# 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`.

<details>

<summary>Help output</summary>

```{program-output} black --help

```

</details>

## 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.

<details>
<summary>Example <code>pyproject.toml</code></summary>

```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)
'''
```

</details>

### 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).