ruff/crates/ruff_python_formatter

Ruff Formatter

The Ruff formatter is an extremely fast Python code formatter that ships as part of the ruff CLI (as of Ruff v0.0.289).

The formatter is currently in an Alpha state. The Alpha is primarily intended for experimentation: our focus is on collecting feedback that we can address prior to a production-ready Beta release later this year. (While we're using the formatter in production on our own projects, the CLI, configuration options, and code style may change arbitrarily between the Alpha and Beta.)

We'd love to hear your feedback.

Goals

The formatter is designed to be a drop-in replacement for Black, but with an excessive focus on performance and direct integration with Ruff.

Specifically, the formatter is intended to emit near-identical output when run over Black-formatted code. When run over extensive Black-formatted projects like Django and Zulip, > 99.9% of lines are formatted identically. When migrating an existing project from Black to Ruff, you should expect to see a few differences on the margins, but the vast majority of your code should be unchanged.

If you identify deviations in your project, spot-check them against the intentional deviations enumerated below, as well as the unintentional deviations filed in the issue tracker. If you've identified a new deviation, please file an issue.

When run over non-Black-formatted code, the formatter makes some different decisions than Black, and so more deviations should be expected, especially around the treatment of end-of-line comments. For details, see Black compatibility.

Getting started

The Ruff formatter shipped in an Alpha state as part of Ruff v0.0.289.

CLI

The Ruff formatter is available as a standalone subcommand on the ruff CLI:

❯ ruff format --help
Run the Ruff formatter on the given files or directories

Usage: ruff format [OPTIONS] [FILES]...

Arguments:
  [FILES]...  List of files or directories to format

Options:
      --check            Avoid writing any formatted files back; instead, exit with a non-zero status code if any files would have been modified, and zero otherwise
      --config <CONFIG>  Path to the `pyproject.toml` or `ruff.toml` file to use for configuration
  -h, --help             Print help

File selection:
      --respect-gitignore  Respect file exclusions via `.gitignore` and other standard ignore files
      --force-exclude      Enforce exclusions, even for paths passed to Ruff directly on the command-line

Miscellaneous:
      --isolated                         Ignore all configuration files
      --stdin-filename <STDIN_FILENAME>  The name of the file when passing it through stdin

Log levels:
  -v, --verbose  Enable verbose logging
  -q, --quiet    Print diagnostics, but nothing else
  -s, --silent   Disable all logging (but still exit with status code "1" upon detecting diagnostics)

Note: ruff format is currently hidden by default and will not be visible when running ruff --help.

Similar to Black, running ruff format /path/to/file.py will format the given file or directory in-place, while ruff format --check /path/to/file.py will avoid writing any formatted files back, instead exiting with a non-zero status code if any files are not already formatted.

VS Code

As of v2023.36.0, the Ruff VS Code extension ships with support for the Ruff formatter. To enable formatting capabilities, set the ruff.enableExperimentalFormatter setting to true in your settings.json, and mark the Ruff extension as your default Python formatter:

{
  "ruff.enableExperimentalFormatter": true,
  "[python]": {
    "editor.defaultFormatter": "charliermarsh.ruff"
  }
}

From there, you can format a file by running the Format Document command, or enable formatting on-save by adding "editor.formatOnSave": true to your settings.json:

{
  "ruff.enableExperimentalFormatter": true,
  "[python]": {
    "editor.defaultFormatter": "charliermarsh.ruff",
    "editor.formatOnSave": true
  }
}

Configuration

The Ruff formatter respects Ruff's line-length setting, which can be provided via a pyproject.toml or ruff.toml file, or on the CLI, as in:

ruff format --line-length 100 /path/to/file.py

In future releases, the Ruff formatter will likely support configuration of:

• Quote style (single vs. double).
• Line endings (LF vs. CRLF).
• Indentation (tabs vs. spaces).
• Tab width.

Excluding code from formatting

Ruff supports Black's # fmt: off, # fmt: on, and # fmt: skip pragmas, with a few caveats.

See Ruff's suppression comment proposal for details.

Black compatibility

The formatter is designed to be a drop-in replacement for Black.

Specifically, the formatter is intended to emit near-identical output when run over Black-formatted code. When migrating an existing project from Black to Ruff, you should expect to see a few differences on the margins, but the vast majority of your code should be formatted identically. Note, however, that the formatter does not yet implement or support Black's preview style.

When run over non-Black-formatted code, the formatter makes some different decisions than Black, and so more deviations should be expected.

