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The Ruff Formatter

The Ruff formatter is an extremely fast Python code formatter designed as a drop-in replacement for Black, available as part of the ruff CLI via ruff format.

ruff format

ruff format is the primary entrypoint to the formatter. It accepts a list of files or directories, and formats all discovered Python files:

ruff format                   # Format all files in the current directory.
ruff format path/to/code/     # Format all files in `path/to/code` (and any subdirectories).
ruff format path/to/file.py   # Format a single file.

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, and instead exit with a non-zero status code upon detecting any unformatted files.

For the full list of supported options, run ruff format --help.

Philosophy

The initial goal of the Ruff formatter is not to innovate on code style, but rather, to innovate on performance, and provide a unified toolchain across Ruff's linter, formatter, and any and all future tools.

As such, the formatter is designed as a drop-in replacement for Black, but with an excessive focus on performance and direct integration with Ruff. Given Black's popularity within the Python ecosystem, targeting Black compatibility ensures that formatter adoption is minimally disruptive for the vast majority of projects.

Specifically, the formatter is intended to emit near-identical output when run over existing Black-formatted code. When run over extensive Black-formatted projects like Django and Zulip, > 99.9% of lines are formatted identically. (See: _Style Guide.)

Given this focus on Black compatibility, the formatter thus adheres to Black's (stable) code style, which aims for "consistency, generality, readability and reducing git diffs". To give you a sense for the enforced code style, here's an example:

# Input
def _make_ssl_transport(
    rawsock, protocol, sslcontext, waiter=None,
    *, server_side=False, server_hostname=None,
    extra=None, server=None,
    ssl_handshake_timeout=None,
    call_connection_made=True):
    '''Make an SSL transport.'''
    if waiter is None:
      waiter = Future(loop=loop)

    if extra is None:
      extra = {}

    ...

# Ruff
def _make_ssl_transport(
    rawsock,
    protocol,
    sslcontext,
    waiter=None,
    *,
    server_side=False,
    server_hostname=None,
    extra=None,
    server=None,
    ssl_handshake_timeout=None,
    call_connection_made=True,
):
    """Make an SSL transport."""
    if waiter is None:
        waiter = Future(loop=loop)

    if extra is None:
        extra = {}

    ...

Like Black, the Ruff formatter does not support extensive code style configuration; however, unlike Black, it does support configuring the desired quote style, indent style, line endings, and more. (See: Configuration.)

While the formatter is designed to be a drop-in replacement for Black, it is not intended to be used interchangeably with Black on an ongoing basis, as the formatter does differ from Black in a few conscious ways (see: Known deviations). In general, deviations are limited to cases in which Ruff's behavior was deemed more consistent, or significantly simpler to support (with negligible end-user impact) given the differences in the underlying implementations between Black and Ruff.

Going forward, the Ruff Formatter will support Black's preview style under Ruff's own preview mode.

Configuration

The Ruff Formatter exposes a small set of configuration options, some of which are also supported by Black (like line width), some of which are unique to Ruff (like quote, indentation style and formatting code examples in docstrings).

For example, to configure the formatter to use single quotes, format code examples in docstrings, a line width of 100, and tab indentation, add the following to your configuration file:

[tool.ruff]
line-length = 100

[tool.ruff.format]
quote-style = "single"
indent-style = "tab"
docstring-code-format = true

line-length = 100

[format]
quote-style = "single"
indent-style = "tab"
docstring-code-format = true

For the full list of supported settings, see Settings. For more on configuring Ruff via pyproject.toml, see Configuring Ruff.

Given the focus on Black compatibility (and unlike formatters like YAPF), Ruff does not currently expose any other configuration options.

Docstring formatting

The Ruff formatter provides an opt-in feature for automatically formatting Python code examples in docstrings. The Ruff formatter currently recognizes code examples in the following formats:

  • The Python doctest format.
  • CommonMark fenced code blocks with the following info strings: python, py, python3, or py3. Fenced code blocks without an info string are assumed to be Python code examples and also formatted.
  • reStructuredText literal blocks. While literal blocks may contain things other than Python, this is meant to reflect a long-standing convention in the Python ecosystem where literal blocks often contain Python code.
  • reStructuredText [code-block and sourcecode directives]. As with Markdown, the language names recognized for Python are python, py, python3, or py3.

