Binary methods using the visitor pattern

visitor_binary_operation.py

class Expr(object):
    def accept(self, visitor):
        method_name = 'visit_{}'.format(self.__class__.__name__.lower())
        visit = getattr(visitor, method_name)
        return visit(self)


class Int(Expr):
    def __init__(self, value):
        self.value = value

    def __str__(self):
        return 'Int {}'.format(self.value)


class Str(Expr):
    def __init__(self, value):
        self.value = value

    def __str__(self):
        return 'Str {}'.format(self.value)


class Visitor(object):
    pass


class IntAdd(Visitor):
    def __init__(self, i):
        self.i = i

    def visit_int(self, i):
        return Int(int(self.i.value) + int(i.value))

    def visit_str(self, s):
        return Str(str(self.i.value) + str(s.value))


class StrAdd(Visitor):
    def __init__(self, s):
        self.s = s

    def visit_int(self, i):
        return Str(str(self.s.value) + str(i.value))

    def visit_str(self, s):
        return Str(str(self.s.value) + str(s.value))


class AddVisitor(Visitor):

    def __init__(self, expr):
        self.expr = expr

    def visit_int(self, i):
        return self.expr.accept(IntAdd(i))

    def visit_str(self, s):
        return self.expr.accept(StrAdd(s))


def add(a, b):
    return a.accept(AddVisitor(b))


def main():
   print(add(Int(4), Int(5)))
   print(add(Str(4), Int(5)))
   print(add(Int(4), Str(5)))
   print(add(Str(4), Str(5)))


if __name__ == '__main__':
    main()