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The Ruff Linter#

The Ruff Linter is an extremely fast Python linter designed as a drop-in replacement for Flake8 (plus dozens of plugins), isort, pydocstyle, pyupgrade, autoflake, and more.

ruff check#

ruff check is the primary entrypoint to the Ruff linter. It accepts a list of files or directories, and lints all discovered Python files, optionally fixing any fixable errors:

ruff check                  # Lint all files in the current directory.
ruff check --fix            # Lint all files in the current directory, and fix any fixable errors.
ruff check --watch          # Lint all files in the current directory, and re-lint on change.
ruff check path/to/code/    # Lint all files in `path/to/code` (and any subdirectories).

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

Note

As of Ruff v0.1.7 the ruff check command uses the current working directory (.) as the default path to check. On older versions, you must provide this manually e.g. ruff check .. See the file discovery documentation for details.

Rule selection#

The set of enabled rules is controlled via the lint.select, lint.extend-select, and lint.ignore settings.

Ruff's linter mirrors Flake8's rule code system, in which each rule code consists of a one-to-three letter prefix, followed by three digits (e.g., F401). The prefix indicates that "source" of the rule (e.g., F for Pyflakes, E for pycodestyle, ANN for flake8-annotations).

Rule selectors like lint.select and lint.ignore accept either a full rule code (e.g., F401) or any valid prefix (e.g., F). For example, given the following configuration file:

[tool.ruff.lint]
select = ["E", "F"]
ignore = ["F401"]

[lint]
select = ["E", "F"]
ignore = ["F401"]

Ruff would enable all rules with the E (pycodestyle) or F (Pyflakes) prefix, with the exception of F401. For more on configuring Ruff via pyproject.toml, see Configuring Ruff.

As a special-case, Ruff also supports the ALL code, which enables all rules. Note that some pydocstyle rules conflict (e.g., D203 and D211) as they represent alternative docstring formats. Ruff will automatically disable any conflicting rules when ALL is enabled.

If you're wondering how to configure Ruff, here are some recommended guidelines:

  • Prefer lint.select over lint.extend-select to make your rule set explicit.
  • Use ALL with discretion. Enabling ALL will implicitly enable new rules whenever you upgrade.
  • Start with a small set of rules (select = ["E", "F"]) and add a category at-a-time. For example, you might consider expanding to select = ["E", "F", "B"] to enable the popular flake8-bugbear extension.

For example, a configuration that enables some of the most popular rules (without being too pedantic) might look like the following:

[tool.ruff.lint]
select = [
    # pycodestyle
    "E",
    # Pyflakes
    "F",
    # pyupgrade
    "UP",
    # flake8-bugbear
    "B",
    # flake8-simplify
    "SIM",
    # isort
    "I",
]

[lint]
select = [
    # pycodestyle
    "E",
    # Pyflakes
    "F",
    # pyupgrade
    "UP",
    # flake8-bugbear
    "B",
    # flake8-simplify
    "SIM",
    # isort
    "I",
]

To resolve the enabled rule set, Ruff may need to reconcile lint.select and lint.ignore from a variety of sources, including the current pyproject.toml, any inherited pyproject.toml files, and the CLI (e.g., --select).

In those scenarios, Ruff uses the "highest-priority" select as the basis for the rule set, and then applies extend-select and ignore adjustments. CLI options are given higher priority than pyproject.toml options, and the current pyproject.toml file is given higher priority than any inherited pyproject.toml files.

For example, given the following configuration file:

[tool.ruff.lint]
select = ["E", "F"]
ignore = ["F401"]

[lint]
select = ["E", "F"]
ignore = ["F401"]

Running ruff check --select F401 would result in Ruff enforcing F401, and no other rules.

Running ruff check --extend-select B would result in Ruff enforcing the E, F, and B rules, with the exception of F401.

Fixes#

Ruff supports automatic fixes for a variety of lint errors. For example, Ruff can remove unused imports, reformat docstrings, rewrite type annotations to use newer Python syntax, and more.

To enable fixes, pass the --fix flag to ruff check:

ruff check --fix

By default, Ruff will fix all violations for which safe fixes are available; to determine whether a rule supports fixing, see Rules.

Fix safety#

Ruff labels fixes as "safe" and "unsafe". The meaning and intent of your code will be retained when applying safe fixes, but the meaning could be changed when applying unsafe fixes.

For example, unnecessary-iterable-allocation-for-first-element (RUF015) is a rule which checks for potentially unperformant use of list(...)[0]. The fix replaces this pattern with next(iter(...)) which can result in a drastic speedup:

$ python -m timeit "head = list(range(99999999))[0]"
1 loop, best of 5: 1.69 sec per loop

$ python -m timeit "head = next(iter(range(99999999)))"
5000000 loops, best of 5: 70.8 nsec per loop

However, when the collection is empty, this changes the raised exception from an IndexError to StopIteration:

$ python -c 'list(range(0))[0]'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
IndexError: list index out of range

$ python -c 'next(iter(range(0)))[0]'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
StopIteration

Since this could break error handling, this fix is categorized as unsafe.