If a code example is recognized and treated as Python, the Ruff formatter will automatically skip it if the code does not parse as valid Python or if the reformatted code would produce an invalid Python program.

Users may also configure the line length limit used for reformatting Python code examples in docstrings. The default is a special value, dynamic, which instructs the formatter to respect the line length limit setting for the surrounding Python code. The dynamic setting ensures that even when code examples are found inside indented docstrings, the line length limit configured for the surrounding Python code will not be exceeded. Users may also configure a fixed line length limit for code examples in docstrings.

For example, this configuration shows how to enable docstring code formatting with a fixed line length limit:

[tool.ruff.format]
docstring-code-format = true
docstring-code-line-length = 20

[format]
docstring-code-format = true
docstring-code-line-length = 20

With the above configuration, this code:

def f(x):
    '''
    Something about `f`. And an example:

    .. code-block:: python

        foo, bar, quux = this_is_a_long_line(lion, hippo, lemur, bear)
    '''
    pass

... will be reformatted (assuming the rest of the options are set to their defaults) as:

def f(x):
    """
    Something about `f`. And an example:

    .. code-block:: python

        (
            foo,
            bar,
            quux,
        ) = this_is_a_long_line(
            lion,
            hippo,
            lemur,
            bear,
        )
    """
    pass

Format suppression

Like Black, Ruff supports # fmt: on, # fmt: off, and # fmt: skip pragma comments, which can be used to temporarily disable formatting for a given code block.

# fmt: on and # fmt: off comments are enforced at the statement level:

# fmt: off
not_formatted=3
also_not_formatted=4
# fmt: on

As such, adding # fmt: on and # fmt: off comments within expressions will have no effect. In the following example, both list entries will be formatted, despite the # fmt: off:

[
    # fmt: off
    '1',
    # fmt: on
    '2',
]

Instead, apply the # fmt: off comment to the entire statement:

# fmt: off
[
    '1',
    '2',
]
# fmt: on

Like Black, Ruff will also recognize YAPF's # yapf: disable and # yapf: enable pragma comments, which are treated equivalently to # fmt: off and # fmt: on, respectively.

# fmt: skip comments suppress formatting for a preceding statement, case header, decorator, function definition, or class definition:

if True:
    pass
elif False: # fmt: skip
    pass

@Test
@Test2 # fmt: skip
def test(): ...

a = [1, 2, 3, 4, 5] # fmt: skip

def test(a, b, c, d, e, f) -> int: # fmt: skip
    pass

As such, adding an # fmt: skip comment at the end of an expression will have no effect. In the following example, the list entry '1' will be formatted, despite the # fmt: skip:

a = call(
    [
        '1',  # fmt: skip
        '2',
    ],
    b
)

Instead, apply the # fmt: skip comment to the entire statement:

a = call(
  [
    '1',
    '2',
  ],
  b
)  # fmt: skip

Conflicting lint rules

Ruff's formatter is designed to be used alongside the linter. However, the linter includes some rules that, when enabled, can cause conflicts with the formatter, leading to unexpected behavior. When configured appropriately, the goal of Ruff's formatter-linter compatibility is such that running the formatter should never introduce new lint errors.

When using Ruff as a formatter, we recommend avoiding the following lint rules:

While the line-too-long (E501) rule can be used alongside the formatter, the formatter only makes a best-effort attempt to wrap lines at the configured line-length. As such, formatted code may exceed the line length, leading to line-too-long (E501) errors.

None of the above are included in Ruff's default configuration. However, if you've enabled any of these rules or their parent categories (like Q), we recommend disabling them via the linter's lint.ignore setting.

Similarly, we recommend avoiding the following isort settings, which are incompatible with the formatter's treatment of import statements when set to non-default values:

If you've configured any of these settings to take on non-default values, we recommend removing them from your Ruff configuration.

When an incompatible lint rule or setting is enabled, ruff format will emit a warning. If your ruff format is free of warnings, you're good to go!

