How can I define algebraic data types in Python?

Question

How can I define algebraic data types in Python (2 or 3)?

Answer 1

The typing module provides Union which, dissimilar to C, is a sum type. You'll need to use mypy to do static type checking, and there's a notable lack of pattern matching, but combined with tuples (product types), that's the two common algebraic types.

from dataclasses import dataclass from typing import Union   @dataclass class Point:     x: float     y: float   @dataclass class Circle:     x: float     y: float     r: float   @dataclass class Rectangle:     x: float     y: float     w: float     h: float   Shape = Union[Point, Circle, Rectangle]   def print_shape(shape: Shape):     if isinstance(shape, Point):         print(f"Point {shape.x} {shape.y}")     elif isinstance(shape, Circle):         print(f"Circle {shape.x} {shape.y} {shape.r}")     elif isinstance(shape, Rectangle):         print(f"Rectangle {shape.x} {shape.y} {shape.w} {shape.h}")   print_shape(Point(1, 2)) print_shape(Circle(3, 5, 7)) print_shape(Rectangle(11, 13, 17, 19)) # print_shape(4)  # mypy type error