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Template Method

Template Method em Python

O Template Method é um padrão de projeto comportamental que permite definir o esqueleto de um algoritmo em uma classe base e permitir que as subclasses substituam as etapas sem alterar a estrutura geral do algoritmo.

Uso do padrão em Python

Complexidade:

Popularidade:

Exemplos de uso: O padrão Template Method é bastante comum nos frameworks Python. Os desenvolvedores costumam usá-lo para fornecer aos usuários do framework um meio simples de estender a funcionalidade padrão usando herança.

Identificação: O Template Method pode ser reconhecido por métodos comportamentais que já possuem um comportamento “padrão” definido pela classe base.

Exemplo conceitual

Este exemplo ilustra a estrutura do padrão de projeto Template Method. Ele se concentra em responder a estas perguntas:

  • De quais classes ele consiste?
  • Quais papéis essas classes desempenham?
  • De que maneira os elementos do padrão estão relacionados?

main.py: Exemplo conceitual

from abc import ABC, abstractmethod


class AbstractClass(ABC):
    """
    The Abstract Class defines a template method that contains a skeleton of
    some algorithm, composed of calls to (usually) abstract primitive
    operations.

    Concrete subclasses should implement these operations, but leave the
    template method itself intact.
    """

    def template_method(self) -> None:
        """
        The template method defines the skeleton of an algorithm.
        """

        self.base_operation1()
        self.required_operations1()
        self.base_operation2()
        self.hook1()
        self.required_operations2()
        self.base_operation3()
        self.hook2()

    # These operations already have implementations.

    def base_operation1(self) -> None:
        print("AbstractClass says: I am doing the bulk of the work")

    def base_operation2(self) -> None:
        print("AbstractClass says: But I let subclasses override some operations")

    def base_operation3(self) -> None:
        print("AbstractClass says: But I am doing the bulk of the work anyway")

    # These operations have to be implemented in subclasses.

    @abstractmethod
    def required_operations1(self) -> None:
        pass

    @abstractmethod
    def required_operations2(self) -> None:
        pass

    # These are "hooks." Subclasses may override them, but it's not mandatory
    # since the hooks already have default (but empty) implementation. Hooks
    # provide additional extension points in some crucial places of the
    # algorithm.

    def hook1(self) -> None:
        pass

    def hook2(self) -> None:
        pass


class ConcreteClass1(AbstractClass):
    """
    Concrete classes have to implement all abstract operations of the base
    class. They can also override some operations with a default implementation.
    """

    def required_operations1(self) -> None:
        print("ConcreteClass1 says: Implemented Operation1")

    def required_operations2(self) -> None:
        print("ConcreteClass1 says: Implemented Operation2")


class ConcreteClass2(AbstractClass):
    """
    Usually, concrete classes override only a fraction of base class'
    operations.
    """

    def required_operations1(self) -> None:
        print("ConcreteClass2 says: Implemented Operation1")

    def required_operations2(self) -> None:
        print("ConcreteClass2 says: Implemented Operation2")

    def hook1(self) -> None:
        print("ConcreteClass2 says: Overridden Hook1")


def client_code(abstract_class: AbstractClass) -> None:
    """
    The client code calls the template method to execute the algorithm. Client
    code does not have to know the concrete class of an object it works with, as
    long as it works with objects through the interface of their base class.
    """

    # ...
    abstract_class.template_method()
    # ...


if __name__ == "__main__":
    print("Same client code can work with different subclasses:")
    client_code(ConcreteClass1())
    print("")

    print("Same client code can work with different subclasses:")
    client_code(ConcreteClass2())

Output.txt: Resultados da execução

Same client code can work with different subclasses:
AbstractClass says: I am doing the bulk of the work
ConcreteClass1 says: Implemented Operation1
AbstractClass says: But I let subclasses override some operations
ConcreteClass1 says: Implemented Operation2
AbstractClass says: But I am doing the bulk of the work anyway

Same client code can work with different subclasses:
AbstractClass says: I am doing the bulk of the work
ConcreteClass2 says: Implemented Operation1
AbstractClass says: But I let subclasses override some operations
ConcreteClass2 says: Overridden Hook1
ConcreteClass2 says: Implemented Operation2
AbstractClass says: But I am doing the bulk of the work anyway

Template Method em outras linguagens

Padrões de Projeto: Template Method em Java Padrões de Projeto: Template Method em C# Padrões de Projeto: Template Method em C++ Padrões de Projeto: Template Method em PHP Padrões de Projeto: Template Method em Ruby Padrões de Projeto: Template Method em Swift Padrões de Projeto: Template Method em TypeScript