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Preview#

Ruff includes an opt-in preview mode to provide an opportunity for community feedback and increase confidence that changes are a net-benefit before enabling them for everyone.

Preview mode enables a collection of unstable features such as new lint rules and fixes, formatter style changes, interface updates, and more. Warnings about deprecated features may turn into errors when using preview mode.

Enabling preview mode does not on its own enable all preview rules. See the rules section for details on selecting preview rules.

Enabling preview mode#

Preview mode can be enabled with the --preview flag on the CLI or by setting preview = true in your Ruff configuration file.

Preview mode can be configured separately for linting and formatting (requires Ruff v0.1.1+). To enable preview lint rules without preview style formatting:

[tool.ruff.lint]
preview = true

[lint]
preview = true

ruff check --preview

To enable preview style formatting without enabling any preview lint rules:

[tool.ruff.format]
preview = true

[format]
preview = true

ruff format --preview

Using rules that are in preview#

If a rule is marked as preview, it can only be selected if preview mode is enabled. For example, consider a hypothetical rule, HYP001. If HYP001 were in preview, it would not be enabled by adding it to the selected rule set.

[tool.ruff.lint]
extend-select = ["HYP001"]

[lint]
extend-select = ["HYP001"]

ruff check --extend-select HYP001

It also would not be enabled by selecting the HYP category, like so:

[tool.ruff.lint]
extend-select = ["HYP"]

[lint]
extend-select = ["HYP"]

ruff check --extend-select HYP

Similarly, it would not be enabled via the ALL selector:

[tool.ruff.lint]
select = ["ALL"]

[lint]
select = ["ALL"]

ruff check --select ALL

However, it would be enabled in any of the above cases if you enabled preview mode:

[tool.ruff.lint]
extend-select = ["HYP"]
preview = true

[lint]
extend-select = ["HYP"]
preview = true

ruff check --extend-select HYP --preview

To see which rules are currently in preview, visit the rules reference.

Selecting single preview rules#

When preview mode is enabled, selecting rule categories or prefixes will include all preview rules that match. If you'd prefer to opt in to each preview rule individually, you can toggle the explicit-preview-rules setting in your configuration file:

[tool.ruff.lint]
preview = true
explicit-preview-rules = true

[lint]
preview = true
explicit-preview-rules = true

In our previous example, --select with ALL HYP, HYP0, or HYP00 would not enable HYP001. Each preview rule will need to be selected with its exact code, e.g. --select ALL,HYP001.

If preview mode is not enabled, this setting has no effect.

Deprecated rules#

When preview mode is enabled, deprecated rules will be disabled. If a deprecated rule is selected explicitly, an error will be raised. Deprecated rules will not be included if selected via a rule category or prefix.