"""
Providing typing utilities.
"""
from __future__ import annotations
from typing import TypeVar, TypeAlias, cast
from betty.docstring import append
_T = TypeVar("_T")
def _internal(target: _T) -> _T:
target.__doc__ = append(
target.__doc__ or "",
"This is internal. It **MAY** be used anywhere in Betty's source code, but **MUST NOT** be used by third-party code.",
)
return target
[docs]
@_internal
def internal(target: _T) -> _T:
"""
Mark a target as internal to Betty.
Anything decorated with ``@internal`` MAY be used anywhere in Betty's source code,
but MUST be considered private by third-party code.
"""
return _internal(target)
[docs]
@internal
def public(target: _T) -> _T:
"""
Mark a target as publicly usable.
This is intended for items nested inside something marked with :py:func:`betty.typing.internal`,
such as class attributes: third-party code **SHOULD NOT** use a class marked ``@internal``
directly, but **MAY** use any of its attributes that are marked ``@public``.
"""
return target
[docs]
class Void:
"""
A sentinel that describes the absence of a value.
Using this sentinel allows for actual values to be ``None``. Like ``None``,
``Void`` is only ever used through its type, and never instantiated.
"""
[docs]
def __new__(cls): # pragma: no cover # noqa D102
raise RuntimeError("The Void sentinel cannot be instantiated.")
Voidable: TypeAlias = _T | type[Void]
[docs]
def void_none(value: _T | None) -> Voidable[_T]:
"""
Passthrough a value, but convert ``None`` to :py:class:`betty.typing.Void`.
"""
return Void if value is None else value
[docs]
def none_void(value: Voidable[_T]) -> _T | None:
"""
Passthrough a value, but convert :py:class:`betty.typing.Void` to ``None``.
"""
return None if value is Void else cast(_T, value)