Visitor in Python

from __future__ import annotations from abc import ABC, abstractmethod from typing import List class Component(ABC): """ The Component interface declares an `accept` method that should take the base visitor interface as an argument. """ @abstractmethod def accept(self, visitor: Visitor) -> None: pass class ConcreteComponentA(Component): """ Each Concrete Component must implement the `accept` method in such a way that it calls the visitor's method corresponding to the component's class. """ def accept(self, visitor: Visitor) -> None: """ Note that we're calling `visitConcreteComponentA`, which matches the current class name. This way we let the visitor know the class of the component it works with. """ visitor.visit_concrete_component_a(self) def exclusive_method_of_concrete_component_a(self) -> str: """ Concrete Components may have special methods that don't exist in their base class or interface. The Visitor is still able to use these methods since it's aware of the component's concrete class. """ return "A" class ConcreteComponentB(Component): """ Same here: visitConcreteComponentB => ConcreteComponentB """ def accept(self, visitor: Visitor): visitor.visit_concrete_component_b(self) def special_method_of_concrete_component_b(self) -> str: return "B" class Visitor(ABC): """ The Visitor Interface declares a set of visiting methods that correspond to component classes. The signature of a visiting method allows the visitor to identify the exact class of the component that it's dealing with. """ @abstractmethod def visit_concrete_component_a(self, element: ConcreteComponentA) -> None: pass @abstractmethod def visit_concrete_component_b(self, element: ConcreteComponentB) -> None: pass """ Concrete Visitors implement several versions of the same algorithm, which can work with all concrete component classes. You can experience the biggest benefit of the Visitor pattern when using it with a complex object structure, such as a Composite tree. In this case, it might be helpful to store some intermediate state of the algorithm while executing visitor's methods over various objects of the structure. """ class ConcreteVisitor1(Visitor): def visit_concrete_component_a(self, element) -> None: print(f"{element.exclusive_method_of_concrete_component_a()} + ConcreteVisitor1") def visit_concrete_component_b(self, element) -> None: print(f"{element.special_method_of_concrete_component_b()} + ConcreteVisitor1") class ConcreteVisitor2(Visitor): def visit_concrete_component_a(self, element) -> None: print(f"{element.exclusive_method_of_concrete_component_a()} + ConcreteVisitor2") def visit_concrete_component_b(self, element) -> None: print(f"{element.special_method_of_concrete_component_b()} + ConcreteVisitor2") def client_code(components: List[Component], visitor: Visitor) -> None: """ The client code can run visitor operations over any set of elements without figuring out their concrete classes. The accept operation directs a call to the appropriate operation in the visitor object. """ # ... for component in components: component.accept(visitor) # ... if __name__ == "__main__": components = [ConcreteComponentA(), ConcreteComponentB()] print("The client code works with all visitors via the base Visitor interface:") visitor1 = ConcreteVisitor1() client_code(components, visitor1) print("It allows the same client code to work with different types of visitors:") visitor2 = ConcreteVisitor2() client_code(components, visitor2) 

The client code works with all visitors via the base Visitor interface: A + ConcreteVisitor1 B + ConcreteVisitor1 It allows the same client code to work with different types of visitors: A + ConcreteVisitor2 B + ConcreteVisitor2