Explicit # fmt: on/off indentation level (#554)

[etc/vim.git] / README.md

![Black Logo](https://raw.githubusercontent.com/ambv/black/master/docs/_static/logo2-readme.png)
<h2 align="center">The Uncompromising Code Formatter</h2>

<p align="center">
<a href="https://travis-ci.org/ambv/black"><img alt="Build Status" src="https://travis-ci.org/ambv/black.svg?branch=master"></a>
<a href="https://black.readthedocs.io/en/stable/?badge=stable"><img alt="Documentation Status" src="https://readthedocs.org/projects/black/badge/?version=stable"></a>
<a href="https://coveralls.io/github/ambv/black?branch=master"><img alt="Coverage Status" src="https://coveralls.io/repos/github/ambv/black/badge.svg?branch=master"></a>
<a href="https://github.com/ambv/black/blob/master/LICENSE"><img alt="License: MIT" src="https://black.readthedocs.io/en/stable/_static/license.svg"></a>
<a href="https://pypi.org/project/black/"><img alt="PyPI" src="https://black.readthedocs.io/en/stable/_static/pypi.svg"></a>
<a href="https://pepy.tech/project/black"><img alt="Downloads" src="https://pepy.tech/badge/black"></a>
<a href="https://github.com/ambv/black"><img alt="Code style: black" src="https://img.shields.io/badge/code%20style-black-000000.svg"></a>
</p>

> “Any color you like.”


*Black* is the uncompromising Python code formatter.  By using it, you
agree to cede control over minutiae of hand-formatting.  In return,
*Black* gives you speed, determinism, and freedom from `pycodestyle`
nagging about formatting.  You will save time and mental energy for
more important matters.

Blackened code looks the same regardless of the project you're reading.
Formatting becomes transparent after a while and you can focus on the
content instead.

*Black* makes code review faster by producing the smallest diffs
possible.

Try it out now using the [Black Playground](https://black.now.sh).

---

*Contents:* **[Installation and usage](#installation-and-usage)** |
**[Code style](#the-black-code-style)** |
**[pyproject.toml](#pyprojecttoml)** |
**[Editor integration](#editor-integration)** |
**[blackd](#blackd)** |
**[Version control integration](#version-control-integration)** |
**[Ignoring unmodified files](#ignoring-unmodified-files)** |
**[Testimonials](#testimonials)** |
**[Show your style](#show-your-style)** |
**[Contributing](#contributing-to-black)** |
**[Change Log](#change-log)** |
**[Authors](#authors)**

---

## Installation and usage

### Installation

*Black* can be installed by running `pip install black`.  It requires
Python 3.6.0+ to run but you can reformat Python 2 code with it, too.


### Usage

To get started right away with sensible defaults:

```
black {source_file_or_directory}
```

### Command line options

*Black* doesn't provide many options.  You can list them by running
`black --help`:

```text
black [OPTIONS] [SRC]...

Options:
  -l, --line-length INTEGER   Where to wrap around.  [default: 88]
  --py36                      Allow using Python 3.6-only syntax on all input
                              files.  This will put trailing commas in function
                              signatures and calls also after *args and
                              **kwargs.  [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.
  -N, --skip-numeric-underscore-normalization
                              Don't normalize underscores in numeric literals.                        
  --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.
  --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. On Windows, use forward
                              slashes for directories.  [default: \.pyi?$]
  --exclude TEXT              A regular expression that matches files and
                              directories that should be excluded on
                              recursive searches. On Windows, use forward
                              slashes for directories.  [default:
                              build/|buck-out/|dist/|_build/|\.git/|\.hg/|
                              \.mypy_cache/|\.nox/|\.tox/|\.venv/]
  -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
                              --exclude=.
  --version                   Show the version and exit.
  --config PATH               Read configuration from PATH.
  --help                      Show this message and exit.
```

*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).


### NOTE: This is a beta product

*Black* is already successfully used by several projects, small and big.
It also sports a decent test suite.  However, it is still very new.
Things will probably be wonky for a while. This is made explicit by the
"Beta" trove classifier, as well as by the "b" in the version number.
What this means for you is that **until the formatter becomes stable,
you should expect some formatting to change in the future**.  That being
said, no drastic stylistic changes are planned, mostly responses to bug
reports.

Also, as a temporary safety measure, *Black* will check that the
reformatted code still produces a valid AST that is equivalent to the
original.  This slows it down.  If you're feeling confident, use
``--fast``.


## The *Black* code style

*Black* reformats entire files in place.  It is not configurable.  It
doesn't take previous formatting into account.  It doesn't reformat
blocks that start with `# fmt: off` and end with `# fmt: on`. `# fmt: on/off`
have to be on the same level of indentation. It also
recognizes [YAPF](https://github.com/google/yapf)'s block comments to
the same effect, as a courtesy for straddling code.


### How *Black* wraps lines

*Black* ignores previous formatting and applies uniform horizontal
and vertical whitespace to your code.  The rules for horizontal
whitespace can be summarized as: do whatever makes `pycodestyle` happy.
The coding style used by *Black* can be viewed as a strict subset of
PEP 8.

