# 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: ```sh black {source_file_or_directory} ``` You can run _Black_ as a package if running it as a script doesn't work: ```sh 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] --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|\. hg|\.mypy_cache|\.nox|\.tox|\.venv|venv|\.svn|_bu ild|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. ```
## 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](https://www.python.org/dev/peps/pep-0518/) defines `pyproject.toml` as a configuration file to store build system requirements for Python projects. With the help of tools like [Poetry](https://python-poetry.org/) or [Flit](https://flit.readthedocs.io/en/latest/) 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](https://github.com/toml-lang/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 ```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](./file_collection_and_discovery.md). Another good choice would be setting up an [integration with your editor](../integrations/editors.md) of choice or with [pre-commit for source version control](../integrations/source_version_control.md).