black 18.3a0

How Black formats files

Black ignores previous formatting and applies uniform horizontal and vertical whitespace to your code. The rules for horizontal whitespace are pretty obvious and can be summarized as: do whatever makes pycodestyle happy.

As for vertical whitespace, Black tries to render one full expression or simple statement per line. If this fits the allotted line length, great.

# in:
l = [1,
     2,
     3,
]

# out:
l = [1, 2, 3]

If not, Black will look at the contents of the first outer matching brackets and put that in a separate indented line.

# in:
l = [[n for n in list_bosses()], [n for n in list_employees()]]

# out:
l = [
    [n for n in list_bosses()], [n for n in list_employees()]
]

If that still doesn’t fit the bill, it will decompose the internal expression further using the same rule, indenting matching brackets every time. If the contents of the matching brackets pair are comma-separated (like an argument list, or a dict literal, and so on) then Black will first try to keep them on the same line with the matching brackets. If that doesn’t work, it will put all of them in separate lines.

# in:
def very_important_function(template: str, *variables, *, file: os.PathLike, debug: bool = False):
    """Applies `variables` to the `template` and writes to `file`."""
    with open(file, 'w') as f:
        ...

# out:
def very_important_function(
    template: str,
    *variables,
    *,
    file: os.PathLike,
    debug: bool = False,
):
    """Applies `variables` to the `template` and writes to `file`."""
    with open(file, 'w') as f:
        ...

You might have noticed that closing brackets are always dedented and that a trailing comma is always added. Such formatting produces smaller diffs; when you add or remove an element, it’s always just one line. Also, having the closing bracket dedented provides a clear delimiter between two distinct sections of the code that otherwise share the same indentation level (like the arguments list and the docstring in the example above).

Unnecessary trailing commas are removed if an expression fits in one line. This makes it 1% more likely that your line won’t exceed the allotted line length limit.

Black avoids spurious vertical whitespace. This is in the spirit of PEP 8 which says that in-function vertical whitespace should only be used sparingly. One exception is control flow statements: Black will always emit an extra empty line after return, raise, break, continue, and yield. This is to make changes in control flow more prominent to readers of your code.

That’s it. The rest of the whitespace formatting rules follow PEP 8 and are designed to keep pycodestyle quiet.

Line length

You probably noticed the peculiar default line length. Black defaults to 88 characters per line, which happens to be 10% over 80. This number was found to produce significantly shorter files than sticking with 80 (the most popular), or even 79 (used by the standard library). In general, 90-ish seems like the wise choice.

If you’re paid by the line of code you write, you can pass --line-length with a lower number. Black will try to respect that. However, sometimes it won’t be able to without breaking other rules. In those rare cases, auto-formatted code will exceed your allotted limit.

You can also increase it, but remember that people with sight disabilities find it harder to work with line lengths exceeding 100 characters. It also adversely affects side-by-side diff review on typical screen resolutions. Long lines also make it harder to present code neatly in documentation or talk slides.

If you’re using Flake8, you can bump max-line-length to 88 and forget about it. Alternatively, use Bugbear’s B950 warning instead of E501 and keep the max line length at 80 which you are probably already using. You’d do it like this:

[flake8]
max-line-length = 80
...
select = C,E,F,W,B,B950
ignore = E501

You’ll find Black’s own .flake8 config file is configured like this. If you’re curious about the reasoning behind B950, Bugbear’s documentation explains it. The tl;dr is “it’s like highway speed limits, we won’t bother you if you overdo it by a few km/h”.

Editor integration

There is currently no integration with any text editors. Vim and Atom/Nuclide integration is planned by the author, others will require external contributions.

Patches welcome! ✨ 🍰 ✨