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Template Method Design Pattern

Video Lecture

Section Video Links
Template Method Overview Template Method Overview Template Method Overview 
Template Method Use Case Template Method Use Case Template Method Use Case 

Overview

In the Template Method pattern, you create an abstract class (template) that contains a Template Method that is a series of instructions that are a combination of abstract and hook methods.

Abstract methods need to be overridden in the subclasses that extend the abstract (template) class.

Hook methods normally have empty bodies in the abstract class. Subclasses can optionally override the hook methods to create custom implementations.

So, what you have, is an abstract class, with several types of methods, being the main template method, and a combination of abstract and/or hooks, that can be extended by different subclasses that all have the option of customizing the behavior of the template class without changing its underlying algorithm structure.

Template methods are useful to help you factor out common behavior within your library classes.

Note that this pattern describes the behavior of a method and how its inner method calls behave.

Hooks are default behavior and can be overridden. They are normally empty by default.

Abstract methods, must be overridden in the concrete class that extends the template class.

Terminology

  • Abstract Class: Defines the template method and the primitive steps as abstract and/or hook methods.
  • Concrete Class: A subclass that extends some or all of the abstract class primitive methods.

Template Method UML Diagram

Template Method UML Diagram

Source Code

Note that in both the concrete classes in this concept example, the template_method() was not overridden since it was already inherited. Only the primitives (abstract or hooks) were optionally overridden.

To create an empty abstract method in your abstract class, that must be overridden in a subclass, then use the ABCMeta @abstractmethod decorator.

./template/template_concept.py

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# pylint: disable=too-few-public-methods
"The Template Method Pattern Concept"
from abc import ABCMeta, abstractmethod

class AbstractClass(metaclass=ABCMeta):
    "A template class containing a template method and primitive methods"

    @staticmethod
    def step_one():
        """
        Hooks are normally empty in the abstract class. The
        implementing class can optionally override providing a custom
        implementation
        """

    @staticmethod
    @abstractmethod
    def step_two():
        """
        An abstract method that must be overridden in the implementing
        class. It has been given `@abstractmethod` decorator so that
        pylint shows the error.
        """

    @staticmethod
    def step_three():
        """
        Hooks can also contain default behavior and can be optionally
        overridden
        """
        print("Step Three is a hook that prints this line by default.")

    @classmethod
    def template_method(cls):
        """
        This is the template method that the subclass will call.
        The subclass (implementing class) doesn't need to override this
        method since it has would have already optionally overridden
        the following methods with its own implementations
        """
        cls.step_one()
        cls.step_two()
        cls.step_three()

class ConcreteClassA(AbstractClass):
    "A concrete class that only overrides step two"
    @staticmethod
    def step_two():
        print("Class_A : Step Two (overridden)")

class ConcreteClassB(AbstractClass):
    "A concrete class that only overrides steps one, two and three"
    @staticmethod
    def step_one():
        print("Class_B : Step One (overridden)")

    @staticmethod
    def step_two():
        print("Class_B : Step Two. (overridden)")

    @staticmethod
    def step_three():
        print("Class_B : Step Three. (overridden)")

# The Client
CLASS_A = ConcreteClassA()
CLASS_A.template_method()

CLASS_B = ConcreteClassB()
CLASS_B.template_method()

Output

python ./template/template_concept.py
Class_A : Step Two (overridden)
Step Three is a hook that prints this line by default.
Class_B : Step One (overridden)
Class_B : Step Two. (overridden)
Class_B : Step Three. (overridden)

SBCODE Editor

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Template Method Example Use Case

In the example use case, there is an AbstractDocument with several methods, some are optional and others must be overridden.

The document will be written out in two different formats.

Depending on the concrete class used, the text() method will wrap new lines with <p> tags and the print() method will format text with tabs, or include HTML tags.

Template Method Use Case UML Diagram

Template Method Use Case UML Diagram

Source Code

./template/client.py

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"The Template Pattern Use Case Example"
from text_document import TextDocument
from html_document import HTMLDocument

TEXT_DOCUMENT = TextDocument()
TEXT_DOCUMENT.create_document("Some Text")

HTML_DOCUMENT = HTMLDocument()
HTML_DOCUMENT.create_document("Line 1\nLine 2")

./template/abstract_document.py

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"An abstract document containing a combination of hooks and abstract methods"
from abc import ABCMeta, abstractmethod

class AbstractDocument(metaclass=ABCMeta):
    "A template class containing a template method and primitive methods"

    @staticmethod
    @abstractmethod
    def title(document):
        "must implement"

    @staticmethod
    def description(document):
        "optional"

    @staticmethod
    def author(document):
        "optional"

    @staticmethod
    def background_colour(document):
        "optional with a default behavior"
        document["background_colour"] = "white"

    @staticmethod
    @abstractmethod
    def text(document, text):
        "must implement"

    @staticmethod
    def footer(document):
        "optional"

    @staticmethod
    def print(document):
        "optional with a default behavior"
        print("----------------------")
        for attribute in document:
            print(f"{attribute}\t: {document[attribute]}")
        print()

    @classmethod
    def create_document(cls, text):
        "The template method"
        _document = {}
        cls.title(_document)
        cls.description(_document)
        cls.author(_document)
        cls.background_colour(_document)
        cls.text(_document, text)
        cls.footer(_document)
        cls.print(_document)

./template/text_document.py

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"A text document concrete class of AbstractDocument"
from abstract_document import AbstractDocument

class TextDocument(AbstractDocument):
    "Prints out a text document"
    @staticmethod
    def title(document):
        document["title"] = "New Text Document"

    @staticmethod
    def text(document, text):
        document["text"] = text

    @staticmethod
    def footer(document):
        document["footer"] = "-- Page 1 --"

./template/html_document.py

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"A HTML document concrete class of AbstractDocument"
from abstract_document import AbstractDocument

class HTMLDocument(AbstractDocument):
    "Prints out a HTML formatted document"
    @staticmethod
    def title(document):
        document["title"] = "New HTML Document"

    @staticmethod
    def text(document, text):
        "Putting multiple lines into there own p tags"
        lines = text.splitlines()
        markup = ""
        for line in lines:
            markup = markup + "    <p>" + f"{line}</p>\n"
        document["text"] = markup[:-1]

    @staticmethod
    def print(document):
        "overriding print to output with html tags"
        print("<html>")
        print("  <head>")
        for attribute in document:
            if attribute in ["title", "description", "author"]:
                print(
                    f"    <{attribute}>{document[attribute]}"
                    f"</{attribute}>"
                )
            if attribute == "background_colour":
                print("    <style>")
                print("      body {")
                print(
                    f"        background-color: "
                    f"{document[attribute]};")
                print("      }")
                print("    </style>")
        print("  </head>")
        print("  <body>")
        print(f"{document['text']}")
        print("  </body>")
        print("</html>")

Output

python ./template/client.py
----------------------
title   : New Text Document
background_colour       : white
text    : Some Text
footer  : -- Page 1 --

<html>
  <head>
    <title>New HTML Document</title>
    <style>
      body {
        background-color: white;
      }
    </style>
  </head>
  <body>
    <p>Line 1</p>
    <p>Line 2</p>
  </body>
</html>

SBCODE Editor

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Summary

  • The Template method defines an algorithm in terms of abstract operations and subclasses override some or all of the methods to create concrete behaviors.

  • Abstract methods must be overridden in the subclasses that extend the abstract class.

  • Hook Methods usually have empty bodies in the super class but can be optionally overridden in the subclass.

  • If a class contains many conditional statements, consider converting it to use the Template Method pattern.