Special ValuesΒΆ

Some special values (“sentinels” values) are defined:

This option cannot be used (set or accessed). Useful primarily in subclasses, to “turn off” options that apply in the superclass, but not the subclass.
Option is not set initially or on a per-instance basis, but may be invoked on a call-by-call basis.
Not used, but explicitly marked as reserved for future use.
If an option is set to Unset, the current value is removed, letting values from higher up the option chain shine through.