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Template Method

Template Method in Python

Template Method is a behavioral design pattern that allows you to defines a skeleton of an algorithm in a base class and let subclasses override the steps without changing the overall algorithm’s structure.

Learn more about Template Method

Usage of the pattern in Python

Complexity:

Popularity:

Usage examples: The Template Method pattern is quite common in Python frameworks. Developers often use it to provide framework users with a simple means of extending standard functionality using inheritance.

Identification: Template Method can be recognized by behavioral methods which already have a “default” behavior defined by the base class.

Conceptual Example

This example illustrates the structure of the Template Method design pattern. It focuses on answering these questions:

  • What classes does it consists of?
  • What roles do these classes play?
  • In what way the elements of the pattern are related?

main.py: Conceptual Example

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: Output

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 in Other Languages

Template Method in Java Template Method in C# Template Method in PHP Template Method in TypeScript