Features that can be explicitly enabled or disabled at build time

These features are unique in that, if they are enabled or disabled at build-time, then we usually do not want to detect them on-the-fly. Instead we trust the value supplied (or detected) at build-time. There is however an overrride to defer these checks to run-time (the classic behavior) for use on binary distros or anywhere it is desirable to enable/disable features without rebuilding sage.

This is an implementation of Option 3 in Github discussion 41067.

class sage.features.build_feature.BuildFeature(*args, **kwds)

Bases: Feature

A class for features that can be enabled or disabled at build-time.

The current implementation refers to build features that are configurable in meson. For example:

option(
  'foo',
  type: 'feature',
  value: 'auto',
  description: 'support for foo'
)

At build time, support for this “foo” will be automatically detected, and either enabled or disabled depending on whether or not its requirements are met. Alternatively, users may pass either -Dfoo=enabled or -Dfoo=disabled to explicitly enable or disable the feature. Features may be disabled regardless of whether or not they are installed, but usually features may only be enabled if their dependencies are present and usable.

In any event, after meson setup, support for “foo” is either enabled or disabled, and a boolean variable called something like foo_enabled is written to sage.config. In your subclass, you should set the member variable _enabled_in_build to the value of that config variable.

The _is_present() method for this class will return the value of that config variable unless defer_feature_checks is set to True in sage.config. If checks are deferred, the _is_present() method will try to return the value of is_present_at_runtime() instead. If your feature can be detected at run-time, you should implement that check in is_present_at_runtime(). Otherwise, leave it unimplemented; and _is_present() will return False.

EXAMPLES:

sage: from sage.features.build_feature import BuildFeature
sage: BuildFeature("foo")
Feature('foo')

is_runtime_detectable()

Return whether or not this feature can (and should) be detected at runtime.

A feature is runtime detectable if both of the following hold:

• Deferred feature checks have been enabled globally by passing -Ddefer_feature_checks=true to meson setup.

• An is_present_at_runtime method has been implemented for the feature.

EXAMPLES:

The method returns False if you have not implemented is_present_at_runtime:

sage: from sage.features.build_feature import BuildFeature
sage: bf = BuildFeature("example")
sage: bf.is_runtime_detectable()
False