darker 1.0.0

Apply Black formatting only in regions changed since last commit

What?

This is a small utility built on top of the black and isort Python code formatters to enable formatting of only regions which have changed in the Git working tree since the last commit.

Why?

You want to start unifying code style in your project using black. But 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 as is when starting a project from scratch.

How?

To install, use:

pip install darker

The darker <myfile.py> command reads the original file, formats it using black, combines original and formatted regions based on edits, and writes back over the original file.

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.

Example:

$ mkdir test && cd test && git init
Initialized empty Git repository in /tmp/test/.git/
$ echo "if True: print('hi')\n\nif False: print('there')" | tee test.py
if True: print('hi')

if False: print('there')
$ git add test.py && git commit -m "Initial commit"
[master (root-commit) a0c7c32] Initial commit
 1 file changed, 3 insertions(+)
 create mode 100644 test.py
$ echo "if True: print('changed')\n\nif False: print('there')" | tee test.py
if True: print('changed')

if False: print('there')
$ darker test.py && cat test.py
if True:
    print("changed")

if False: print('there')

Customizing Black and isort behavior

Project-specific default options for 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:

• Black documentation about pyproject.toml

• isort documentation about config files

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

-c PATH, --config PATH
                      Ask `black` and `isort` to read configuration from PATH.
-S, --skip-string-normalization
                      Don't normalize string quotes or prefixes
-l LINE_LENGTH, --line-length LINE_LENGTH
                      How many characters per line to allow [default: 88]

New in version 1.0.0: The -c, -S and -l command line options.

New in version 1.0.0: isort is configured with -c and -l, too.

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:

   "python.formatting.provider": "black",
   "python.formatting.blackPath": "<install_location_from_step_2>"

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.

License

BSD. See LICENSE.rst.

Prior art

• black-macchiato

• darken (deprecated in favor of Darker; thanks Carreau for inspiration!)

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