As for vertical whitespace, *Black* tries to render one full expression
or simple statement per line.  If this fits the allotted line length,
great.
```py3
# in:

l = [1,
     2,
     3,
]

# out:

l = [1, 2, 3]
```

If not, *Black* will look at the contents of the first outer matching
brackets and put that in a separate indented line.
```py3
# in:

TracebackException.from_exception(exc, limit, lookup_lines, capture_locals)

# out:

TracebackException.from_exception(
    exc, limit, lookup_lines, capture_locals
)
```

If that still doesn't fit the bill, it will decompose the internal
expression further using the same rule, indenting matching brackets
every time.  If the contents of the matching brackets pair are
comma-separated (like an argument list, or a dict literal, and so on)
then *Black* will first try to keep them on the same line with the
matching brackets.  If that doesn't work, it will put all of them in
separate lines.
```py3
# in:

def very_important_function(template: str, *variables, file: os.PathLike, debug: bool = False):
    """Applies `variables` to the `template` and writes to `file`."""
    with open(file, 'w') as f:
        ...

# out:

def very_important_function(
    template: str,
    *variables,
    file: os.PathLike,
    debug: bool = False,
):
    """Applies `variables` to the `template` and writes to `file`."""
    with open(file, "w") as f:
        ...
```

You might have noticed that closing brackets are always dedented and
that a trailing comma is always added.  Such formatting produces smaller
diffs; when you add or remove an element, it's always just one line.
Also, having the closing bracket dedented provides a clear delimiter
between two distinct sections of the code that otherwise share the same
indentation level (like the arguments list and the docstring in the
example above).

If a data structure literal (tuple, list, set, dict) or a line of "from"
imports cannot fit in the allotted length, it's always split into one
element per line.  This minimizes diffs as well as enables readers of
code to find which commit introduced a particular entry.  This also
makes *Black* compatible with [isort](https://pypi.org/p/isort/) with
the following configuration.

<details>
<summary>A compatible `.isort.cfg`</summary>

```
[settings]
multi_line_output=3
include_trailing_comma=True
force_grid_wrap=0
combine_as_imports=True
line_length=88
```

The equivalent command line is:
```
$ isort --multi-line=3 --trailing-comma --force-grid-wrap=0 --combine-as --line-width=88 [ file.py ]
```
</details>

### Line length

You probably noticed the peculiar default line length.  *Black* defaults
to 88 characters per line, which happens to be 10% over 80.  This number
was found to produce significantly shorter files than sticking with 80
(the most popular), or even 79 (used by the standard library).  In
general, [90-ish seems like the wise choice](https://youtu.be/wf-BqAjZb8M?t=260).

If you're paid by the line of code you write, you can pass
`--line-length` with a lower number.  *Black* will try to respect that.
However, sometimes it won't be able to without breaking other rules.  In
those rare cases, auto-formatted code will exceed your allotted limit.

You can also increase it, but remember that people with sight disabilities
find it harder to work with line lengths exceeding 100 characters.
It also adversely affects side-by-side diff review  on typical screen
resolutions.  Long lines also make it harder to present code neatly
in documentation or talk slides.

If you're using Flake8, you can bump `max-line-length` to 88 and forget
about it.  Alternatively, use [Bugbear](https://github.com/PyCQA/flake8-bugbear)'s
B950 warning instead of E501 and keep the max line length at 80 which
you are probably already using.  You'd do it like this:
```ini
[flake8]
max-line-length = 80
...
select = C,E,F,W,B,B950
ignore = E501
```

You'll find *Black*'s own .flake8 config file is configured like this.
If you're curious about the reasoning behind B950, Bugbear's documentation
explains it.  The tl;dr is "it's like highway speed limits, we won't
bother you if you overdo it by a few km/h".


### Empty lines

*Black* avoids spurious vertical whitespace.  This is in the spirit of
PEP 8 which says that in-function vertical whitespace should only be
used sparingly.

*Black* will allow single empty lines inside functions, and single and
double empty lines on module level left by the original editors, except
when they're within parenthesized expressions.  Since such expressions
are always reformatted to fit minimal space, this whitespace is lost.

It will also insert proper spacing before and after function definitions.
It's one line before and after inner functions and two lines before and
after module-level functions and classes.  *Black* will not put empty
lines between function/class definitions and standalone comments that
immediately precede the given function/class.

*Black* will enforce single empty lines between a class-level docstring
and the first following field or method.  This conforms to
[PEP 257](https://www.python.org/dev/peps/pep-0257/#multi-line-docstrings).

*Black* won't insert empty lines after function docstrings unless that
empty line is required due to an inner function starting immediately
after.


### Trailing commas

*Black* will add trailing commas to expressions that are split
by comma where each element is on its own line.  This includes function
signatures.

Unnecessary trailing commas are removed if an expression fits in one
line.  This makes it 1% more likely that your line won't exceed the
allotted line length limit.  Moreover, in this scenario, if you added
another argument to your call, you'd probably fit it in the same line
anyway.  That doesn't make diffs any larger.

One exception to removing trailing commas is tuple expressions with
just one element.  In this case *Black* won't touch the single trailing
comma as this would unexpectedly change the underlying data type.  Note
that this is also the case when commas are used while indexing.  This is
a tuple in disguise: ```numpy_array[3, ]```.

