019-04-15 15:55:13 -0400 3)     with open(file, "r+") as f:
7a1ae265 (John Smith 2019-04-15 15:55:13 -0400 4)         f.write(formatted)

You can even configure git to automatically ignore revisions listed in a file on every call to git blame.

$ git config blame.ignoreRevsFile .git-blame-ignore-revs

The one caveat is that GitHub and GitLab do not yet support ignoring revisions using their native UI of blame. So blame information will be cluttered with a reformatting commit on those platforms. (If you'd like this feature, there's an open issue for GitLab and please let GitHub know!)

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 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 is a PEP 8 compliant opinionated formatter. Black reformats entire files in place. It is not configurable. It doesn't take previous formatting into account. Your main option of configuring Black is that 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. To learn more about Black's opinions, to go the_black_code_style.

Please refer to this document before submitting an issue. What seems like a bug might be intended behaviour.

Pragmatism

Early versions of Black used to be absolutist in some respects. They took after its initial author. This was fine at the time as it made the implementation simpler and there were not many users anyway. Not many edge cases were reported. As a mature tool, Black does make some exceptions to rules it otherwise holds. This section of the_black_code_style describes what those exceptions are and why this is the case.

Please refer to this document before submitting an issue just like with the document above. What seems like a bug might be intended behaviour.

pyproject.toml

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 --include and --exclude 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 pyproject.toml file?

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 need for setup.py and setup.cfg files.

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 .hg directory, 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 want with --config. In this situation Black will not look for any other file.

If you're running with --verbose, you will see a blue message if a file was found and used.

Please note blackd will not use pyproject.toml configuration.

Configuration format

As the file extension suggests, pyproject.toml is a TOML file. It contains separate sections for different tools. Black is using the [tool.black] 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.

Example pyproject.toml 
[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
)
'''

Lookup hierarchy

Command-line options have defaults that you can see in --help. A 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.

Editor integration

Black can be integrated into many editors with plugins. They let you run Black on your code with the ease of doing it in your editor. To get started using Black in your editor of choice, please see editor_integration.

Patches are welcome for editors without an editor integration or plugin! More information can be found in editor_integration.

blackd

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 a file. Please refer to blackd to get the ball rolling.

black-primer

black-primer is a tool built for CI (and humans) to have Black --check a number of (configured in primer.json) Git accessible projects in parallel. black_primer has more information regarding its usage and configuration.

(A PR adding Mercurial support will be accepted.)

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/psf/black
    rev: 19.10b0 # Replace by any tag/version: https://github.com/psf/black/tags
    hooks:
      - id: black
        language_version: python3 # Should be a command that runs python3.6+

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

Avoid using args in the hook. Instead, store necessary configuration in 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 language_version accordingly. Finally, stable is a branch that tracks the latest release on PyPI. If you'd rather run on master, this is also an option.

GitHub Actions

Create a file named .github/workflows/black.yml inside your repository with:

name: Lint

on: [push, pull_request]

jobs:
  lint:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
      - uses: actions/setup-python@v2
      - uses: psf/black@stable

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>.<file-mode>.pickle
  • macOS: /Users/<username>/Library/Caches/black/<version>/cache.<line-length>.<file-mode>.pickle
  • Linux: /home/<username>/.cache/black/<version>/cache.<line-length>.<file-mode>.pickle

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 .cache/black/<version>/.

Used by

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, Bandersnatch, Pipenv, virtualenv), pandas, Pillow, every Datadog Agent Integration, Home Assistant.

The following organizations use Black: Facebook, Dropbox, Mozilla, Quora.

Are we missing anyone? Let us know.

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/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

Looks like this: Code style: black

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

The log's become rather long. It moved to its own file.

See CHANGES.

Authors

Glued together by Łukasz Langa.

Maintained with Carol Willing, Carl Meyer, Jelle Zijlstra, Mika Naylor, Zsolt Dollenstein, and Cooper Lees.

Multiple contributions by:

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