Ruff only enables safe fixes by default. Unsafe fixes can be enabled by settings unsafe-fixes in your configuration file or passing the --unsafe-fixes flag to ruff check:

# Show unsafe fixes
ruff check --unsafe-fixes

# Apply unsafe fixes
ruff check --fix --unsafe-fixes

By default, Ruff will display a hint when unsafe fixes are available but not enabled. The suggestion can be silenced by setting the unsafe-fixes setting to false or using the --no-unsafe-fixes flag.

The safety of fixes can be adjusted per rule using the lint.extend-safe-fixes and lint.extend-unsafe-fixes settings.

For example, the following configuration would promote unsafe fixes for F601 to safe fixes and demote safe fixes for UP034 to unsafe fixes:

[tool.ruff.lint]
extend-safe-fixes = ["F601"]
extend-unsafe-fixes = ["UP034"]

[lint]
extend-safe-fixes = ["F601"]
extend-unsafe-fixes = ["UP034"]

You may use prefixes to select rules as well, e.g., F can be used to promote fixes for all rules in Pyflakes to safe.

Note

All fixes will always be displayed by Ruff when using the json output format. The safety of each fix is available under the applicability field.

Disabling fixes#

To limit the set of rules that Ruff should fix, use the lint.fixable and lint.unfixable settings, along with their lint.extend-fixable and lint.extend-unfixable variants.

For example, the following configuration would enable fixes for all rules except unused-imports (F401):

[tool.ruff.lint]
fixable = ["ALL"]
unfixable = ["F401"]

[lint]
fixable = ["ALL"]
unfixable = ["F401"]

Conversely, the following configuration would only enable fixes for F401:

[tool.ruff.lint]
fixable = ["F401"]

[lint]
fixable = ["F401"]

Error suppression#

Ruff supports several mechanisms for suppressing lint errors, be they false positives or permissible violations.

To omit a lint rule entirely, add it to the "ignore" list via the lint.ignore setting, either on the command-line or in your pyproject.toml or ruff.toml file.

To suppress a violation inline, Ruff uses a noqa system similar to Flake8. To ignore an individual violation, add # noqa: {code} to the end of the line, like so:

# Ignore F841.
x = 1  # noqa: F841

# Ignore E741 and F841.
i = 1  # noqa: E741, F841

# Ignore _all_ violations.
x = 1  # noqa

For multi-line strings (like docstrings), the noqa directive should come at the end of the string (after the closing triple quote), and will apply to the entire string, like so:

"""Lorem ipsum dolor sit amet.

Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor.
"""  # noqa: E501

For import sorting, the noqa should come at the end of the first line in the import block, and will apply to all imports in the block, like so:

import os  # noqa: I001
import abc

To ignore all violations across an entire file, add the line # ruff: noqa anywhere in the file, preferably towards the top, like so:

# ruff: noqa

To ignore a specific rule across an entire file, add the line # ruff: noqa: {code} anywhere in the file, preferably towards the top, like so:

# ruff: noqa: F841

Or see the lint.per-file-ignores setting, which enables the same functionality from within your pyproject.toml or ruff.toml file.

Global noqa comments must be on their own line to disambiguate from comments which ignore violations on a single line.

Note that Ruff will also respect Flake8's # flake8: noqa directive, and will treat it as equivalent to # ruff: noqa.

Detecting unused suppression comments#

Ruff implements a special rule, unused-noqa, under the RUF100 code, to enforce that your noqa directives are "valid", in that the violations they say they ignore are actually being triggered on that line (and thus suppressed). To flag unused noqa directives, run: ruff check /path/to/file.py --extend-select RUF100.

Ruff can also remove any unused noqa directives via its fix functionality. To remove any unused noqa directives, run: ruff check /path/to/file.py --extend-select RUF100 --fix.

Inserting necessary suppression comments#

Ruff can automatically add noqa directives to all lines that contain violations, which is useful when migrating a new codebase to Ruff. To automatically add noqa directives to all relevant lines (with the appropriate rule codes), run: ruff check /path/to/file.py --add-noqa.

Action comments#

Ruff respects isort's action comments (# isort: skip_file, # isort: on, # isort: off, # isort: skip, and # isort: split), which enable selectively enabling and disabling import sorting for blocks of code and other inline configuration.

Ruff will also respect variants of these action comments with a # ruff: prefix (e.g., # ruff: isort: skip_file, # ruff: isort: on, and so on). These variants more clearly convey that the action comment is intended for Ruff, but are functionally equivalent to the isort variants.

Unlike isort, Ruff does not respect action comments within docstrings.

See the isort documentation for more.

Exit codes#

By default, ruff check exits with the following status codes:

  • 0 if no violations were found, or if all present violations were fixed automatically.
  • 1 if violations were found.
  • 2 if Ruff terminates abnormally due to invalid configuration, invalid CLI options, or an internal error.

This convention mirrors that of tools like ESLint, Prettier, and RuboCop.

ruff check supports two command-line flags that alter its exit code behavior:

  • --exit-zero will cause Ruff to exit with a status code of 0 even if violations were found. Note that Ruff will still exit with a status code of 2 if it terminates abnormally.
  • --exit-non-zero-on-fix will cause Ruff to exit with a status code of 1 if violations were found, even if all such violations were fixed automatically. Note that the use of --exit-non-zero-on-fix can result in a non-zero exit code even if no violations remain after fixing.