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Apply Black formatting only in regions changed since last commit

Project description

master branch build status BSD 3 Clause license Latest release on PyPI Number of downloads Source code formatted using Black Change log


This utility reformats and checks Python source code files in a Git repository. However, it only applies reformatting and reports errors in regions which have changed in the Git working tree since the last commit.

The reformatters and linters supported are:

  • Black for code reformatting

  • isort for sorting imports

  • Mypy for static type checking

  • Pylint for generic static checking of code

  • Flake8 for style guide enforcement

New in version 1.1.0: Support for Mypy, Pylint and other linters.


You want to start unifying code style in your project using Black. Maybe you also like to standardize on how to order your imports, or do static type checking or other static analysis for your code.

However, instead of formatting the whole code base in one giant commit, you’d like to only change formatting when you’re touching the code for other reasons.

This can also be useful when contributing to upstream codebases that are not under your complete control.

However, partial formatting is not supported by Black itself, for various good reasons, and it won’t be implemented either (134, 142, 245, 370, 511, 830).

This is where darker enters the stage. This tool is for those who want to do partial formatting anyway.

Note that this tool is meant for special situations when dealing with existing code bases. You should just use Black and isort as is when starting a project from scratch.


To install, use:

pip install darker

Or, if you’re using Conda for package management:

conda install -c conda-forge darker isort

The darker <> or darker <directory> command reads the original file(s), formats them using Black, combines original and formatted regions based on edits, and writes back over the original file(s).

Alternatively, you can invoke the module directly through the python executable, which may be preferable depending on your setup. Use python -m darker instead of darker in that case.

By default, darker just runs Black to reformat the code. You can enable additional features with command line options:

  • -i / --isort: Reorder imports using isort

  • -L <linter> / --lint <linter>: Run a supported linter:

    • -L mypy: do static type checking using Mypy

    • -L pylint: analyze code using Pylint

    • -L flake8: enforce the Python style guide using Flake8

New in version 1.1.0: The -L / --lint option. New in version 1.2.2: Package available in conda-forge.


This example walks you through a minimal practical use case for Darker.

First, create an empty Git repository:

$ mkdir /tmp/test
$ cd /tmp/test
$ git init
Initialized empty Git repository in /tmp/test/.git/

In the root of that directory, create the ill-formatted Python file

if True: print('hi')
if False: print('there')

Commit that file:

$ git add
$ git commit -m "Initial commit"
[master (root-commit) a0c7c32] Initial commit
 1 file changed, 3 insertions(+)
 create mode 100644

Now modify the first line in that file:

if True: print('CHANGED TEXT')
if False: print('there')

You can ask Darker to show the diff for minimal reformatting which makes edited lines conform to Black rules:

$ darker --diff
@@ -1,3 +1,4 @@
-if True: print('CHANGED TEXT')
+if True:
+    print("CHANGED TEXT")
if False: print('there')

Alternatively, Darker can output the full reformatted file (works only when a single Python file is provided on the command line):

$ darker --stdout
if True:
    print("CHANGED TEXT")
if False: print('there')

If you omit the --diff and --stdout options, Darker replaces the files listed on the command line with partially reformatted ones as shown above:

$ darker

Now the contents of will have changed. Note that the original print() and if False: ... lines have not been reformatted since they had not been edited!

if True:
    print("CHANGED TEXT")
if False: print('there')

You can also ask Darker to reformat edited lines in all Python files in the repository:

$ darker .

Or, if you want to compare to another branch (or, in fact, any commit) instead of the last commit:

$ darker --revision master .

Customizing darker, Black and isort behavior

Project-specific default options for darker, Black and isort are read from the project’s pyproject.toml file in the repository root. isort also looks for a few other places for configuration.

For more details, see:

The following command line arguments can also be used to modify the defaults:

 -r REVISION, --revision REVISION
                       Git revision against which to compare the working tree.
                       Tags, branch names, commit hashes, and other expressions
                       like HEAD~5 work here. Also a range like master...HEAD or
                       master... can be used to compare the best common ancestor.
                       With the magic value :PRE-COMMIT:, Darker expects the
                       revision range from the PRE_COMMIT_FROM_REF and
                       PRE_COMMIT_TO_REF environment variables.

 --diff                Don't write the files back, just output a diff for
                       each file on stdout. Highlight syntax on screen if
                       the `pygments` package is available.

 -d, --stdout          Force complete reformatted output to stdout, instead of
                       in-place. Only valid if there's just one file to reformat.

 --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.

 -i, --isort           Also sort imports using the `isort` package

 -L CMD, --lint CMD    Also run a linter on changed files. CMD can be a name
                       of path of the linter binary, or a full quoted command
 -c PATH, --config PATH
                       Ask `black` and `isort` to read configuration from PATH.
 -S, --skip-string-normalization
                       Don't normalize string quotes or prefixes
                       Normalize string quotes or prefixes. This can be used
                       to override `skip_string_normalization = true` from a
                       configuration file.
                       Skip adding trailing commas to expressions that are
                       split by comma where each element is on its own line.
                       This includes function signatures. This can be used to override
                       `skip_magic_trailing_comma` from a configuration file.
 -l LINE_LENGTH, --line-length LINE_LENGTH
                       How many characters per line to allow [default: 88]

To change default values for these options for a given project, add a [tool.darker] section to pyproject.toml in the project’s root directory. For example:

src = [
revision = "master"
diff = true
check = true
isort = true
lint = [
log_level = "INFO"

New in version 1.0.0:

  • The -c, -S and -l command line options.

