Apply Black formatting only in regions changed since last commit

## What?

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

The reformatters supported are:

• Black for code reformatting

• isort for sorting imports

See Using linters below for the list of supported linters.

To easily run Darker as a Pytest plugin, see pytest-darker.

To integrate Darker with your IDE or with pre-commit, see the relevant sections below in this document.

## Why?

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.

Partial formatting is not supported by Black itself, for various good reasons, and so far there hasn’t been a plan to implemented it either (134, 142, 245, 370, 511, 830). However, in September 2021 Black developers started to hint towards adding this feature after all (1352). This might at least simplify Darker’s algorithm substantially.

But for the time being, this is where darker enters the stage. This tool is for those who want to do partial formatting right now.

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.

You may also want to still consider whether reformatting the whole code base in one commit would make sense in your particular case. You can ignore a reformatting commit in git blame using the blame.ignoreRevsFile config option or --ignore-rev on the command line. For a deeper dive into this topic, see Avoiding ruining git blame in Black documentation, or the article Why does Black insist on reformatting my entire project? from Łukasz Langa (@ambv, the creator of Black). Here’s an excerpt:

“When you make this single reformatting commit, everything that comes after is semantic changes so your commit history is clean in the sense that it actually shows what changed in terms of meaning, not style. There are tools like darker that allow you to only reformat lines that were touched since the last commit. However, by doing that you forever expose yourself to commits that are a mix of semantic changes with stylistic changes, making it much harder to see what changed.”

## How?

To install, use:

pip install darker

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

conda install -c conda-forge darker isort

The darker <myfile.py> 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. Note that isort must be run in the same Python environment as the packages to process, as it imports your modules to determine whether they are first or third party modules.

• -L <linter> / --lint <linter>: Run a supported linter (see Using linters)

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

## Example

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 our_file.py: if True: print('hi') print() if False: print('there') Commit that file: $ git add our_file.py
$git commit -m "Initial commit" [master (root-commit) a0c7c32] Initial commit 1 file changed, 3 insertions(+) create mode 100644 our_file.py Now modify the first line in that file: if True: print('CHANGED TEXT') print() 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 our_file.py
--- our_file.py
+++ our_file.py
@@ -1,3 +1,4 @@
-if True: print('CHANGED TEXT')
+if True:
+    print("CHANGED TEXT")
print()
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 our_file.py if True: print("CHANGED TEXT") print() 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 our_file.py

Now the contents of our_file.py 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")
print()
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.

Darker does honor exclusion options in Black configuration files when recursing directories, but the exclusions are only applied to Black reformatting. Isort and linters are still run on excluded files. Also, individual files explicitly listed on the command line are still reformatted even if they match exclusion patterns.

For more details, see:

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

-r REV, --revision REV

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 works in pre-commit compatible mode. Darker expects the revision range from the PRE_COMMIT_FROM_REF and PRE_COMMIT_TO_REF environment variables. If those are not found, Darker works against HEAD.

--diff

Don’t write the files back, just output a diff for each file on stdout. Highlight syntax if on a terminal and the pygments package is available, or if enabled by configuration.

-d, --stdout

Force complete reformatted output to stdout, instead of in-place. Only valid if there’s just one file to reformat. Highlight syntax if on a terminal and the pygments package is available, or if enabled by configuration.

--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 line. Highlight linter output syntax if on a terminal and the pygments package is available, or if enabled by configuration.

-c PATH, --config PATH

-v, --verbose

Show steps taken and summarize modifications

-q, --quiet

Reduce amount of output

--color

Enable syntax highlighting even for non-terminal output. Overrides the environment variable PY_COLORS=0

--no-color

Disable syntax highlighting even for terminal output. Overrides the environment variable PY_COLORS=1

-S, --skip-string-normalization

Don’t normalize string quotes or prefixes

--no-skip-string-normalization

Normalize string quotes or prefixes. This can be used to override skip_string_normalization = true from a configuration file.

--skip-magic-trailing-comma

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 = true from a configuration file.

-l LENGTH, --line-length LENGTH

How many characters per line to allow [default: 88]

-W WORKERS, --workers WORKERS

How many parallel workers to allow, or 0 for one per core [default: 1]

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:

[tool.darker]
src = [
"src/mypackage",
]
revision = "master"
diff = true
check = true
isort = true
lint = [
"pylint",
]
log_level = "INFO"

Be careful to not use options which generate output which is unexpected for other tools. For example, VSCode only expects the reformat diff, so lint = [ ... ] can’t be used with it.

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

New in version 1.5.0: The -W / --workers command line option

New in version 1.5.0: The --color and --no-color command line options

## 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": [],

VSCode will always add --diff --quiet as arguments to Darker, but you can also pass additional arguments in the blackArgs option (e.g. ["--isort", "--revision=master..."]). Be sure to not enable any linters here or in pyproject.toml since VSCode won’t be able to understand output from them.

Note that VSCode first copies the file to reformat into a temporary <filename>.py.<hash>.tmp file, then calls Black (or Darker in this case) on that file, and brings the changes in the modified files back into the editor. Darker is aware of this behavior, and will correctly compare .py.<hash>.tmp files to corresponding .py files from earlier repository revisions.

### Vim

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.

### Emacs

You can integrate with Emacs using Steve Purcell’s emacs-reformatter library.

