import cobble

@cobble.data
class Song(object):
    name = cobble.field()
    artist = cobble.field()
    album = cobble.field(default=None)


song = Song("MFEO", artist="Jack's Mannequin")

print(song) # Prints "Song(name='MFEO', artist="Jack's Mannequin", album=None)"

class Expression(object):
    pass

@cobble.data
class Literal(Expression):
    value = cobble.field()

@cobble.data
class Add(Expression):
    left = cobble.field()
    right = cobble.field()

class Evaluator(cobble.visitor(Expression)):
    def visit_literal(self, literal):
        return literal.value

    def visit_add(self, add):
        return self.visit(add.left) + self.visit(add.right)

Evaluator().visit(Add(Literal(2), Literal(4))) # 6

class Expression(object):
    pass

@cobble.visitable
class Literal(Expression):
    def __init__(self, value):
        self.value = value

@cobble.visitable
class Add(Expression):
    def __init__(self, left, right):
        self.left = left
        self.right = right

class Evaluator(cobble.visitor(Expression)):
    def visit_literal(self, literal):
        return literal.value

    def visit_add(self, add):
        return self.visit(add.left) + self.visit(add.right)

Evaluator().visit(Add(Literal(2), Literal(4))) # 6