Classes

Ordinary classes work when their fields are known statically, and a misspelled method is a compile-time error (E1202). __init__ declares the fields, annotations confirm them, and every attribute access is resolved on a type the checker knows.

Run it

python  classes.ppy
ppy     classes.ppy
ppy run classes.ppy

Fields from annotations, or from __init__

class Circle(Shape):
    radius: float

    def __init__(self, radius: float) -> None:
        super().__init__("circle")
        self.radius = radius

    def area(self) -> float:
        return 3.141592653589793 * self.radius * self.radius

Shape.name is declared in the class body and assigned in __init__, and so is Circle.radius. Inheritance and super() work as in Python, and area is dispatched on the static type where it can be.

Box is a @dataclass of two floats, the one kind of class native code can take apart: an all-scalar dataclass flattens into scalar arguments at the boundary (value classes).

Narrowing a union

@ppy.pure
def describe(value: int | str | None) -> str:
    if value is None:
        return "none"
    if isinstance(value, str):
        return "text of " + str(len(value))
    return "number " + str(value)

After is None fails, value is int | str. After isinstance(value, str) fails, it is int. Each branch is typed with what is left, so len(value) and str(value) both check.

Declarative construction

The checker judges a class from what it can read, not from what running it would do. These are E1507:

• a class body that runs statements
• a body value constructing a project descriptor whose __set_name__ runs at creation
• a base whose __init_subclass__ does real work

The safe class hoister in ppy convert uses the same facts.

What it prints

python classes.ppy, ppy classes.ppy, ppy run classes.ppy

circle 12.5664 12.0
none text of 5 number 7

Read on: Narrowing · The subset

classes.ppy is hand-written; there is no .py source and no conversion step.

04_classes/classes.ppy

from dataclasses import dataclass

import ppy


class Shape:
    name: str

    def __init__(self, name: str) -> None:
        self.name = name

    def area(self) -> float:
        return 0.0


class Circle(Shape):
    radius: float

    def __init__(self, radius: float) -> None:
        super().__init__("circle")
        self.radius = radius

    def area(self) -> float:
        return 3.141592653589793 * self.radius * self.radius


@dataclass
class Box:
    width: float
    height: float

    def area(self) -> float:
        return self.width * self.height


@ppy.pure
def describe(value: int | str | None) -> str:
    if value is None:
        return "none"
    if isinstance(value, str):
        return "text of " + str(len(value))
    return "number " + str(value)


def main() -> None:
    circle = Circle(2.0)
    box = Box(3.0, 4.0)
    print(circle.name, round(circle.area(), 4), box.area())
    print(describe(None), describe("hello"), describe(7))


if __name__ == "__main__":
    main()

Source: examples/04_classes.