One exception to adding trailing commas is function signatures
containing `*`, `*args`, or `**kwargs`.  In this case a trailing comma
is only safe to use on Python 3.6.  *Black* will detect if your file is
already 3.6+ only and use trailing commas in this situation.  If you
wonder how it knows, it looks for f-strings and existing use of trailing
commas in function signatures that have stars in them.  In other words,
if you'd like a trailing comma in this situation and *Black* didn't
recognize it was safe to do so, put it there manually and *Black* will
keep it.


### Strings

*Black* prefers double quotes (`"` and `"""`) over single quotes (`'`
and `'''`).  It will replace the latter with the former as long as it
does not result in more backslash escapes than before.

*Black* also standardizes string prefixes, making them always lowercase.
On top of that, if your code is already Python 3.6+ only or it's using
the `unicode_literals` future import, *Black* will remove `u` from the
string prefix as it is meaningless in those scenarios.

The main reason to standardize on a single form of quotes is aesthetics.
Having one kind of quotes everywhere reduces reader distraction.
It will also enable a future version of *Black* to merge consecutive
string literals that ended up on the same line (see
[#26](https://github.com/ambv/black/issues/26) for details).

Why settle on double quotes?  They anticipate apostrophes in English
text.  They match the docstring standard described in PEP 257.  An
empty string in double quotes (`""`) is impossible to confuse with
a one double-quote regardless of fonts and syntax highlighting used.
On top of this, double quotes for strings are consistent with C which
Python interacts a lot with.

On certain keyboard layouts like US English, typing single quotes is
a bit easier than double quotes.  The latter requires use of the Shift
key.  My recommendation here is to keep using whatever is faster to type
and let *Black* handle the transformation.

If you are adopting *Black* in a large project with pre-existing string
conventions (like the popular ["single quotes for data, double quotes for
human-readable strings"](https://stackoverflow.com/a/56190)), you can
pass `--skip-string-normalization` on the command line.  This is meant as
an adoption helper, avoid using this for new projects.

### Numeric literals

*Black* standardizes most numeric literals to use lowercase letters for the
syntactic parts and uppercase letters for the digits themselves: `0xAB`
instead of `0XAB` and `1e10` instead of `1E10`. Python 2 long literals are
styled as `2L` instead of `2l` to avoid confusion between `l` and `1`. In
Python 3.6+, *Black* adds underscores to long numeric literals to aid
readability: `100000000` becomes `100_000_000`.

For regions where numerals are grouped differently (like [India](https://en.wikipedia.org/wiki/Indian_numbering_system)
and [China](https://en.wikipedia.org/wiki/Chinese_numerals#Whole_numbers)),
the `-N` or `--skip-numeric-underscore-normalization` command line option
makes *Black* preserve underscores in numeric literals.

### Line breaks & binary operators

*Black* will break a line before a binary operator when splitting a block
of code over multiple lines. This is so that *Black* is compliant with the
recent changes in the [PEP 8](https://www.python.org/dev/peps/pep-0008/#should-a-line-break-before-or-after-a-binary-operator)
style guide, which emphasizes that this approach improves readability.

This behaviour may raise ``W503 line break before binary operator`` warnings in
style guide enforcement tools like Flake8. Since ``W503`` is not PEP 8 compliant,
you should tell Flake8 to ignore these warnings.


### Slices

PEP 8 [recommends](https://www.python.org/dev/peps/pep-0008/#whitespace-in-expressions-and-statements)
to treat ``:`` in slices as a binary operator with the lowest priority, and to
leave an equal amount of space on either side, except if a parameter is omitted
(e.g. ``ham[1 + 1 :]``). It also states that for extended slices, both ``:``
operators have to have the same amount of spacing, except if a parameter is
omitted (``ham[1 + 1 ::]``). *Black* enforces these rules consistently.

This behaviour may raise ``E203 whitespace before ':'`` warnings in style guide
enforcement tools like Flake8. Since ``E203`` is not PEP 8 compliant, you should
tell Flake8 to ignore these warnings.


### Parentheses

Some parentheses are optional in the Python grammar.  Any expression can
be wrapped in a pair of parentheses to form an atom.  There are a few
interesting cases:

- `if (...):`
- `while (...):`
- `for (...) in (...):`
- `assert (...), (...)`
- `from X import (...)`
- assignments like:
  - `target = (...)`
  - `target: type = (...)`
  - `some, *un, packing = (...)`
  - `augmented += (...)`

In those cases, parentheses are removed when the entire statement fits
in one line, or if the inner expression doesn't have any delimiters to
further split on.  If there is only a single delimiter and the expression
starts or ends with a bracket, the parenthesis can also be successfully
omitted since the existing bracket pair will organize the expression
neatly anyway.  Otherwise, the parentheses are added.