Intentional deviations

This section enumerates the known, intentional deviations between the Ruff formatter and Black's stable style. (Unintentional deviations are tracked in the issue tracker.)

Trailing end-of-line comments

Black's priority is to fit an entire statement on a line, even if it contains end-of-line comments. In such cases, Black collapses the statement, and moves the comment to the end of the collapsed statement:

# Input
while (
    cond1  # almost always true
    and cond2  # almost never true
):
    print("Do something")

# Black
while cond1 and cond2:  # almost always true  # almost never true
    print("Do something")

Ruff, like Prettier, expands any statement that contains trailing end-of-line comments. For example, Ruff would avoid collapsing the while test in the snippet above. This ensures that the comments remain close to their original position and retain their original intent, at the cost of retaining additional vertical space.

This deviation only impacts unformatted code, in that Ruff's output should not deviate for code that has already been formatted by Black.

Pragma comments are ignored when computing line width

Pragma comments (# type, # noqa, # pyright, # pylint, etc.) are ignored when computing the width of a line. This prevents Ruff from moving pragma comments around, thereby modifying their meaning and behavior:

See Ruff's pragma comment handling proposal for details.

This is similar to Pyink but a deviation from Black. Black avoids splitting any lines that contain a # type comment (#997), but otherwise avoids special-casing pragma comments.

As Ruff expands trailing end-of-line comments, Ruff will also avoid moving pragma comments in cases like the following, where moving the # noqa to the end of the line causes it to suppress errors on both first() and second():

# Input
[
    first(),  # noqa
    second()
]

# Black
[first(), second()]  # noqa

# Ruff
[
    first(),  # noqa
    second(),
]

Line width vs. line length

Ruff uses the Unicode width of a line to determine if a line fits. Black's stable style uses character width, while Black's preview style uses Unicode width for strings (#3445), and character width for all other tokens. Ruff's behavior is closer to Black's preview style than Black's stable style, although Ruff also uses Unicode width for identifiers and comments.

Walruses in slice expressions

Black avoids inserting space around := operators within slices. For example, the following adheres to Black stable style:

# Input
x[y:=1]

# Black
x[y:=1]

Ruff will instead add space around the := operator:

# Input
x[y:=1]

# Ruff
x[y := 1]

This will likely be incorporated into Black's preview style (#3823).

global and nonlocal names are broken across multiple lines by continuations

If a global or nonlocal statement includes multiple names, and exceeds the configured line width, Ruff will break them across multiple lines using continuations:

# Input
global analyze_featuremap_layer, analyze_featuremapcompression_layer, analyze_latencies_post, analyze_motions_layer, analyze_size_model

# Ruff
global \
    analyze_featuremap_layer, \
    analyze_featuremapcompression_layer, \
    analyze_latencies_post, \
    analyze_motions_layer, \
    analyze_size_model

Newlines are inserted after all class docstrings

Black typically enforces a single newline after a class docstring. However, it does not apply such formatting if the docstring is single-quoted rather than triple-quoted, while Ruff enforces a single newline in both cases:

# Input
class IntFromGeom(GEOSFuncFactory):
    "Argument is a geometry, return type is an integer."
    argtypes = [GEOM_PTR]
    restype = c_int
    errcheck = staticmethod(check_minus_one)

# Black
class IntFromGeom(GEOSFuncFactory):
    "Argument is a geometry, return type is an integer."
    argtypes = [GEOM_PTR]
    restype = c_int
    errcheck = staticmethod(check_minus_one)

# Ruff
class IntFromGeom(GEOSFuncFactory):
    "Argument is a geometry, return type is an integer."

    argtypes = [GEOM_PTR]
    restype = c_int
    errcheck = staticmethod(check_minus_one)

Trailing own-line comments on imports are not moved to the next line

Black enforces a single empty line between an import and a trailing own-line comment. Ruff leaves such comments in-place:

# Input
import os
# comment

import sys

# Black
import os

# comment

import sys

# Ruff
import os
# comment

import sys

Parentheses around awaited collections are not preserved

Black preserves parentheses around awaited collections:

await ([1, 2, 3])

Ruff will instead remove them:

await [1, 2, 3]

This is more consistent to the formatting of other awaited expressions: Ruff and Black both remove parentheses around, e.g., await (1), only retaining them when syntactically required, as in, e.g., await (x := 1).

