None-like singleton in Python (mypy-compatible)

novalue.py

"""
None-like singleton in Python (mypy-compatible).

https://github.com/lemon24/reader/issues/177#issuecomment-674786498

"""

from typing import Union, cast


class NoValueType:
    
    def __new__(cls) -> 'NoValueType':
        return NoValue

    def __repr__(self) -> str:
        return 'NoValue'

    def __bool__(self) -> bool:
        # to prevent "if thing" when "if thing is False" was meant;
        raise TypeError("cannot use NoValue in boolean context")


NoValue = cast(NoValueType, object.__new__(NoValueType))

assert NoValueType() is NoValue


"""
# alternative way to make a singleton based on https://stackoverflow.com/a/6798042

from typing import Dict, Any

class Singleton(type):

    _instances: Dict['Singleton', 'Singleton'] = {}

    def __call__(cls, *args: Any, **kwargs: Any) -> 'Singleton':
        
        if cls not in cls._instances:
            cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
        return cls._instances[cls]

class NoValueType(metaclass=Singleton):

    ... # repr etc.

"""


# we could define a MyOptional[T] == Union[NoValueType, T], but it's too complicated


one: Union[NoValueType, str] = 'aaa'
two: Union[NoValueType, str] = NoValue

for maybe_str in one, two:
    # sadly, mypy can't understand "maybe_str is NoValue", like it does for None
    if isinstance(maybe_str, str):
        print(maybe_str.lower())
    else:
        print(repr(maybe_str))

# error: Item "NoValueType" of "Union[NoValueType, str]" has no attribute "lower"
maybe_str.lower()


aaa: Union[NoValueType, bool] = True
bbb: Union[NoValueType, bool] = NoValue

def negate(a: bool) -> bool:
    return not a

for maybe_bool in aaa, bbb:
    if isinstance(maybe_bool, bool):
        print(negate(maybe_bool))
    else:
        print(repr(maybe_bool))

# error: Argument 1 to "negate" has incompatible type "Union[NoValueType, bool]"; expected "bool"
negate(maybe_bool)