Please note that *Black* does not add or remove any additional nested
parentheses that you might want to have for clarity or further
code organization.  For example those parentheses are not going to be
removed:
```py3
return not (this or that)
decision = (maybe.this() and values > 0) or (maybe.that() and values < 0)
```


### Call chains

Some popular APIs, like ORMs, use call chaining.  This API style is known
as a [fluent interface](https://en.wikipedia.org/wiki/Fluent_interface).
*Black* formats those by treating dots that follow a call or an indexing
operation like a very low priority delimiter.  It's easier to show the
behavior than to explain it.  Look at the example:
```py3
def example(session):
    result = (
        session.query(models.Customer.id)
        .filter(
            models.Customer.account_id == account_id,
            models.Customer.email == email_address,
        )
        .order_by(models.Customer.id.asc())
        .all()
    )
```


### Typing stub files

PEP 484 describes the syntax for type hints in Python.  One of the
use cases for typing is providing type annotations for modules which
cannot contain them directly (they might be written in C, or they might
be third-party, or their implementation may be overly dynamic, and so on).

To solve this, [stub files with the `.pyi` file
extension](https://www.python.org/dev/peps/pep-0484/#stub-files) can be
used to describe typing information for an external module.  Those stub
files omit the implementation of classes and functions they
describe, instead they only contain the structure of the file (listing
globals, functions, and classes with their members).  The recommended
code style for those files is more terse than PEP 8:

* prefer `...` on the same line as the class/function signature;
* avoid vertical whitespace between consecutive module-level functions,
  names, or methods and fields within a single class;
* use a single blank line between top-level class definitions, or none
  if the classes are very small.

*Black* enforces the above rules.  There are additional guidelines for
formatting `.pyi` file that are not enforced yet but might be in
a future version of the formatter:

* all function bodies should be empty (contain `...` instead of the body);
* do not use docstrings;
* prefer `...` over `pass`;
* for arguments with a default, use `...` instead of the actual default;
* avoid using string literals in type annotations, stub files support
  forward references natively (like Python 3.7 code with `from __future__
  import annotations`);
* use variable annotations instead of type comments, even for stubs that
  target older versions of Python;
* for arguments that default to `None`, use `Optional[]` explicitly;
* use `float` instead of `Union[int, float]`.


## pyproject.toml

*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`
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.


### 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://poetry.eustace.io/) 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 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 `pyproject.toml`</summary>

```toml
[tool.black]
line-length = 88
py36 = true
include = '\.pyi?$'
exclude = '''
/(
    \.git
  | \.hg
  | \.mypy_cache
  | \.tox
  | \.venv
  | _build
  | buck-out
  | build
  | dist

  # The following are specific to Black, you probably don't want those.
  | blib2to3
  | tests/data
)/
'''
```

</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.


## Editor integration

### Emacs

Use [proofit404/blacken](https://github.com/proofit404/blacken).


### PyCharm

1. Install `black`.

```console
$ pip install black
```

2. Locate your `black` installation folder.

  On macOS / Linux / BSD:

```console
$ which black
/usr/local/bin/black  # possible location
```

  On Windows:

```console
$ where black
%LocalAppData%\Programs\Python\Python36-32\Scripts\black.exe  # possible location
```

3. Open External tools in PyCharm with `File -> Settings -> Tools -> External Tools`.

4. Click the + icon to add a new external tool with the following values:
    - Name: Black
    - Description: Black is the uncompromising Python code formatter.
    - Program: <install_location_from_step_2>
    - Arguments: `$FilePath$`

5. Format the currently opened file by selecting `Tools -> External Tools -> black`.
    - Alternatively, you can set a keyboard shortcut by navigating to `Preferences -> Keymap -> External Tools -> External Tools - Black`.

6. Optionally, run Black on every file save:

    1. Make sure you have the [File Watcher](https://plugins.jetbrains.com/plugin/7177-file-watchers) plugin installed.
    2. Go to `Preferences -> Tools -> File Watchers` and click `+` to add a new watcher:
        - Name: Black
        - File type: Python
        - Scope: Project Files
        - Program: <install_location_from_step_2>
        - Arguments: `$FilePath$`
        - Output paths to refresh: `$FilePathRelativeToProjectRoot$`
        - Working directory: `$ProjectFileDir$`

### Vim

Commands and shortcuts:

* `:Black` to format the entire file (ranges not supported);
* `:BlackUpgrade` to upgrade *Black* inside the virtualenv;
* `:BlackVersion` to get the current version of *Black* inside the
  virtualenv.

Configuration:
* `g:black_fast` (defaults to `0`)
* `g:black_linelength` (defaults to `88`)
* `g:black_skip_string_normalization` (defaults to `0`)
* `g:black_virtualenv` (defaults to `~/.vim/black`)

To install with [vim-plug](https://github.com/junegunn/vim-plug):

```
Plug 'ambv/black',
```

or with [Vundle](https://github.com/VundleVim/Vundle.vim):

```
Plugin 'ambv/black'
```

or you can copy the plugin from [plugin/black.vim](https://github.com/ambv/black/tree/master/plugin/black.vim).
Let me know if this requires any changes to work with Vim 8's builtin
`packadd`, or Pathogen, and so on.

This plugin **requires Vim 7.0+ built with Python 3.6+ support**.  It
needs Python 3.6 to be able to run *Black* inside the Vim process which
is much faster than calling an external command.

On first run, the plugin creates its own virtualenv using the right
Python version and automatically installs *Black*. You can upgrade it later
by calling `:BlackUpgrade` and restarting Vim.

