Magic Values Code

magic_values.py

from typing import Optional, List, Type, Dict, Any


class MetaConstant(type):
    def __repr__(cls) -> str:
        """Constants' class name with additional context.
        Ex. DataCenter -> Location.DataCenter
        Ex. fra4 -> DataCenter.fra4("fra4")
        Ex. Constant -> Constant
        """
        parent = cls.mro()[1]
        if parent == object:
            return cls.__name__
        else:
            repr = f"{parent.__name__}.{cls.__name__}"
            if hasattr(cls, "_value"):
                repr += f'("{cls._value}")'
            return repr


class Constant(metaclass=MetaConstant):
    _value: str
    def __new__(
        cls, val: Optional[str] = None, _visited: List[Type] = []
    ) -> "Constant":
        _value: Optional[str] = getattr(cls, "_value", None)
        if _value is None and val is None:
            msg = f'Abstract {cls.__bases__[0].__name__} "{cls.__name__}" cannot be instantiated'
            raise TypeError(msg)
        elif _value is None:
            # DFS through the class tree to find a matching subclass.
            for child in cls.__subclasses__():
                try:
                    if match := child.__new__(child, val, _visited=_visited):
                        return match
                except:
                    continue
            # Compile a list of the subclassese we've tried for a better exception message.
            valids = ", ".join([f"{c}" for c in _visited])
            msg = f"invalid {cls.__name__}:'{val}' is not valid for any {cls.__name__}s: [{valids}]"
            raise ValueError(msg)
        elif val is None:
            # Default case.
            return super(Constant, cls).__new__(cls)
        else:
            # Base case for DFS.
            if _value == val:
                return super(Constant, cls).__new__(cls)
            else:
                _visited.append(cls)
                msg = f"invalid value for {cls}: '{val}'"
                raise ValueError(msg)

    def __init_subclass__(cls) -> None:
        """Register subclass as parent attribute
        Ex. Datacenter.fra4
        """
        setattr(type(cls), cls.__name__, cls)

    @property
    def value(self) -> str:
        return self._value

    def __repr__(self) -> str:
        return "Value: "+self._value

    def __str__(self) -> str:
        return self._value

    def __eq__(self, other:Any) -> bool:
        return self._value == other._value