Using use-package:

(use-package reformatter
:hook ((python-mode . darker-reformat-on-save-mode))
:config
(reformatter-define darker-reformat
:program "darker"
:stdin nil
:stdout nil
:args (list "-q" input-file))

This will automatically reformat the buffer on save.

You have multiple functions available to launch it manually:

• darker-reformat

• darker-reformat-region

• darker-reformat-buffer

## 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).

2. Create a base pre-commit configuration:

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

-   repo: https://github.com/akaihola/darker
rev: 1.5.0
hooks:
-   id: darker
4. install the Git hook scripts:

pre-commit install

### Using arguments

You can provide arguments, such as enabling isort, by specifying args. Note the inclusion of the isort Python package under additional_dependencies:

-   repo: https://github.com/akaihola/darker
rev: 1.5.0
hooks:
-   id: darker
args: [--isort]
-   isort~=5.9

## GitHub Actions integration

You can use Darker within a GitHub Actions workflow without setting your own Python environment. Great for enforcing that modifications and additions to your code match the Black code style.

### Compatibility

This action is known to support all GitHub-hosted runner OSes. In addition, only published versions of Darker are supported (i.e. whatever is available on PyPI).

### Usage

Create a file named .github/workflows/darker.yml inside your repository with:

name: Lint

on: [push, pull_request]

jobs:
lint:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout@v3
with:
fetch-depth: 0
- uses: actions/setup-python@v2
- uses: akaihola/darker@1.5.0
with:
options: "--check --diff --color"
revision: "master..."
src: "./src"
version: "1.5.0"
lint: "flake8,pylint==2.13.1"

There needs to be a working Python environment, set up using actions/setup-python in the above example. Darker will be installed in an isolated virtualenv to prevent conflicts with other workflows.

"uses:" specifies which Darker release to get the GitHub Action definition from. We recommend to pin this to a specific release. "version:" specifies which version of Darker to run in the GitHub Action. It defaults to the same version as in "uses:", but you can force it to use a different version as well. Darker versions available from PyPI are supported, as well as commit SHAs or branch names, prefixed with an @ symbol (e.g. version: "@master").

The revision: "master..." (or "main...") option instructs Darker to compare the current branch to the branching point from main branch when determining which source code lines have been changed. If omitted, the Darker GitHub Action will determine the commit range automatically.

"src:" defines the root directory to run Darker for. This is typically the source tree, but you can use "." (the default) to also reformat Python files like "setup.py" in the root of the whole repository.

You can also configure other arguments passed to Darker via "options:". It defaults to "--check --diff --color". You can e.g. add "--isort" to sort imports, or "--verbose" for debug logging.

To run linters through Darker, you can provide a comma separated list of linters using the lint: option. Only flake8, pylint and mypy are supported. Versions can be constrained using pip syntax, e.g. "flake8>=3.9.2".

New in version 1.1.0: GitHub Actions integration. Modeled after how Black does it, thanks to Black authors for the example!

New in version 1.4.1: The revision: option, with smart default value if omitted.

New in version 1.5.0: The lint: option.

## Using linters

One way to use Darker is to filter linter output to modified lines only. Darker supports any linter with output in one of the following formats:

<file>:<linenum>: <description>
<file>:<linenum>:<col>: <description>

Most notably, the following linters/checkers have been verified to work with Darker:

New in version 1.1.0: Support for Mypy, Pylint, Flake8 and compatible linters.

New in version 1.2.0: Support for test coverage output using cov_to_lint.py.

To run a linter, use the --lint / -L command line option:

• -L mypy: do static type checking using Mypy

• -L pylint: analyze code using Pylint

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

• -L cov_to_lint.py: read .coverage and list non-covered modified lines

Darker also groups linter output into blocks of consecutive lines separated by blank lines. Here’s an example of cov_to_lint.py output:

\$ darker --revision 0.1.0.. --check --lint cov_to_lint.py src
src/darker/__main__.py:94:  no coverage:             logger.debug("No changes in %s after isort", src)
src/darker/__main__.py:95:  no coverage:             break

src/darker/__main__.py:125: no coverage:         except NotEquivalentError:

src/darker/__main__.py:130: no coverage:             if context_lines == max_context_lines:
src/darker/__main__.py:131: no coverage:                 raise
src/darker/__main__.py:132: no coverage:             logger.debug(

⚠ NOTE ⚠

Don’t enable linting on the command line or in the configuration when running Darker as a reformatter in VSCode. You will confuse VSCode with unexpected output from Darker, as it only expect black’s output

## Syntax highlighting

Darker automatically enables syntax highlighting for the --diff, -d/--stdout and -L/--lint options if it’s running on a terminal and the Pygments package is installed.

You can force enable syntax highlighting on non-terminal output using

• the color = true option in the [tool.darker] section of pyproject.toml of your Python project’s root directory,

• the PY_COLORS=1 environment variable, and

• the --color command line option for darker.

You can force disable syntax highlighting on terminal output using

• the color = false option in pyproject.toml,

• the PY_COLORS=0 environment variable, and

• the --no-color command line option.

In the above lists, latter configuration methods override earlier ones, so the command line options always take highest precedence.

## 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.

## 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

• yapf – Google’s Python formatter

• yapf_diff – apply yapf or other formatters to modified lines only

## Contributors ✨

See README.rst for the list of contributors.

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

## Project details

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