If you need to do anything special to make your virtualenv work and
install *Black* (for example you want to run a version from master),
create a virtualenv manually and point `g:black_virtualenv` to it.
The plugin will use it.

To run *Black* on save, add the following line to `.vimrc` or `init.vim`:

```
autocmd BufWritePre *.py execute ':Black'
```

**How to get Vim with Python 3.6?**
On Ubuntu 17.10 Vim comes with Python 3.6 by default.
On macOS with Homebrew run: `brew install vim --with-python3`.
When building Vim from source, use:
`./configure --enable-python3interp=yes`. There's many guides online how
to do this.


### Visual Studio Code

Use the [Python extension](https://marketplace.visualstudio.com/items?itemName=ms-python.python)
([instructions](https://code.visualstudio.com/docs/python/editing#_formatting)).


### SublimeText 3

Use [sublack plugin](https://github.com/jgirardet/sublack).


### IPython Notebook Magic

Use [blackcellmagic](https://github.com/csurfer/blackcellmagic).


### Python Language Server

If your editor supports the [Language Server Protocol](https://langserver.org/)
(Atom, Sublime Text, Visual Studio Code and many more), you can use
the [Python Language Server](https://github.com/palantir/python-language-server) with the
[pyls-black](https://github.com/rupert/pyls-black) plugin.


### Atom/Nuclide

Use [python-black](https://atom.io/packages/python-black).


### Other editors

Other editors will require external contributions.

Patches welcome! ✨ 🍰 ✨

Any tool that can pipe code through *Black* using its stdio mode (just
[use `-` as the file name](https://www.tldp.org/LDP/abs/html/special-chars.html#DASHREF2)).
The formatted code will be returned on stdout (unless `--check` was
passed).  *Black* will still emit messages on stderr but that shouldn't
affect your use case.

This can be used for example with PyCharm's [File Watchers](https://www.jetbrains.com/help/pycharm/file-watchers.html).

## blackd

`blackd` is a small HTTP server that exposes *Black*'s functionality over
a simple protocol. The main benefit of using it is to avoid paying the
cost of starting up a new *Black* process every time you want to blacken
a file.

### Usage

`blackd` is not packaged alongside *Black* by default because it has additional
dependencies. You will need to do `pip install black[d]` to install it.

You can start the server on the default port, binding only to the local interface
by running `blackd`. You will see a single line mentioning the server's version,
and the host and port it's listening on. `blackd` will then print an access log
similar to most web servers on standard output, merged with any exception traces
caused by invalid formatting requests.

`blackd` provides even less options than *Black*. You can see them by running
`blackd --help`:

```text
Usage: blackd [OPTIONS]

Options:
  --bind-host TEXT                Address to bind the server to.
  --bind-port INTEGER             Port to listen on
  --version                       Show the version and exit.
  -h, --help                      Show this message and exit.
```

### Protocol

`blackd` only accepts `POST` requests at the `/` path. The body of the request
should contain the python source code to be formatted, encoded 
according to the `charset` field in the `Content-Type` request header. If no
`charset` is specified, `blackd` assumes `UTF-8`.

There are a few HTTP headers that control how the source is formatted. These
correspond to command line flags for *Black*. There is one exception to this:
`X-Protocol-Version` which if present, should have the value `1`, otherwise the
request is rejected with `HTTP 501` (Not Implemented). 

The headers controlling how code is formatted are:

 - `X-Line-Length`: corresponds to the `--line-length` command line flag.
 - `X-Skip-String-Normalization`: corresponds to the `--skip-string-normalization`
    command line flag. If present and its value is not the empty string, no string
    normalization will be performed.
 - `X-Skip-Numeric-Underscore-Normalization`: corresponds to the
    `--skip-numeric-underscore-normalization` command line flag.
 - `X-Fast-Or-Safe`: if set to `fast`, `blackd` will act as *Black* does when
    passed the `--fast` command line flag.
 - `X-Python-Variant`: if set to `pyi`, `blackd` will act as *Black* does when
    passed the `--pyi` command line flag. Otherwise, its value must correspond to
    a Python version. If this value represents at least Python 3.6, `blackd` will
    act as *Black* does when passed the `--py36` command line flag.

If any of these headers are set to invalid values, `blackd` returns a `HTTP 400`
error response, mentioning the name of the problematic header in the message body.

Apart from the above, `blackd` can produce the following response codes:

 - `HTTP 204`: If the input is already well-formatted. The response body is
        empty.
 - `HTTP 200`: If formatting was needed on the input. The response body
        contains the blackened Python code, and the `Content-Type` header is set
        accordingly.
 - `HTTP 400`: If the input contains a syntax error. Details of the error are
        returned in the response body.
 - `HTTP 500`: If there was any kind of error while trying to format the input.
        The response body contains a textual representation of the error.

## Version control integration

Use [pre-commit](https://pre-commit.com/). Once you [have it
installed](https://pre-commit.com/#install), add this to the
`.pre-commit-config.yaml` in your repository:
```yaml
repos:
-   repo: https://github.com/ambv/black
    rev: stable
    hooks:
    - id: black
      language_version: python3.6
```
Then run `pre-commit install` and you're ready to go.

