black/docs/usage_and_configuration/the_basics.md

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

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.