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This patch changes the preview style so that string splitters respect
Unicode East Asian Width[^1] property.  If you are not familiar to CJK
languages it is not clear immediately.  Let me elaborate with some

Traditionally, East Asian characters (including punctuation) have
taken up space twice than European letters and stops when they are
rendered in monospace typeset.  Compare the following characters:


The characters at the first line are half-width, and the second line
are full-width.  (Also note that the last character with a small
circle, the East Asian period, is also full-width.)  Therefore, if we
want to prevent those full-width characters to exceed the maximum
columns per line, we need to count their *width* rather than the number
of characters.  Again, the following characters:


These are just 4 characters, but their total width is 8.

Suppose we want to maintain up to 4 columns per line with the following


How should it be then?  We want it to look like:


However, Black currently turns it into like this:


It's because Black currently counts the number of characters in the line
instead of measuring their width. So, how could we measure the width?
How can we tell if a character is full- or half-width? What if half-width
characters and full-width ones are mixed in a line? That's why Unicode
defined an attribute named `East_Asian_Width`. Unicode grouped every
single character according to their width in fixed-width typeset.

This partially addresses #1197, but only for string splitters. The other
parts need to be fixed as well in future patches.

This was implemented by copying rich's own approach to handling wide
characters: generate a table using wcwidth, check it into source
control, and use in to drive helper functions in Black's logic. This
gets us the best of both worlds: accuracy and performance (and let's us
update as per our stability policy too!).

Co-authored-by: Jelle Zijlstra <>

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March 14, 2018 12:55
March 18, 2023 10:41

Black Logo

The Uncompromising Code Formatter

Actions Status Documentation Status Coverage Status License: MIT PyPI Downloads conda-forge Code style: black

“Any color you like.”

Black is the uncompromising Python code formatter. By using it, you agree to cede 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.

Try it out now using the Black Playground. Watch the PyCon 2019 talk to learn more.

Read the documentation on ReadTheDocs!

Installation and usage


Black can be installed by running pip install black. It requires Python 3.7+ to run. If you want to format Jupyter Notebooks, install with pip install "black[jupyter]".

If you can't wait for the latest hotness and want to install from GitHub, use:

pip install git+


To get started right away with sensible defaults:

black {source_file_or_directory}

You can run Black as a package if running it as a script doesn't work:

python -m black {source_file_or_directory}

Further information can be found in our docs:

Black is already successfully used by many projects, small and big. Black has a comprehensive test suite, with efficient parallel tests, and our own auto formatting and parallel Continuous Integration runner. Now that we have become stable, you should not expect large formatting changes in the future. Stylistic changes will mostly be responses to bug reports and support for new Python syntax. For more information please refer to the The Black Code Style.

Also, as a safety measure which slows down processing, Black will check that the reformatted code still produces a valid AST that is effectively equivalent to the original (see the Pragmatism section for details). If you're feeling confident, use --fast.

The Black code style

Black is a PEP 8 compliant opinionated formatter. Black reformats entire files in place. Style configuration options are deliberately limited and rarely added. It doesn't take previous formatting into account (see Pragmatism for exceptions).

Our documentation covers the current Black code style, but planned changes to it are also documented. They're both worth taking a look:

Changes to the Black code style are bound by the Stability Policy:

Please refer to this document before submitting an issue. What seems like a bug might be intended behaviour.


Early versions of Black used to be absolutist in some respects. They took after its initial author. This was fine at the time as it made the implementation simpler and there were not many users anyway. Not many edge cases were reported. As a mature tool, Black does make some exceptions to rules it otherwise holds.

Please refer to this document before submitting an issue just like with the document above. What seems like a bug might be intended behaviour.


Black is able to read project-specific default values for its command line options from a pyproject.toml file. This is especially useful for specifying custom --include and --exclude/--force-exclude/--extend-exclude patterns for your project.

You can find more details in our documentation:

And if you're looking for more general configuration documentation:

Pro-tip: If you're asking yourself "Do I need to configure anything?" the answer is "No". Black is all about sensible defaults. Applying those defaults will have your code in compliance with many other Black formatted projects.

Used by

The following notable open-source projects trust Black with enforcing a consistent code style: pytest, tox, Pyramid, Django, Django Channels, Hypothesis, attrs, SQLAlchemy, Poetry, PyPA applications (Warehouse, Bandersnatch, Pipenv, virtualenv), pandas, Pillow, Twisted, LocalStack, every Datadog Agent Integration, Home Assistant, Zulip, Kedro, OpenOA, FLORIS, ORBIT, WOMBAT, and many more.

The following organizations use Black: Facebook, Dropbox, KeepTruckin, Mozilla, Quora, Duolingo, QuantumBlack, Tesla, Archer Aviation.

Are we missing anyone? Let us know.


Mike Bayer, author of SQLAlchemy:

I can't think of any single tool in my entire programming career that has given me a bigger productivity increase by its introduction. I can now do refactorings in about 1% of the keystrokes that it would have taken me previously when we had no way for code to format itself.

Dusty Phillips, writer:

Black is opinionated so you don't have to be.

Hynek Schlawack, creator of attrs, core developer of Twisted and CPython:

An auto-formatter that doesn't suck is all I want for Xmas!

Carl Meyer, Django core developer:

At least the name is good.

Kenneth Reitz, creator of requests and pipenv:

This vastly improves the formatting of our code. Thanks a ton!

Show your style

Use the badge in your project's

[![Code style: black](](

Using the badge in README.rst:

.. image::

Looks like this: Code style: black




Welcome! Happy to see you willing to make the project better. You can get started by reading this:

You can also take a look at the rest of the contributing docs or talk with the developers:

Change log

The log has become rather long. It moved to its own file.



The author list is quite long nowadays, so it lives in its own file.


Code of Conduct

Everyone participating in the Black project, and in particular in the issue tracker, pull requests, and social media activity, is expected to treat other people with respect and more generally to follow the guidelines articulated in the Python Community Code of Conduct.

At the same time, humor is encouraged. In fact, basic familiarity with Monty Python's Flying Circus is expected. We are not savages.

And if you really need to slap somebody, do it with a fish while dancing.