match_lambda_hack.py

from dataclasses import dataclass

def lambda_match_wrapper(instance, _locals):
	return type("MatchWrapper", (), {
		"__getattribute__": (lambda self, name:
			type("EqWrapper", (), {
				"__eq__": (lambda self, o:
					eval(o, None, dict(_locals, self=instance)) )})()
			if name == "_lambda" else
				instance.__getattribute__(name) )})()

class Tree(): pass

@dataclass
class Leaf(Tree):
	v: int

@dataclass
class Node(Tree):
	left: Tree
	right: Tree

def exists_leaf(tree: Tree, test):
	match lambda_match_wrapper(tree, locals()):
		case Leaf(_lambda="test(self.v)") \
			| Node(_lambda=
				"exists_leaf(self.left, test)"
				| "exists_leaf(self.right, test)"):
			return True
	return False

def has_even_leaf(tree: Tree):
	return exists_leaf(tree, lambda n: n % 2 == 0)

print(has_even_leaf(Leaf(0)))
print(has_even_leaf(Node(Leaf(3), Node(Leaf(7), Leaf(6)))))
print(has_even_leaf(Node(Leaf(3), Node(Node(Leaf(5), Leaf(27)), Leaf(3)))))