The basics

Foundational knowledge on using and configuring Black.

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).

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 has quite a few knobs these days, although Black is opinionated so style configuration options are deliberately limited and rarely added. You can list them by running black --help.

Help output
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|py39]
                                  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.
  -C, --skip-magic-trailing-comma
                                  Don't use trailing commas as a reason to
                                  split lines.
  --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]
  --required-version TEXT         Require a specific version of Black to be
                                  running (useful for unifying results across
                                  many environments e.g. with a pyproject.toml
                                  file).
  --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: /(\.direnv|\.eggs|\.git|\.h
                                  g|\.mypy_cache|\.nox|\.tox|\.venv|venv|\.svn
                                  |_build|buck-out|build|dist)/]
  --extend-exclude TEXT           Like --exclude, but adds additional files
                                  and directories on top of the excluded ones.
                                  (Useful if you simply want to add to the
                                  default)
  --force-exclude TEXT            Like --exclude, but files and directories
                                  matching this regex will be excluded even
                                  when they are passed explicitly as
                                  arguments.
  --stdin-filename TEXT           The name of the file when passing it through
                                  stdin. Useful to make sure Black will
                                  respect --force-exclude option on some
                                  editors that rely on using stdin.
  -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
                                  exclusion patterns.
  --version                       Show the version and exit.
  --config FILE                   Read configuration from FILE path.
  -h, --help                      Show this message and exit.

Code input alternatives

Standard Input

Black supports formatting code via stdin, with the result being printed to stdout. Just let Black know with - as the path.

$ echo "print ( 'hello, world' )" | black -
print("hello, world")
reformatted -
All done! ✨ 🍰 ✨
1 file reformatted.

Tip: if you need Black to treat stdin input as a file passed directly via the CLI, use --stdin-filename. Useful to make sure Black will respect the --force-exclude option on some editors that rely on using stdin.

As a string

You can also pass code as a string using the -c / --code option.

$ black --code "print ( 'hello, world' )"
print("hello, world")

Writeback and reporting

By default Black reformats the files given and/or found in place. Sometimes you need Black to just tell you what it would do without actually rewriting the Python files.

There’s two variations to this mode that are independently enabled by their respective flags. Both variations can be enabled at once.

Exit code

Passing --check will make Black exit with:

  • code 0 if nothing would change;

  • code 1 if some files would be reformatted; or

  • code 123 if there was an internal error

$ black test.py --check
All done! ✨ 🍰 ✨
1 file would be left unchanged.
$ echo $?
0

$ black test.py --check
would reformat test.py
Oh no! 💥 💔 💥
1 file would be reformatted.
$ echo $?
1

$ black test.py --check
error: cannot format test.py: INTERNAL ERROR: Black produced code that is not equivalent to the source.  Please report a bug on https://github.com/psf/black/issues.  This diff might be helpful: /tmp/blk_kjdr1oog.log
Oh no! 💥 💔 💥
1 file would fail to reformat.
$ echo $?
123

Diffs

Passing --diff will make Black print out diffs that indicate what changes Black would’ve made. They are printed to stdout so capturing them is simple.

If you’d like colored diffs, you can enable them with the --color.

$ black test.py --diff
--- test.py     2021-03-08 22:23:40.848954 +0000
+++ test.py     2021-03-08 22:23:47.126319 +0000
@@ -1 +1 @@
-print ( 'hello, world' )
+print("hello, world")
would reformat test.py
All done! ✨ 🍰 ✨
1 file would be reformatted.

Output verbosity

Black in general tries to produce the right amount of output, balancing between usefulness and conciseness. By default, Black emits files modified and error messages, plus a short summary.

$ black src/
error: cannot format src/black_primer/cli.py: Cannot parse: 5:6: mport asyncio
reformatted src/black_primer/lib.py
reformatted src/blackd/__init__.py
reformatted src/black/__init__.py
Oh no! 💥 💔 💥
3 files reformatted, 2 files left unchanged, 1 file failed to reformat.

Passing -v / --verbose will cause Black to also emit messages about files that were not changed or were ignored due to exclusion patterns. If Black is using a configuration file, a blue message detailing which one it is using will be emitted.

$ black src/ -v
Using configuration from /tmp/pyproject.toml.
src/blib2to3 ignored: matches the --extend-exclude regular expression
src/_black_version.py wasn't modified on disk since last run.
src/black/__main__.py wasn't modified on disk since last run.
error: cannot format src/black_primer/cli.py: Cannot parse: 5:6: mport asyncio
reformatted src/black_primer/lib.py
reformatted src/blackd/__init__.py
reformatted src/black/__init__.py
Oh no! 💥 💔 💥
3 files reformatted, 2 files left unchanged, 1 file failed to reformat

Passing -q / --quiet will cause Black to stop emitting all non-critial output. Error messages will still be emitted (which can silenced by 2>/dev/null).

$ black src/ -q
error: cannot format src/black_primer/cli.py: Cannot parse: 5:6: mport asyncio

Versions

You can check the version of Black you have installed using the --version flag.

$ black --version
black, version 21.5b0

An option to require a specific version to be running is also provided.

$ black --required-version 21.5b2 -c "format = 'this'"
format = "this"
$ black --required-version 31.5b2 -c "still = 'beta?!'"
Oh no! 💥 💔 💥 The required version does not match the running version!

This is useful for example when running Black in multiple environments that haven’t necessarily installed the correct version. This option can be set in a configuration file for consistent results across environments.

Configuration via a file

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/--force-exclude/--extend-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. Applying those defaults will have your code in compliance with many other Black formatted projects.

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 use a “global” configuration, stored in a specific location in your home directory. This will be used as a fallback configuration, that is, it will be used if and only if Black doesn’t find any configuration as mentioned above. Depending on your operating system, this configuration file should be stored as:

  • Windows: ~\.black

  • Unix-like (Linux, MacOS, etc.): $XDG_CONFIG_HOME/black (~/.config/black if the XDG_CONFIG_HOME environment variable is not set)

Note that these are paths to the TOML file itself (meaning that they shouldn’t be named as pyproject.toml), not directories where you store the configuration. Here, ~ refers to the path to your home directory. On Windows, this will be something like C:\\Users\UserName.

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?$'
extend-exclude = '''
# A regex preceded with ^/ will apply only to files and directories
# in the root of the project.
^/foo.py  # exclude a file named foo.py in the root of the project (in addition to the defaults)
'''

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.

Next steps

You’ve probably noted that not all of the options you can pass to Black have been covered. Don’t worry, the rest will be covered in a later section.

A good next step would be configuring auto-discovery so black . is all you need instead of laborously listing every file or directory. You can get started by heading over to File collection and discovery.

Another good choice would be setting up an integration with your editor of choice or with pre-commit for source version control.