Avoid using `args` in the hook.  Instead, store necessary configuration
in `pyproject.toml` so that editors and command-line usage of Black all
behave consistently for your project.  See *Black*'s own `pyproject.toml`
for an example.

If you're already using Python 3.7, switch the `language_version`
accordingly. Finally, `stable` is a tag that is pinned to the latest
release on PyPI.  If you'd rather run on master, this is also an option.


## Ignoring unmodified files

*Black* remembers files it has already formatted, unless the `--diff` flag is used or
code is passed via standard input. This information is stored per-user. The exact
location of the file depends on the *Black* version and the system on which *Black*
is run. The file is non-portable. The standard location on common operating systems
is:

* Windows: `C:\\Users\<username>\AppData\Local\black\black\Cache\<version>\cache.<line-length>.<file-mode>.pickle`
* macOS: `/Users/<username>/Library/Caches/black/<version>/cache.<line-length>.<file-mode>.pickle`
* Linux: `/home/<username>/.cache/black/<version>/cache.<line-length>.<file-mode>.pickle`

`file-mode` is an int flag that determines whether the file was formatted as 3.6+ only,
as .pyi, and whether string normalization was omitted.


## Testimonials

**Dusty Phillips**, [writer](https://smile.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=dusty+phillips):

> *Black* is opinionated so you don't have to be.

**Hynek Schlawack**, [creator of `attrs`](https://www.attrs.org/), core
developer of Twisted and CPython:

> An auto-formatter that doesn't suck is all I want for Xmas!

**Carl Meyer**, [Django](https://www.djangoproject.com/) core developer:

> At least the name is good.

**Kenneth Reitz**, creator of [`requests`](http://python-requests.org/)
and [`pipenv`](https://docs.pipenv.org/):

> This vastly improves the formatting of our code. Thanks a ton!


## Show your style

Use the badge in your project's README.md:

```markdown
[![Code style: black](https://img.shields.io/badge/code%20style-black-000000.svg)](https://github.com/ambv/black)
```

Using the badge in README.rst:
```
.. image:: https://img.shields.io/badge/code%20style-black-000000.svg
    :target: https://github.com/ambv/black
```

Looks like this: [![Code style: black](https://img.shields.io/badge/code%20style-black-000000.svg)](https://github.com/ambv/black)


## License

MIT


## Contributing to *Black*

In terms of inspiration, *Black* is about as configurable as *gofmt*.
This is deliberate.

Bug reports and fixes are always welcome!  However, before you suggest a
new feature or configuration knob, ask yourself why you want it.  If it
enables better integration with some workflow, fixes an inconsistency,
speeds things up, and so on - go for it!  On the other hand, if your
answer is "because I don't like a particular formatting" then you're not
ready to embrace *Black* yet. Such changes are unlikely to get accepted.
You can still try but prepare to be disappointed.

More details can be found in [CONTRIBUTING](CONTRIBUTING.md).


## Change Log

### 18.9b0

* numeric literals are now formatted by *Black* (#452, #461, #464, #469):

  * numeric literals are normalized to include `_` separators on Python 3.6+ code

  * added `--skip-numeric-underscore-normalization` to disable the above behavior and
    leave numeric underscores as they were in the input

  * code with `_` in numeric literals is recognized as Python 3.6+

  * most letters in numeric literals are lowercased (e.g., in `1e10`, `0x01`)

  * hexadecimal digits are always uppercased (e.g. `0xBADC0DE`)

* added `blackd`, see [its documentation](#blackd) for more info (#349)

* adjacent string literals are now correctly split into multiple lines (#463)

* trailing comma is now added to single imports that don't fit on a line (#250)

* cache is now populated when `--check` is successful for a file which speeds up
  consecutive checks of properly formatted unmodified files (#448)

* whitespace at the beginning of the file is now removed (#399)

* fixed mangling [pweave](http://mpastell.com/pweave/) and
  [Spyder IDE](https://pythonhosted.org/spyder/) special comments (#532)

* fixed unstable formatting when unpacking big tuples (#267)

* fixed parsing of `__future__` imports with renames (#389)

* fixed scope of `# fmt: off` when directly preceding `yield` and other nodes (#385)

* fixed formatting of lambda expressions with default arguments (#468)

* fixed ``async for`` statements: *Black* no longer breaks them into separate
  lines (#372)

* note: the Vim plugin stopped registering ``,=`` as a default chord as it turned out
  to be a bad idea (#415)


### 18.6b4

* hotfix: don't freeze when multiple comments directly precede `# fmt: off` (#371)


### 18.6b3

* typing stub files (`.pyi`) now have blank lines added after constants (#340)

* `# fmt: off` and `# fmt: on` are now much more dependable:

  * they now work also within bracket pairs (#329)

  * they now correctly work across function/class boundaries (#335)

  * they now work when an indentation block starts with empty lines or misaligned
    comments (#334)

* made Click not fail on invalid environments; note that Click is right but the
  likelihood we'll need to access non-ASCII file paths when dealing with Python source
  code is low (#277)

* fixed improper formatting of f-strings with quotes inside interpolated
  expressions (#322)

