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I will let the code do the talking here. With OOP, one can add more AST methods and subclasses for the below toy calculator program, as the needs grow. Without OOP, I would have to do many if or isinstance checks to perform the right operation based on the operator.
class Node:
def evaluate(self):
raise NotImplementedError
def pretty(self):
raise NotImplementedError
class Integer(Node):
def __init__(self, value):
self.value = value
def evaluate(self):
return self.value
def pretty(self):
return repr(self.value)
class Add(Node):
def __init__(self, l, r):
self.l = l
self.r = r
def evaluate(self):
return self.l.evaluate() + self.r.evaluate()
def pretty(self):
pretty_l = self.l.pretty()
pretty_r = self.r.pretty()
return f"({pretty_l} + {pretty_r})"
# Similarly can have `Multiply`, `Divide` and other ops
class Mul(Node):
...
class Div(Node):
...
tree = Add(Add(Integer(3), Integer(4)), Integer(5))
print(tree.pretty()) # ((3 + 4) + 5)
print(tree.evaluate()) # 12