Snippets Computer Science / Design Patterns / Factory

Factory Design Pattern

By Marcelo Fernandes Jul 13, 2017

Check List

  1. If you have an inheritance hierarchy that exercises polymorphism, consider adding a polymorphic creation capability by defining a static factory method in the base class.
  2. Design the arguments to the factory method. What qualities or characteristics are necessary and sufficient to identify the correct derived class to instantiate?
  3. Consider designing an internal "object pool" that will allow objects to be reused instead of created from scratch.
  4. Consider making all constructors private or protected.

Rules of Thumb

  • Abstract Factory classes are often implemented with Factory Methods, but they can be implemented using Prototype.
  • Factory Methods are usually called within Template Methods.
  • Factory Method: creation through inheritance. Prototype: creation through delegation.
  • Often, designs start out using Factory Method (less complicated, more customizable, subclasses proliferate) and evolve toward Abstract Factory, Prototype, or Builder (more flexible, more complex) as the designer discovers where more flexibility is needed.
  • Prototype doesn't require subclassing, but it does require an Initialize operation. Factory Method requires subclassing, but doesn't require Initialize.
  • The advantage of a Factory Method is that it can return the same instance multiple times, or can return a subclass rather than an object of that exact type.
  • Some Factory Method advocates recommend that as a matter of language design (or failing that, as a matter of style) absolutely all constructors should be private or protected. It's no one else's business whether a class manufactures a new object or recycles an old one.
  • The new operator considered harmful. There is a difference between requesting an object and creating one. The new operator always creates an object, and fails to encapsulate object creation. A Factory Method enforces that encapsulation, and allows an object to be requested without inextricable coupling to the act of creation.



"""
Define an interface for creating an object, but let subclasses decide
which class to instantiate. Factory Method lets a class defer
instantiation to subclasses.
"""

import abc


class Creator(metaclass=abc.ABCMeta):
    """
    Declare the factory method, which returns an object of type Product.
    Creator may also define a default implementation of the factory
    method that returns a default ConcreteProduct object.
    Call the factory method to create a Product object.
    """

    def __init__(self):
        self.product = self._factory_method()

    @abc.abstractmethod
    def _factory_method(self):
        pass

    def some_operation(self):
        self.product.interface()


class ConcreteCreator1(Creator):
    """
    Override the factory method to return an instance of a
    ConcreteProduct1.
    """

    def _factory_method(self):
        return ConcreteProduct1()


class ConcreteCreator2(Creator):
    """
    Override the factory method to return an instance of a
    ConcreteProduct2.
    """

    def _factory_method(self):
        return ConcreteProduct2()


class Product(metaclass=abc.ABCMeta):
    """
    Define the interface of objects the factory method creates.
    """

    @abc.abstractmethod
    def interface(self):
        pass


class ConcreteProduct1(Product):
    """
    Implement the Product interface.
    """

    def interface(self):
        pass


class ConcreteProduct2(Product):
    """
    Implement the Product interface.
    """

    def interface(self):
        pass


def main():
    concrete_creator = ConcreteCreator1()
    concrete_creator.product.interface()
    concrete_creator.some_operation()


if __name__ == "__main__":
    main()


Notes


References:

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