The basics#
Foundational knowledge on using and configuring Black.
Black is a well-behaved Unix-style command-line tool:
it does nothing if it finds no sources to format;
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 a CLI option prompted it.
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#
The CLI options of Black can be displayed by expanding the view below or by running
black --help. While Black has quite a few knobs these days, it is still opinionated
so style options are deliberately limited and rarely added.
CLI reference
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 [py33|py34|py35|py36|py37|py38|py39|py310|py311]
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).
--ipynb Format all input files like Jupyter
Notebooks regardless of file extension
(useful when piping source on standard
input).
--python-cell-magics TEXT When processing Jupyter Notebooks, add the
given magic to the list of known python-
magics (timeit, capture, prun, pypy, python,
time, python3). Useful for formatting cells
with custom python magics.
-x, --skip-source-first-line Skip the first line of the source code.
-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.
--preview Enable potentially disruptive style changes
that may be added to Black's main
functionality in the next major release.
--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). It can be either a major version
number or an exact version.
--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
|\.ipynb_checkpoints|_build|buck-
out|build|dist|__pypackages__)/]
--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.
-W, --workers INTEGER RANGE Number of parallel workers [default: number
of CPUs in the system] [x>=1]
-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 22.10.0
An option to require a specific version to be running is also provided.
$ black --required-version 21.9b0 -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,
Flit, or
Hatch 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:
~\.blackUnix-like (Linux, MacOS, etc.):
$XDG_CONFIG_HOME/black(~/.config/blackif theXDG_CONFIG_HOMEenvironment variable is not set)
Note that these are paths to the TOML file itself (meaning that they shouldn’t be named
as pyproject.toml), not directories where you store the configuration. Here, ~
refers to the path to your home directory. On Windows, this will be something like
C:\\Users\UserName.
You can also explicitly specify the path to a particular file that you want with
--config. In this situation Black will not look for any other file.
If you’re running with --verbose, you will see a blue message if a file was found and
used.
Please note blackd will not use pyproject.toml configuration.
Configuration format#
As the file extension suggests, pyproject.toml is a
TOML file. It contains separate sections for
different tools. Black is using the [tool.black] section. The option keys are the
same as long names of options on the command line.
Note that you have to use single-quoted strings in TOML for regular expressions. It’s
the equivalent of r-strings in Python. Multiline strings are treated as verbose regular
expressions by Black. Use [ ] to denote a significant space character.
Example pyproject.toml
[tool.black]
line-length = 88
target-version = ['py37']
include = '\.pyi?$'
# 'extend-exclude' excludes files or directories in addition to the defaults
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
| .*_pb2.py # exclude autogenerated Protocol Buffer files anywhere in 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.
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.