The Black code style

Code style

Black reformats entire files in place. Style configuration options are deliberately limited and rarely added. It doesn’t take previous formatting into account, except for the magic trailing comma and preserving newlines. It doesn’t reformat blocks that start with # fmt: off and end with # fmt: on, or lines that ends with # fmt: skip. # fmt: on/off have to be on the same level of indentation. It also recognizes YAPF’s block comments to the same effect, as a courtesy for straddling code.

How Black wraps lines

Black ignores previous formatting and applies uniform horizontal and vertical whitespace to your code. The rules for horizontal whitespace can be summarized as: do whatever makes pycodestyle happy. The coding style used by Black can be viewed as a strict subset of PEP 8.

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:

j = [1,
     2,
     3
]

# out:

j = [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:

ImportantClass.important_method(exc, limit, lookup_lines, capture_locals, extra_argument)

# out:

ImportantClass.important_method(
    exc, limit, lookup_lines, capture_locals, extra_argument
)

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, engine: str, header: bool = True, 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,
    engine: str,
    header: bool = True,
    debug: bool = False,
):
    """Applies `variables` to the `template` and writes to `file`."""
    with open(file, "w") as f:
        ...

Black prefers parentheses over backslashes, and will remove backslashes if found.

# in:

if some_short_rule1 \
  and some_short_rule2:
      ...

# out:

if some_short_rule1 and some_short_rule2:
  ...


# in:

if some_long_rule1 \
  and some_long_rule2:
    ...

# out:

if (
    some_long_rule1
    and some_long_rule2
):
    ...

Backslashes and multiline strings are one of the two places in the Python grammar that break significant indentation. You never need backslashes, they are used to force the grammar to accept breaks that would otherwise be parse errors. That makes them confusing to look at and brittle to modify. This is why Black always gets rid of them.

If you’re reaching for backslashes, that’s a clear signal that you can do better if you slightly refactor your code. I hope some of the examples above show you that there are many ways in which you can do it.

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).

If a data structure literal (tuple, list, set, dict) or a line of “from” imports cannot fit in the allotted length, it’s always split into one element per line. This minimizes diffs as well as enables readers of code to find which commit introduced a particular entry. This also makes Black compatible with isort with the ready-made black profile or manual configuration.

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 mostly forget about it. However, it’s better if you use Bugbear’s B950 warning instead of E501, and bump the max line length to 88 (or the --line-length you used for black), which will align more with black’s “try to respect --line-length, but don’t become crazy if you can’t”. You’d do it like this:

[flake8]
max-line-length = 88
...
select = C,E,F,W,B,B950
extend-ignore = E203, E501

Explanation of why E203 is disabled can be found further in this documentation. And 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”.

If you’re looking for a minimal, black-compatible flake8 configuration:

[flake8]
max-line-length = 88
extend-ignore = E203

Empty lines

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.

Black will allow single empty lines inside functions, and single and double empty lines on module level left by the original editors, except when they’re within parenthesized expressions. Since such expressions are always reformatted to fit minimal space, this whitespace is lost.

It will also insert proper spacing before and after function definitions. It’s one line before and after inner functions and two lines before and after module-level functions and classes. Black will not put empty lines between function/class definitions and standalone comments that immediately precede the given function/class.

Black will enforce single empty lines between a class-level docstring and the first following field or method. This conforms to PEP 257.

Black won’t insert empty lines after function docstrings unless that empty line is required due to an inner function starting immediately after.

Comments

Black does not format comment contents, but it enforces two spaces between code and a comment on the same line, and a space before the comment text begins. Some types of comments that require specific spacing rules are respected: doc comments (#: comment), section comments with long runs of hashes, and Spyder cells. Non-breaking spaces after hashes are also preserved. Comments may sometimes be moved because of formatting changes, which can break tools that assign special meaning to them. See AST before and after formatting for more discussion.

Trailing commas

Black will add trailing commas to expressions that are split by comma where each element is on its own line. This includes function signatures.

One exception to adding trailing commas is function signatures containing *, *args, or **kwargs. In this case a trailing comma is only safe to use on Python 3.6. Black will detect if your file is already 3.6+ only and use trailing commas in this situation. If you wonder how it knows, it looks for f-strings and existing use of trailing commas in function signatures that have stars in them. In other words, if you’d like a trailing comma in this situation and Black didn’t recognize it was safe to do so, put it there manually and Black will keep it.

