shows a possible but not very useful way to operate on an expression

evaluating_expression_with_type_checking.py

from dataclasses import dataclass


class Expression:
    """Represents an ADT expression"""


@dataclass
class Literal(Expression):
    value: float


@dataclass
class Add(Expression):
    x: Expression
    y: Expression


@dataclass
class Subtract(Expression):
    x: Expression
    y: Expression


@dataclass
class Multiply(Expression):
    x: Expression
    y: Expression


@dataclass
class Divide(Expression):
    x: Expression
    y: Expression


def interpret(e: Expression) -> float:
    if isinstance(e, Add):
        return interpret(e.x) + interpret(e.y)
    if isinstance(e, Multiply):
        return interpret(e.x) * interpret(e.y)
    if isinstance(e, Subtract):
        return interpret(e.x) - interpret(e.y)
    if isinstance(e, Divide):
        return interpret(e.x) / interpret(e.y)
    return e.value


def test_add_is_interpreted_properly():
    mult = Add(x=Multiply(x=Literal(value=5), y=Literal(value=3)), y=Literal(2))
    result = interpret(mult)
    assert result == 17.0