Exit codes

ruff format exits with the following status codes:

  • 0 if Ruff terminates successfully, regardless of whether any files were formatted.
  • 2 if Ruff terminates abnormally due to invalid configuration, invalid CLI options, or an internal error.

Meanwhile, ruff format --check exits with the following status codes:

  • 0 if Ruff terminates successfully, and no files would be formatted if --check were not specified.
  • 1 if Ruff terminates successfully, and one or more files would be formatted if --check were not specified.
  • 2 if Ruff terminates abnormally due to invalid configuration, invalid CLI options, or an internal error.

Style Guide

The formatter is designed to be a drop-in replacement for Black. This section documents the areas where the Ruff formatter goes beyond Black in terms of code style.

Intentional deviations

While the Ruff formatter aims to be a drop-in replacement for Black, it does differ from Black in a few known ways. Some of these differences emerge from conscious attempts to improve upon Black's code style, while others fall out of differences in the underlying implementations.

For a complete enumeration of these intentional deviations, see Known deviations.

Unintentional deviations from Black are tracked in the issue tracker. If you've identified a new deviation, please file an issue.

Preview style

Similar to Black, Ruff implements formatting changes under the preview flag, promoting them to stable through minor releases, in accordance with our versioning policy.

F-string formatting

Stabilized in Ruff 0.9.0

Unlike Black, Ruff formats the expression parts of f-strings which are the parts inside the curly braces {...}. This is a known deviation from Black.

Ruff employs several heuristics to determine how an f-string should be formatted which are detailed below.

Quotes

Ruff will use the configured quote style for the f-string expression unless doing so would result in invalid syntax for the target Python version or requires more backslash escapes than the original expression. Specifically, Ruff will preserve the original quote style for the following cases:

When the target Python version is < 3.12 and a self-documenting f-string contains a string literal with the configured quote style:

# format.quote-style = "double"

f'{10 + len("hello")=}'
# This f-string cannot be formatted as follows when targeting Python < 3.12
f"{10 + len("hello")=}"

When the target Python version is < 3.12 and an f-string contains any triple-quoted string, byte or f-string literal that contains the configured quote style:

# format.quote-style = "double"

f'{"""nested " """}'
# This f-string cannot be formatted as follows when targeting Python < 3.12
f"{'''nested " '''}"

For all target Python versions, when a self-documenting f-string contains an expression between the curly braces ({...}) with a format specifier containing the configured quote style:

# format.quote-style = "double"

f'{1=:"foo}'
# This f-string cannot be formatted as follows for all target Python versions
f"{1=:"foo}"

For nested f-strings, Ruff alternates quote styles, starting with the configured quote style for the outermost f-string. For example, consider the following f-string:

# format.quote-style = "double"

f"outer f-string {f"nested f-string {f"another nested f-string"} end"} end"

Ruff formats it as:

f"outer f-string {f'nested f-string {f"another nested f-string"} end'} end"

Line breaks

Starting with Python 3.12 (PEP 701), the expression parts of an f-string can span multiple lines. Ruff needs to decide when to introduce a line break in an f-string expression. This depends on the semantic content of the expression parts of an f-string - for example, introducing a line break in the middle of a natural-language sentence is undesirable. Since Ruff doesn't have enough information to make that decision, it adopts a heuristic similar to Prettier: it will only split the expression parts of an f-string across multiple lines if there was already a line break within any of the expression parts.

For example, the following code:

f"this f-string has a multiline expression {
  ['red', 'green', 'blue', 'yellow',]} and does not fit within the line length"

... is formatted as:

# The list expression is split across multiple lines because of the trailing comma
f"this f-string has a multiline expression {
    [
        'red',
        'green',
        'blue',
        'yellow',
    ]
} and does not fit within the line length"

But, the following will not be split across multiple lines even though it exceeds the line length:

f"this f-string has a multiline expression {['red', 'green', 'blue', 'yellow']} and does not fit within the line length"

If you want Ruff to split an f-string across multiple lines, ensure there's a linebreak somewhere within the {...} parts of an f-string.

Sorting imports

Currently, the Ruff formatter does not sort imports. In order to both sort imports and format, call the Ruff linter and then the formatter:

ruff check --select I --fix
ruff format

A unified command for both linting and formatting is planned.