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Preview features

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

Enabling preview features

To enable all preview features, use the --preview flag:

$ fyn run --preview ...

Or, set the UV_PREVIEW environment variable:

$ UV_PREVIEW=1 fyn run ...

To enable specific preview features, use the --preview-features flag:

$ fyn run --preview-features foo ...

The --preview-features flag can be repeated to enable multiple features:

$ fyn run --preview-features foo --preview-features bar ...

Or, features can be provided in a comma separated list:

$ fyn run --preview-features foo,bar ...

The UV_PREVIEW_FEATURES environment variable can be used similarly, e.g.:

$ UV_PREVIEW_FEATURES=foo,bar fyn run ...

For backwards compatibility, enabling preview features that do not exist will warn, but not error.

Using preview features

Often, preview features can be used without changing any preview settings if the behavior change is gated by some sort of user interaction, For example, while pylock.toml support is in preview, you can use fyn pip install with a pylock.toml file without additional configuration because specifying the pylock.toml file indicates you want to use the feature. However, a warning will be displayed that the feature is in preview. The preview feature can be enabled to silence the warning.

Available preview features

The following preview features are available:

  • add-bounds: Allows configuring the default bounds for fyn add invocations.
  • json-output: Allows --output-format json for various fyn commands.
  • package-conflicts: Allows defining workspace conflicts at the package level.
  • pylock: Allows installing from pylock.toml files.
  • format: Allows using fyn format.
  • index-exclude-newer: Allows using exclude-newer on [[tool.fyn.index]].
  • native-auth: Enables storage of credentials in a system-native location.
  • workspace-metadata: Allows using fyn workspace metadata.
  • workspace-dir: Allows using fyn workspace dir.
  • workspace-list: Allows using fyn workspace list.

Disabling preview features

The --no-preview option can be used to disable preview features.