A pre-existing trailing comma informs Black to always explode contents of the current bracket pair into one item per line. Read more about this in the Pragmatism section below.

Strings

Black prefers double quotes (" and """) over single quotes (' and '''). It will replace the latter with the former as long as it does not result in more backslash escapes than before.

Black also standardizes string prefixes, making them always lowercase. On top of that, if your code is already Python 3.6+ only or it’s using the unicode_literals future import, Black will remove u from the string prefix as it is meaningless in those scenarios.

The main reason to standardize on a single form of quotes is aesthetics. Having one kind of quotes everywhere reduces reader distraction. It will also enable a future version of Black to merge consecutive string literals that ended up on the same line (see #26 for details).

Why settle on double quotes? They anticipate apostrophes in English text. They match the docstring standard described in PEP 257. An empty string in double quotes ("") is impossible to confuse with a one double-quote regardless of fonts and syntax highlighting used. On top of this, double quotes for strings are consistent with C which Python interacts a lot with.

On certain keyboard layouts like US English, typing single quotes is a bit easier than double quotes. The latter requires use of the Shift key. My recommendation here is to keep using whatever is faster to type and let Black handle the transformation.

If you are adopting Black in a large project with pre-existing string conventions (like the popular “single quotes for data, double quotes for human-readable strings”), you can pass --skip-string-normalization on the command line. This is meant as an adoption helper, avoid using this for new projects.

As an experimental option (can be enabled by --experimental-string-processing), Black splits long strings (using parentheses where appropriate) and merges short ones. When split, parts of f-strings that don’t need formatting are converted to plain strings. User-made splits are respected when they do not exceed the line length limit. Line continuation backslashes are converted into parenthesized strings. Unnecessary parentheses are stripped. Because the functionality is experimental, feedback and issue reports are highly encouraged!

Black also processes docstrings. Firstly the indentation of docstrings is corrected for both quotations and the text within, although relative indentation in the text is preserved. Superfluous trailing whitespace on each line and unnecessary new lines at the end of the docstring are removed. All leading tabs are converted to spaces, but tabs inside text are preserved. Whitespace leading and trailing one-line docstrings is removed. The quotations of an empty docstring are separated with one space.

Numeric literals

Black standardizes most numeric literals to use lowercase letters for the syntactic parts and uppercase letters for the digits themselves: 0xAB instead of 0XAB and 1e10 instead of 1E10. Python 2 long literals are styled as 2L instead of 2l to avoid confusion between l and 1.

Line breaks & binary operators

Black will break a line before a binary operator when splitting a block of code over multiple lines. This is so that Black is compliant with the recent changes in the PEP 8 style guide, which emphasizes that this approach improves readability.

Slices

PEP 8 recommends to treat : in slices as a binary operator with the lowest priority, and to leave an equal amount of space on either side, except if a parameter is omitted (e.g. ham[1 + 1 :]). It recommends no spaces around : operators for “simple expressions” (ham[lower:upper]), and extra space for “complex expressions” (ham[lower : upper + offset]). Black treats anything more than variable names as “complex” (ham[lower : upper + 1]). It also states that for extended slices, both : operators have to have the same amount of spacing, except if a parameter is omitted (ham[1 + 1 ::]). Black enforces these rules consistently.

This behaviour may raise E203 whitespace before ':' warnings in style guide enforcement tools like Flake8. Since E203 is not PEP 8 compliant, you should tell Flake8 to ignore these warnings.

Parentheses

Some parentheses are optional in the Python grammar. Any expression can be wrapped in a pair of parentheses to form an atom. There are a few interesting cases:

  • if (...):

  • while (...):

  • for (...) in (...):

  • assert (...), (...)

  • from X import (...)

  • assignments like:

    • target = (...)

    • target: type = (...)

    • some, *un, packing = (...)

    • augmented += (...)

In those cases, parentheses are removed when the entire statement fits in one line, or if the inner expression doesn’t have any delimiters to further split on. If there is only a single delimiter and the expression starts or ends with a bracket, the parentheses can also be successfully omitted since the existing bracket pair will organize the expression neatly anyway. Otherwise, the parentheses are added.

