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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="http://black.readthedocs.io/en/latest/?badge=latest"><img alt="Documentation Status" src="http://readthedocs.org/projects/black/badge/?version=latest"></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="http://black.readthedocs.io/en/latest/_static/license.svg"></a> <a href="https://pypi.python.org/pypi/black"><img alt="PyPI" src="http://black.readthedocs.io/en/latest/_static/pypi.svg"></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 cease 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. ## 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] --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] -q, --quiet Don't emit non-error messages to stderr. Errors are still emitted, silence those with 2>/dev/null. --version Show the version and exit. --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 occured (or `--check` was used). ### NOTE: This is an early pre-release *Black* can already successfully format itself and the standard library. 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 "Alpha" trove classifier, as well as by the "a" 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**. 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`. 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 are pretty obvious and 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: l = [[n for n in list_bosses()], [n for n in list_employees()]] # out: l = [ [n for n in list_bosses()], [n for n in list_employees()] ] ``` 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). ### 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. One exception is control flow statements: *Black* will always emit an extra empty line after ``return``, ``raise``, ``break``, ``continue``, and ``yield``. This is to make changes in control flow more prominent to readers of your code. *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. *Black* will put those empty lines also between the function definition and any standalone comments that immediately precede the given function. If you want to comment on the entire function, use a docstring or put a leading comment in the function body. ### 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. 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. ### 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. ### 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 (...)` 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. Otherwise, the parentheses are always added. ## Editor integration ### Emacs Use [proofit404/blacken](https://github.com/proofit404/blacken). ### PyCharm 1. Install `black`. $ pip install black 2. Locate your `black` installation folder. On MacOS / Linux / BSD: $ which black /usr/local/bin/black # possible location On Windows: $ 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`. ### Vim Commands and shortcuts: * `,=` or `: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_virtualenv` (defaults to `~/.vim/black`) To install, copy the plugin from [vim/plugin/black.vim](https://github.com/ambv/black/tree/master/vim/plugin/black.vim). Let me know if this requires any changes to work with Vim 8's builtin `packadd`, or Pathogen, or Vundle, 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), just create a virtualenv manually and point `g:black_virtualenv` to it. The plugin will use it. **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 [joslarson.black-vscode](https://marketplace.visualstudio.com/items?itemName=joslarson.black-vscode). ### Other editors Atom/Nuclide integration is planned by the author, others will require external contributions. Patches welcome! ✨ 🍰 ✨ Any tool that can pipe code through *Black* using its stdio mode (just [use `-` as the file name](http://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). ## 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 args: [--line-length=88, --safe] python_version: python3.6 ``` Then run `pre-commit install` and you're ready to go. `args` in the above config is optional but shows you how you can change the line length if you really need to. If you're already using Python 3.7, switch the `python_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. ## 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`](http://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) ``` 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* and *rustfmt* are. 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 ### 18.4a1 * 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 PEP8](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), [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) * [Daniel M. Capella](mailto:polycitizen@gmail.com) * [Eli Treuherz](mailto:eli.treuherz@cgi.com) * Hugo van Kemenade * [Ivan Katanić](mailto:ivan.katanic@gmail.com) * [Osaetin Daniel](mailto:osaetindaniel@gmail.com)