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:

black [OPTIONS] [SRC]...

Options:
  -l, --line-length INTEGER       How many characters per line to allow.
                                  [default: 88]
  -t, --target-version [py27|py33|py34|py35|py36|py37|py38]
                                  Python versions that should be supported by
                                  Black's output. [default: per-file auto-
                                  detection]
  --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. Deprecated; use
                                  --target-version instead. [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.
  --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.  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: /(\.eggs|\.git|\.hg|\.mypy
                                  _cache|\.nox|\.tox|\.venv|_build|buck-
                                  out|build|dist)/]
  -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.
  -h, --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.