Factory Method – Python Design Patterns

Factory Method is a Creational Design Pattern that allows an interface or a class to create an object, but lets subclasses decide which class or object to instantiate. Using the Factory method, we have the best ways to create an object. Here, objects are created without exposing the logic to the client, and for creating the new type of object, the client uses the same common interface.
 

Problems we face without Factory Method:

Imagine you are having your own startup which provides ridesharing in different parts of the country. The initial version of the app only provides Two-Wheeler ridesharing but as time passes, your app becomes popular and now you want to add Three and Four-Wheeler ridesharing also. 
It’s a piece of great news! but what about the software developers of your startup. They have to change the whole code because now most part of the code is coupled with the Two-Wheeler class and developers have to make changes to the entire codebase. 
After being done with all these changes, either the developers end with the messy code or with the resignation letter. 
 

Localizer app

Diagrammatic representation of Problems without using Factory Method

Let’s understand the concept with one more example which is related to the translations and localization of the different languages. 
Suppose we have created an app whose main purpose is to translate one language into another and currently our app works with 10 languages only. Now our app has become widely popular among people but the demand has grown suddenly to include 5 more languages. 
It’s a piece of great news! only for the owner not for the developers. They have to change the whole code because now most part of the code is coupled with the existing languages only and that’s why developers have to make changes to the entire codebase which is really a difficult task to do.
Let’s look at the code for the problem which we may face without using the factory method.
 

Note: Following code is written without using the Factory method. 

# Python Code for Object
# Oriented Concepts without
# using Factory method 

class FrenchLocalizer:

    """ it simply returns the french version """

    def __init__(self):

        self.translations = {"car": "voiture", "bike": "bicyclette",
                             "cycle":"cyclette"}

    def localize(self, msg):

        """change the message using translations"""
        return self.translations.get(msg, msg)

class SpanishLocalizer:
    """it simply returns the spanish version"""

    def __init__(self):

        self.translations = {"car": "coche", "bike": "bicicleta",
                             "cycle":"ciclo"}

    def localize(self, msg):

        """change the message using translations"""
        return self.translations.get(msg, msg)

class EnglishLocalizer:
    """Simply return the same message"""

    def localize(self, msg):
        return msg

if __name__ == "__main__":

    # main method to call others
    f = FrenchLocalizer()
    e = EnglishLocalizer()
    s = SpanishLocalizer()

    # list of strings
    message = ["car", "bike", "cycle"]

    for msg in message:
        print(f.localize(msg))
        print(e.localize(msg))
        print(s.localize(msg))

Solution by Factory Method:

Its solution is to replace the straightforward object construction calls with calls to the special factory method. Actually, there will be no difference in the object creation but they are being called within the factory method.
For example Our Two_Wheeler, Three_Wheeler, and Four_wheeler classes should implement the ridesharing interface which will declare a method called a ride. Each class will implement this method uniquely.
 

solution_factory_diagram

Now let us understand the factory method with the help of an example:

# Python Code for factory method 
# it comes under the creational 
# Design Pattern

class FrenchLocalizer:

    """ it simply returns the french version """

    def __init__(self):

        self.translations = {"car": "voiture", "bike": "bicyclette",
                             "cycle":"cyclette"}

    def localize(self, msg):

        """change the message using translations"""
        return self.translations.get(msg, msg)

class SpanishLocalizer:
    """it simply returns the spanish version"""

    def __init__(self):
        self.translations = {"car": "coche", "bike": "bicicleta",
                             "cycle":"ciclo"}

    def localize(self, msg):

        """change the message using translations"""
        return self.translations.get(msg, msg)

class EnglishLocalizer:
    """Simply return the same message"""

    def localize(self, msg):
        return msg

def Factory(language ="English"):

    """Factory Method"""
    localizers = {
        "French": FrenchLocalizer,
        "English": EnglishLocalizer,
        "Spanish": SpanishLocalizer,
    }

    return localizers[language]()

if __name__ == "__main__":

    f = Factory("French")
    e = Factory("English")
    s = Factory("Spanish")

    message = ["car", "bike", "cycle"]

    for msg in message:
        print(f.localize(msg))
        print(e.localize(msg))
        print(s.localize(msg))

Class Diagram for Factory Method:

Let’s look at the class diagram considering the example of ride-sharing. 
 

Factory_pattern_class_diagram

Advantages of using Factory method: 

1. We can easily add new types of products without disturbing the existing client code.
2. Generally, tight coupling is being avoided between the products and the creator classes and objects.

Disadvantages of using Factory method:

1. To create a particular concrete product object, the client might have to sub-class the creator class.
2. You end up with a huge number of small files i.e, cluttering the files.
   • In a Graphics system, depending upon the user’s input it can draw different shapes like rectangles, Square, Circle, etc. But for the ease of both developers as well as the client, we can use the factory method to create the instance depending upon the user’s input. Then we don’t have to change the client code for adding a new shape.
   • On a Hotel booking site, we can book a slot for 1 room, 2 rooms, 3 rooms, etc. Here user can input the number of rooms he wants to book. Using the factory method, we can create a factory class Any Rooms which will help us to create the instance depending upon the user’s input. Again we don’t have to change the client’s code for adding the new facility.

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