Use Flag Guide

Use flags can be used to control optional dependencies and optional configurations which a user may want to choose between.

Use flags should be declared in IUSE and described using the use top level field in the mod’s metadata.yaml.


Build File

class Package(Pybuild1):
    IUSE = "foo bar"
    RDEPEND = """
        foo? ( !!cat/bar cat/foo )
        bar? ( !!cat/foo cat/bar )
    REQUIRED_USE = "?? ( foo bar )"


    foo: description of foo
    bar: description of bar

Use flags can be used in use-conditional expressions, that is, either flag? or !flag?, followed by brackets to indicate the scope of the conditional. RDEPEND and SRC_URI both support including use-conditional expressions.

Use flags can also be used in the pybuild.Pybuild1.REQUIRED_USE field.

In the above example, we declare flags foo and bar. From RDEPEND we see that if the user enables flag foo, the mod will pull in cat/foo as a dependency, and block cat/bar from being installed. On the other hand, if the user enables bar, the reverse will happen. We then declare in REQUIRED_USE, that at most one of foo and bar can be set at once (?? allows zero or one of the flags in the set to be enabled). You might see a configuration like this for a mod that has two mutually exclusive optional dependencies.

Note that the pybuild.InstallDir object as well as the pybuild.File object, also have REQUIRED_USE fields. In their cases, the InstallDir, or File will only be included if REQUIRED_USE is satisfied. You could, for example, disable an InstallDir when a use flag is not enabled by including that use flag in its REQUIRED_USE field.

Global and Local Use Flags

Global use flags are described in the /profiles/use.yaml file and should not be included in a mod’s specific metadata.yaml file. They do however need to be included in IUSE. If a use flag is to be made global, it should be used by multiple mods for more or less the same purpose.

All use flags that are not declared in /profiles/use.yaml, and are not use expand flags (see below), are considered to be local use flags.

Use Flag Defaults

You can add a + before a flag in pybuild.Pybuild1.IUSE to enable it by default. This affects both local and global flags, though only for this package in particular.

Defaults can also be configured for each profile by modifying the USE variable in the profile (for global flags), or setting the flag for specific packages in package.use.

Use Flag Aliases

Use flags can be set up to default to enabled when a particular package is installed, or disabled when the package is not installed (or going to be installed).

This can be useful for configuring patches for mods which are only necessary to allow it to work with another mod.

To create a use flag alias, add the flag and the package to profiles/use.alias.yaml (also see Package Repositories).

E.g. profiles/use.alias.yaml
tr: landmasses/tamriel-rebuilt

Use flag aliases must always be global use flags.


This is a loose relationship. Users can still override aliased flags, the feature just removes the need to update package.use when a package is installed or removed. This means that you must still add the relevant dependencies for the use flags, which usually come in one of two forms:

Mandatory patches: when a flag is used to enable a patch which otherwise causes a conflict. In this case it is necessary to also add a blocker to the dependency string. E.g.

   tr? ( >=landmasses/tamriel-rebuilt-8 )
   !tr? ( !!landmasses/tamriel-rebuilt )

Optional patches: when a flag is used to enable a patch which adds behaviour to match or extend another package, but would not cause issues if it is not installed. E.g.

   tr? ( landmasses/tamriel-rebuilt )

USE Expand variables

USE Expand variables are a certain class of automatically generated use flags set by the user’s global configuration. Every variable specified in the USE_EXPAND profile variable causes the values of the profile variable of the same name to generate use flags of the form {use-expand-name}_{value}.

Supported flags should then be described in /profiles/desc/{use}.yaml (where {use} is the lowercased use flag group name).

E.g. profiles/base/defaults.conf (path varies depending on profile setup)
E.g. profiles/desc/screen_aspect.yaml
4x3: Use 4x3 screen aspect ratio
16x9: Use 16x9 screen aspect ratio
16x10: Use 16x10 screen aspect ratio

This produces the flags screen_aspect_4x3, screen_aspect_16x9 and screen_aspect_16x10, which can be enabled by the user declaring something such as the following in their config file:



The TEXTURE_SIZE variable is a special variable which behaves similarly to use-expand variables, but has two special properties

  1. Values for a package are declared in TEXTURE_SIZES, instead of IUSE, and are implicitly added to REQUIRED_USE as part of a ^^ (exactly-one-of) relationship.

  2. The flag enabled is determined using the user’s TEXTURE_SIZE configuration variable.

They can be used in use-conditionals and REQUIRED_USE just like other use flags.

Use Flag Priority

Use flags values can be defined in several different places. The priority for the different ways of specifying a value is as follows, from lowest priority (always overridden), to highest priority (never overridden).

Generally speaking, the IUSE default is only useful for flags which are specific to a certain package. For use flags which are shared between many packages with a consistent meaning, the default should be set globally in the profile.

The profile package.use should only need to be used for flags which are set globally in the profile, but need to be overridden for a particular package (since the IUSE default cannot be used in such cases).

External Resources