Implicit string concatenations in attribute accesses (#7052)

Given the following unformatted code:

print("aaaaaaaaaaaaaaaa" "aaaaaaaaaaaaaaaa".format(bbbbbbbbbbbbbbbbbb + bbbbbbbbbbbbbbbbbb))

Internally, Black's logic will first expand the outermost print call:

print(
    "aaaaaaaaaaaaaaaa" "aaaaaaaaaaaaaaaa".format(bbbbbbbbbbbbbbbbbb + bbbbbbbbbbbbbbbbbb)
)

Since the argument is still too long, Black will then split on the operator with the highest split precedence. In this case, Black splits on the implicit string concatenation, to produce the following Black-formatted code:

print(
    "aaaaaaaaaaaaaaaa"
    "aaaaaaaaaaaaaaaa".format(bbbbbbbbbbbbbbbbbb + bbbbbbbbbbbbbbbbbb)
)

Ruff gives implicit concatenations a "lower" priority when breaking lines. As a result, Ruff would instead format the above as:

print(
    "aaaaaaaaaaaaaaaa" "aaaaaaaaaaaaaaaa".format(
        bbbbbbbbbbbbbbbbbb + bbbbbbbbbbbbbbbbbb
    )
)

In general, Black splits implicit string concatenations over multiple lines more often than Ruff, even if those concatenations can fit on a single line. Ruff instead avoids splitting such concatenations unless doing so is necessary to fit within the configured line width.

Own-line comments on expressions don't cause the expression to expand (#7314)

Given an expression like:

(
    # A comment in the middle
    some_example_var and some_example_var not in some_example_var
)

Black associates the comment with some_example_var, thus splitting it over two lines:

(
    # A comment in the middle
    some_example_var
    and some_example_var not in some_example_var
)

Ruff will instead associate the comment with the entire boolean expression, thus preserving the initial formatting:

(
    # A comment in the middle
    some_example_var and some_example_var not in some_example_var
)

Tuples are parenthesized when expanded (#7317)

Ruff tends towards parenthesizing tuples (with a few exceptions), while Black tends to remove tuple parentheses more often.

In particular, Ruff will always insert parentheses around tuples that expand over multiple lines:

# Input
(a, b), (c, d,)

# Black
(a, b), (
    c,
    d,
)

# Ruff
(
    (a, b),
    (
        c,
        c,
    ),
)

There's one exception here. In for loops, both Ruff and Black will avoid inserting unnecessary parentheses:

# Input
for a, f(b,) in c:
    pass

# Black
for a, f(
    b,
) in c:
    pass

# Ruff
for a, f(
    b,
) in c:
    pass

Single-element tuples are always parenthesized

Ruff always inserts parentheses around single-element tuples, while Black will omit them in some cases:

# Input
(a, b),

# Black
(a, b),

# Ruff
((a, b),)

Adding parentheses around single-element tuples adds visual distinction and helps avoid "accidental" tuples created by extraneous trailing commas (see, e.g., #17181).

Trailing commas are inserted when expanding a function definition with a single argument (#7323)

When a function definition with a single argument is expanded over multiple lines, Black will add a trailing comma in some cases, depending on whether the argument includes a type annotation and/or a default value.

For example, Black will add a trailing comma to the first and second function definitions below, but not the third:

def func(
    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa,
) -> None:
    ...


def func(
    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa=1,
) -> None:
    ...


def func(
    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa: Argument(
        "network_messages.pickle",
        help="The path of the pickle file that will contain the network messages",
    ) = 1
) -> None:
    ...

Ruff will instead insert a trailing comma in all such cases for consistency.

Parentheses around call-chain assignment values are not preserved (#7320)

Given:

def update_emission_strength():
    (
        get_rgbw_emission_node_tree(self)
        .nodes["Emission"]
        .inputs["Strength"]
        .default_value
    ) = (self.emission_strength * 2)

Black will preserve the parentheses in (self.emission_strength * 2), whereas Ruff will remove them.

Both Black and Ruff remove such parentheses in simpler assignments, like:

# Input
def update_emission_strength():
    value = (self.emission_strength * 2)

# Black
def update_emission_strength():
    value = self.emission_strength * 2

# Ruff
def update_emission_strength():
    value = self.emission_strength * 2

Type annotations may be parenthesized when expanded (#7315)

Black will avoid parenthesizing type annotations in an annotated assignment, while Ruff will insert parentheses in some cases.

For example:

# Black
StartElementHandler: Callable[[str, dict[str, str]], Any] | Callable[[str, list[str]], Any] | Callable[
    [str, dict[str, str], list[str]], Any
] | None

# Ruff
StartElementHandler: (
    Callable[[str, dict[str, str]], Any]
    | Callable[[str, list[str]], Any]
    | Callable[[str, dict[str, str], list[str]], Any]
    | None
)