Coding Trees in Python 3 (https://www.youtube.com/watch?v=7tCNu4CnjVc)

coding_trees.py

class Expr:
    def eval(self, env):
        raise NotImplementedError

class BinaryOperator(Expr):
    def __init__(self, left, right):
        super().__init__()
        self.left = left
        self.right = right

    def __str__(self):
        return f'({self.left} {self.operator} {self.right})'

class Times(BinaryOperator):
    def __init__(self, left, right): 
        super().__init__(left, right)
        self.operator = '*'

    def eval(self, env):
        return self.left.eval(env) * self.right.eval(env)

class Plus(BinaryOperator):
    def __init__(self, left, right): 
        super().__init__(left, right)
        self.operator = '+'

    def eval(self, env):
        return self.left.eval(env) + self.right.eval(env)

class Minus(BinaryOperator):
    def __init__(self, left, right): 
        super().__init__(left, right)
        self.operator = '-'

    def eval(self, env):
        return self.left.eval(env) - self.right.eval(env)

class Divide(BinaryOperator):
    def __init__(self, left, right): 
        super().__init__(left, right)
        self.operator = '/'

    def eval(self, env):
        return self.left.eval(env) / self.right.eval(env)

class Const(Expr):
    def __init__(self, val):
        self.val = val

    def __str__(self):
        return str(self.val)

    def eval(self, _env):
        return self.val

class Var(Expr):
    def __init__(self, name):
        self.name = name

    def __str__(self):
        return self.name

    def eval(self, _env):
        return env[self.name]

e1 = Times(Const(3), Plus(Var('x'), Var('y')))
print(e1)

e2 = Plus(Times(Const(3), Var('y')), Var('x'))
print(e2)

env = { 'x': 2, 'y': 4 }

print(e1.eval(env))

print(e2.eval(env))