* fixed unnecessary slowdown when long list literals where found in a file

* fixed unnecessary slowdown on AST nodes with very many siblings

* fixed cannibalizing backslashes during string normalization

* fixed a crash due to symbolic links pointing outside of the project directory (#338)


### 18.6b2

* added `--config` (#65)

* added `-h` equivalent to `--help` (#316)

* fixed improper unmodified file caching when `-S` was used

* fixed extra space in string unpacking (#305)

* fixed formatting of empty triple quoted strings (#313)

* fixed unnecessary slowdown in comment placement calculation on lines without
  comments


### 18.6b1

* hotfix: don't output human-facing information on stdout (#299)

* hotfix: don't output cake emoji on non-zero return code (#300)


### 18.6b0

* added `--include` and `--exclude` (#270)

* added `--skip-string-normalization` (#118)

* added `--verbose` (#283)

* the header output in `--diff` now actually conforms to the unified diff spec

* fixed long trivial assignments being wrapped in unnecessary parentheses (#273)

* fixed unnecessary parentheses when a line contained multiline strings (#232)

* fixed stdin handling not working correctly if an old version of Click was
  used (#276)

* *Black* now preserves line endings when formatting a file in place (#258)


### 18.5b1

* added `--pyi` (#249)

* added `--py36` (#249)

* Python grammar pickle caches are stored with the formatting caches, making
  *Black* work in environments where site-packages is not user-writable (#192)

* *Black* now enforces a PEP 257 empty line after a class-level docstring
  (and/or fields) and the first method

* fixed invalid code produced when standalone comments were present in a trailer
  that was omitted from line splitting on a large expression (#237)

* fixed optional parentheses being removed within `# fmt: off` sections (#224)

* fixed invalid code produced when stars in very long imports were incorrectly
  wrapped in optional parentheses (#234)

* fixed unstable formatting when inline comments were moved around in
  a trailer that was omitted from line splitting on a large expression
  (#238)

* fixed extra empty line between a class declaration and the first
  method if no class docstring or fields are present (#219)

* fixed extra empty line between a function signature and an inner
  function or inner class (#196)


### 18.5b0

* call chains are now formatted according to the
  [fluent interfaces](https://en.wikipedia.org/wiki/Fluent_interface)
  style (#67)

* data structure literals (tuples, lists, dictionaries, and sets) are
  now also always exploded like imports when they don't fit in a single
  line (#152)

* slices are now formatted according to PEP 8 (#178)

* parentheses are now also managed automatically on the right-hand side
  of assignments and return statements (#140)

* math operators now use their respective priorities for delimiting multiline
  expressions (#148)

* optional parentheses are now omitted on expressions that start or end
  with a bracket and only contain a single operator (#177)

* empty parentheses in a class definition are now removed (#145, #180)

* string prefixes are now standardized to lowercase and `u` is removed
  on Python 3.6+ only code and Python 2.7+ code with the `unicode_literals`
  future import (#188, #198, #199)

* typing stub files (`.pyi`) are now formatted in a style that is consistent
  with PEP 484 (#207, #210)

* progress when reformatting many files is now reported incrementally

* fixed trailers (content with brackets) being unnecessarily exploded
  into their own lines after a dedented closing bracket (#119)

* fixed an invalid trailing comma sometimes left in imports (#185)

* fixed non-deterministic formatting when multiple pairs of removable parentheses
  were used (#183)

* fixed multiline strings being unnecessarily wrapped in optional
  parentheses in long assignments (#215)

* fixed not splitting long from-imports with only a single name

* fixed Python 3.6+ file discovery by also looking at function calls with
  unpacking. This fixed non-deterministic formatting if trailing commas
  where used both in function signatures with stars and function calls
  with stars but the former would be reformatted to a single line.

* fixed crash on dealing with optional parentheses (#193)

* fixed "is", "is not", "in", and "not in" not considered operators for
  splitting purposes

* fixed crash when dead symlinks where encountered


### 18.4a4

* don't populate the cache on `--check` (#175)


### 18.4a3

* added a "cache"; files already reformatted that haven't changed on disk
  won't be reformatted again (#109)

* `--check` and `--diff` are no longer mutually exclusive (#149)

* generalized star expression handling, including double stars; this
  fixes multiplication making expressions "unsafe" for trailing commas (#132)

* *Black* no longer enforces putting empty lines behind control flow statements
  (#90)

* *Black* now splits imports like "Mode 3 + trailing comma" of isort (#127)

* fixed comment indentation when a standalone comment closes a block (#16, #32)

* fixed standalone comments receiving extra empty lines if immediately preceding
  a class, def, or decorator (#56, #154)

* fixed `--diff` not showing entire path (#130)

* fixed parsing of complex expressions after star and double stars in
  function calls (#2)

* fixed invalid splitting on comma in lambda arguments (#133)

* fixed missing splits of ternary expressions (#141)


### 18.4a2

* fixed parsing of unaligned standalone comments (#99, #112)

* fixed placement of dictionary unpacking inside dictionary literals (#111)

* Vim plugin now works on Windows, too

* fixed unstable formatting when encountering unnecessarily escaped quotes
  in a string (#120)


### 18.4a1

* added `--quiet` (#78)