Please note that Black does not add or remove any additional nested parentheses that you might want to have for clarity or further code organization. For example those parentheses are not going to be removed:

return not (this or that)
decision = (maybe.this() and values > 0) or (maybe.that() and values < 0)

Call chains

Some popular APIs, like ORMs, use call chaining. This API style is known as a fluent interface. Black formats those by treating dots that follow a call or an indexing operation like a very low priority delimiter. It’s easier to show the behavior than to explain it. Look at the example:

def example(session):
    result = (
        session.query(models.Customer.id)
        .filter(
            models.Customer.account_id == account_id,
            models.Customer.email == email_address,
        )
        .order_by(models.Customer.id.asc())
        .all()
    )

Typing stub files

PEP 484 describes the syntax for type hints in Python. One of the use cases for typing is providing type annotations for modules which cannot contain them directly (they might be written in C, or they might be third-party, or their implementation may be overly dynamic, and so on).

To solve this, stub files with the .pyi file extension can be used to describe typing information for an external module. Those stub files omit the implementation of classes and functions they describe, instead they only contain the structure of the file (listing globals, functions, and classes with their members). The recommended code style for those files is more terse than PEP 8:

  • prefer ... on the same line as the class/function signature;

  • avoid vertical whitespace between consecutive module-level functions, names, or methods and fields within a single class;

  • use a single blank line between top-level class definitions, or none if the classes are very small.

Black enforces the above rules. There are additional guidelines for formatting .pyi file that are not enforced yet but might be in a future version of the formatter:

  • all function bodies should be empty (contain ... instead of the body);

  • do not use docstrings;

  • prefer ... over pass;

  • for arguments with a default, use ... instead of the actual default;

  • avoid using string literals in type annotations, stub files support forward references natively (like Python 3.7 code with from __future__ import annotations);

  • use variable annotations instead of type comments, even for stubs that target older versions of Python;

  • for arguments that default to None, use Optional[] explicitly;

  • use float instead of Union[int, float].

Pragmatism

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. This section documents what those exceptions are and why this is the case.

The magic trailing comma

Black in general does not take existing formatting into account.

However, there are cases where you put a short collection or function call in your code but you anticipate it will grow in the future.

For example:

TRANSLATIONS = {
    "en_us": "English (US)",
    "pl_pl": "polski",
}

Early versions of Black used to ruthlessly collapse those into one line (it fits!). Now, you can communicate that you don’t want that by putting a trailing comma in the collection yourself. When you do, Black will know to always explode your collection into one item per line.

How do you make it stop? Just delete that trailing comma and Black will collapse your collection into one line if it fits.

If you must, you can recover the behaviour of early versions of Black with the option --skip-magic-trailing-comma / -C.

r”strings” and R”strings”

Black normalizes string quotes as well as string prefixes, making them lowercase. One exception to this rule is r-strings. It turns out that the very popular MagicPython syntax highlighter, used by default by (among others) GitHub and Visual Studio Code, differentiates between r-strings and R-strings. The former are syntax highlighted as regular expressions while the latter are treated as true raw strings with no special semantics.

AST before and after formatting

When run with --safe, Black checks that the code before and after is semantically equivalent. This check is done by comparing the AST of the source with the AST of the target. There are three limited cases in which the AST does differ:

  1. Black cleans up leading and trailing whitespace of docstrings, re-indenting them if needed. It’s been one of the most popular user-reported features for the formatter to fix whitespace issues with docstrings. While the result is technically an AST difference, due to the various possibilities of forming docstrings, all realtime use of docstrings that we’re aware of sanitizes indentation and leading/trailing whitespace anyway.

  2. Black manages optional parentheses for some statements. In the case of the del statement, presence of wrapping parentheses or lack of thereof changes the resulting AST but is semantically equivalent in the interpreter.

  3. Black might move comments around, which includes type comments. Those are part of the AST as of Python 3.8. While the tool implements a number of special cases for those comments, there is no guarantee they will remain where they were in the source. Note that this doesn’t change runtime behavior of the source code.

To put things in perspective, the code equivalence check is a feature of Black which other formatters don’t implement at all. It is of crucial importance to us to ensure code behaves the way it did before it got reformatted. We treat this as a feature and there are no plans to relax this in the future. The exceptions enumerated above stem from either user feedback or implementation details of the tool. In each case we made due diligence to ensure that the AST divergence is of no practical consequence.