TP0234

Error: abstract class instantiated (M28)

Messages

• cannot instantiate abstract class '<C>', with help it does not implement '<B>.<m>' ('<B>.<m>' and '<B>.<n>' for more)
• 'cls(…)' may construct the abstract class '<C>'
• '<C>' cannot add abstract method '<m>', since '<B>.<cm>' calls 'cls(…)' (M28)

Example

From the golden case tests/cases/m28/tp0234_abstract.tpn:

tp0234_abstract.tpn

# Abstract classes can't be instantiated (SPEC §4.8).
class Shape(ABC):
    @abstractmethod
    def area(self) -> float: ...

    @abstractmethod
    def perimeter(self) -> float: ...

    @classmethod
    def unit(cls) -> Shape:
        return cls()

class Square(Shape):
    @override
    def area(self) -> float:
        return 1.0

class Full(Square):
    @override
    def perimeter(self) -> float:
        return 4.0

let a = Shape()
let b = Square()
let c = Full()

class Maker(ABC):
    def __init__(self):
        pass

    @classmethod
    def make(cls) -> Maker:
        return cls()

class Late(Maker):
    @abstractmethod
    def run(self) -> int: ...

The compiler reports:

tp0234_abstract.tpn:11:16: error TP0234: 'cls(…)' may construct the abstract class 'Shape'
tp0234_abstract.tpn:23:9: error TP0234: cannot instantiate abstract class 'Shape'
  help: it does not implement 'Shape.area' and 'Shape.perimeter'
tp0234_abstract.tpn:24:9: error TP0234: cannot instantiate abstract class 'Square'
  help: it does not implement 'Shape.perimeter'
tp0234_abstract.tpn:37:9: error TP0234: 'Late' cannot add abstract method 'run', since 'Maker.make' calls 'cls(…)'

The specification lists every code.