  • isort is configured with -c and -l, too.

New in version 1.1.0: The command line options

  • -r / --revision

  • --diff

  • --check

  • --no-skip-string-normalization

  • -L / --lint

New in version 1.2.0: Support for

  • commit ranges in -r / --revision.

  • a [tool.darker] section in pyproject.toml.

New in version 1.2.2: Support for -r :PRE-COMMIT: / --revision=:PRE_COMMIT:

New in version 1.3.0: Support for command line option --skip-magic-trailing-comma

New in version 1.3.0: The -d / --stdout command line option

Editor integration

Many editors have plugins or recipes for integrating Black. You may be able to adapt them to be used with darker. See editor integration in the Black documentation.

PyCharm/IntelliJ IDEA

  1. Install darker:

    $ pip install darker
  2. Locate your darker installation folder.

    On macOS / Linux / BSD:

    $ which darker
    /usr/local/bin/darker  # possible location

    On Windows:

    $ where darker
    %LocalAppData%\Programs\Python\Python36-32\Scripts\darker.exe  # possible location
  3. Open External tools in PyCharm/IntelliJ IDEA

    On macOS:

    PyCharm -> Preferences -> Tools -> External Tools

    On Windows / Linux / BSD:

    File -> Settings -> Tools -> External Tools

  4. Click the + icon to add a new external tool with the following values:

    • Name: Darker

    • Description: Use Black to auto-format regions changed since the last git commit.

    • Program: <install_location_from_step_2>

    • Arguments: "$FilePath$"

    If you need any extra command line arguments like the ones which change Black behavior, you can add them to the Arguments field, e.g.:

    --config /home/myself/black.cfg "$FilePath$"
  5. Format the currently opened file by selecting Tools -> External Tools -> Darker.

    • Alternatively, you can set a keyboard shortcut by navigating to Preferences or Settings -> Keymap -> External Tools -> External Tools - Darker

  6. Optionally, run darker on every file save:

    1. Make sure you have the File Watcher plugin installed.

    2. Go to Preferences or Settings -> Tools -> File Watchers and click + to add a new watcher:

      • Name: Darker

      • File type: Python

      • Scope: Project Files

      • Program: <install_location_from_step_2>

      • Arguments: $FilePath$

      • Output paths to refresh: $FilePath$

      • Working directory: $ProjectFileDir$

    3. Uncheck “Auto-save edited files to trigger the watcher”

Visual Studio Code

  1. Install darker:

    $ pip install darker
  2. Locate your darker installation folder.

    On macOS / Linux / BSD:

    $ which darker
    /usr/local/bin/darker  # possible location

    On Windows:

    $ where darker
    %LocalAppData%\Programs\Python\Python36-32\Scripts\darker.exe  # possible location
  3. Add these configuration options to VS code, Cmd-Shift-P, Open Settings (JSON):

    "python.formatting.provider": "black",
    "python.formatting.blackPath": "<install_location_from_step_2>",
    "python.formatting.blackArgs": ["--diff"],

You can pass additional arguments to darker in the blackArgs option (e.g. ["--diff", "--isort"]), but make sure at least --diff is included.


Unlike Black and many other formatters, darker needs access to the Git history. Therefore it does not work properly with classical auto reformat plugins.

You can though ask vim to run darker on file save with the following in your .vimrc:

set autoread
autocmd BufWritePost *.py silent :!darker %
  • BufWritePost to run darker once the file has been saved,

  • silent to not ask for confirmation each time,

  • :! to run an external command,

  • % for current file name.

Vim should automatically reload the file.

Using as a pre-commit hook

New in version 1.2.1

To use Darker locally as a Git pre-commit hook for a Python project, do the following:

  1. Install pre-commit in your environment (see pre-commit Installation for details).

  1. Create a base pre-commit configuration:

    pre-commit sample-config >.pre-commit-config.yaml
  1. Append to the created .pre-commit-config.yaml the following lines:

    -   repo:
        rev: 1.3.0
        -   id: darker
  2. install the Git hook scripts:

    pre-commit install

How does it work?

Darker takes a git diff of your Python files, records which lines of current files have been edited or added since the last commit. It then runs Black and notes which chunks of lines were reformatted. Finally, only those reformatted chunks on which edited lines fall (even partially) are applied to the edited file.

Also, in case the --isort option was specified, isort is run on each edited file before applying Black. Similarly, each linter requested using the –lint <command> option is run, and only linting errors/warnings on modified lines are displayed.



Prior art

Interesting code formatting and analysis projects to watch

The following projects are related to Black or Darker in some way or another. Some of them we might want to integrate to be part of a Darker run.

  • blacken-docs – Run Black on Python code blocks in documentation files

  • blackdoc – Run Black on documentation code snippets

  • velin – Reformat docstrings that follow the numpydoc convention

  • diff-cov-lint – Pylint and coverage reports for git diff only

  • xenon – Monitor code complexity

  • pyupgrade – Upgrade syntax for newer versions of the language (see #51)

Contributors ✨

See README.rst for the list of contributors.

This project follows the all-contributors specification. Contributions of any kind are welcome!

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