* added automatic parentheses management (#4)

* added [pre-commit](https://pre-commit.com) integration (#103, #104)

* fixed reporting on `--check` with multiple files (#101, #102)

* fixed removing backslash escapes from raw strings (#100, #105)


### 18.4a0

* added `--diff` (#87)

* add line breaks before all delimiters, except in cases like commas, to
  better comply with PEP 8 (#73)

* standardize string literals to use double quotes (almost) everywhere
  (#75)

* fixed handling of standalone comments within nested bracketed
  expressions; *Black* will no longer produce super long lines or put all
  standalone comments at the end of the expression (#22)

* fixed 18.3a4 regression: don't crash and burn on empty lines with
  trailing whitespace (#80)

* fixed 18.3a4 regression: `# yapf: disable` usage as trailing comment
  would cause *Black* to not emit the rest of the file (#95)

* when CTRL+C is pressed while formatting many files, *Black* no longer
  freaks out with a flurry of asyncio-related exceptions

* only allow up to two empty lines on module level and only single empty
  lines within functions (#74)


### 18.3a4

* `# fmt: off` and `# fmt: on` are implemented (#5)

* automatic detection of deprecated Python 2 forms of print statements
  and exec statements in the formatted file (#49)

* use proper spaces for complex expressions in default values of typed
  function arguments (#60)

* only return exit code 1 when --check is used (#50)

* don't remove single trailing commas from square bracket indexing
  (#59)

* don't omit whitespace if the previous factor leaf wasn't a math
  operator (#55)

* omit extra space in kwarg unpacking if it's the first argument (#46)

* omit extra space in [Sphinx auto-attribute comments](http://www.sphinx-doc.org/en/stable/ext/autodoc.html#directive-autoattribute)
  (#68)


### 18.3a3

* don't remove single empty lines outside of bracketed expressions
  (#19)

* added ability to pipe formatting from stdin to stdin (#25)

* restored ability to format code with legacy usage of `async` as
  a name (#20, #42)

* even better handling of numpy-style array indexing (#33, again)


### 18.3a2

* changed positioning of binary operators to occur at beginning of lines
  instead of at the end, following [a recent change to PEP 8](https://github.com/python/peps/commit/c59c4376ad233a62ca4b3a6060c81368bd21e85b)
  (#21)

* ignore empty bracket pairs while splitting. This avoids very weirdly
  looking formattings (#34, #35)

* remove a trailing comma if there is a single argument to a call

* if top level functions were separated by a comment, don't put four
  empty lines after the upper function

* fixed unstable formatting of newlines with imports

* fixed unintentional folding of post scriptum standalone comments
  into last statement if it was a simple statement (#18, #28)

* fixed missing space in numpy-style array indexing (#33)

* fixed spurious space after star-based unary expressions (#31)


### 18.3a1

* added `--check`

* only put trailing commas in function signatures and calls if it's
  safe to do so. If the file is Python 3.6+ it's always safe, otherwise
  only safe if there are no `*args` or `**kwargs` used in the signature
  or call. (#8)

* fixed invalid spacing of dots in relative imports (#6, #13)

* fixed invalid splitting after comma on unpacked variables in for-loops
  (#23)

* fixed spurious space in parenthesized set expressions (#7)

* fixed spurious space after opening parentheses and in default
  arguments (#14, #17)

* fixed spurious space after unary operators when the operand was
  a complex expression (#15)


### 18.3a0

* first published version, Happy 🍰 Day 2018!

* alpha quality

* date-versioned (see: https://calver.org/)


## Authors

Glued together by [Łukasz Langa](mailto:lukasz@langa.pl).

Maintained with [Carol Willing](mailto:carolcode@willingconsulting.com),
[Carl Meyer](mailto:carl@oddbird.net),
[Jelle Zijlstra](mailto:jelle.zijlstra@gmail.com),
[Mika Naylor](mailto:mail@autophagy.io), and
[Zsolt Dollenstein](mailto:zsol.zsol@gmail.com).

Multiple contributions by:
* [Anthony Sottile](mailto:asottile@umich.edu)
* [Artem Malyshev](mailto:proofit404@gmail.com)
* [Christian Heimes](mailto:christian@python.org)
* [Daniel M. Capella](mailto:polycitizen@gmail.com)
* [Eli Treuherz](mailto:eli@treuherz.com)
* Hugo van Kemenade
* [Ivan Katanić](mailto:ivan.katanic@gmail.com)
* [Jonas Obrist](mailto:ojiidotch@gmail.com)
* [Luka Sterbic](mailto:luka.sterbic@gmail.com)
* [Miguel Gaiowski](mailto:miggaiowski@gmail.com)
* [Miroslav Shubernetskiy](mailto:miroslav@miki725.com)
* [Neraste](neraste.herr10@gmail.com)
* [Osaetin Daniel](mailto:osaetindaniel@gmail.com)
* [Peter Bengtsson](mailto:mail@peterbe.com)
* [Stavros Korokithakis](mailto:hi@stavros.io)
* [Sunil Kapil](mailto:snlkapil@gmail.com)
* [Vishwas B Sharma](mailto:sharma.vishwas88@gmail.com)