The code listings in this book can be found under chapter02 directory at https://github.com/PacktPublishing/Hands-On-Enterprise-Application-Development-with-Python.

The code samples can be cloned by running the following command:

git clone https://github.com/PacktPublishing/Hands-On-Enterprise-Application-Development-with-Python

The instructions to run the code samples can be found under the README.md file present inside the chapter directory.

A design pattern defines a way in which we can organize our solution to a given problem. It does not define algorithms that can be used to solve the problem, but rather provides an abstraction about how, for example, the code should be organized, what classes need to be defined, what their granularity will be, and how the different objects will be created.

The design patterns have gained a lot of traction, and the book Design Patterns: Elements of Reusable Object-Oriented Software, though published in 1994, still serves as a de facto reference when trying to understand design patterns.

A design pattern will usually consist of the following elements:

• A problem statement: A problem statement describes what we want to solve and hence also defines the design patterns we can use. The problem statement will tell us about the scope of the design that we are...

Object-oriented programming (OOP) refers to the organization of code in a format where we are not concerned with the organization of methods, but rather we are concerned with the objects, their properties, and their behavior.

An object may represent any logical entity, such as an animal, vehicle, and furniture, and will contain properties and behaviors describing them.

The basic building block of an OOP-based language is the class that often groups the logically related entities together into a single unit. When we need to work with this unit, we create a new instance of this unit known as the class object, and manipulate the object using the public interfaces exposed by the object.

Object-oriented programming in Python differs quite a lot from what a person might see in C++ or Java, and these differences also affect the way in which we implement the different...

The Singleton pattern is one of the patterns that finds its place in the book by Gang of Four, and which can have various uses where all we want is a class to have a single instance throughout an application.

The Singleton pattern enforces that a class will have only one instance that will be used by any of the components/modules inside an application. This kind of enforcement can be useful when we want to control the access to a resource using only one object. These type of resources can be log files, databases, crash-handling mechanisms, and so on.

In most of the OOP-based languages, to implement the Singleton pattern, the first step is to make the class constructor private and then use a static method inside a class to return the same instance whenever some part of the code needs to use the functionality of the class. In Python, we do not have the functionality...

During the development of large applications, there are certain cases where we might want to initialize a class dynamically, based upon the user input or some other dynamic factor. To achieve this, either we can initialize all the possible objects during the class instantiation and return the one that is required based on the inputs from the environment, or we can altogether defer the creation of class objects until an input has been received.

The Factory pattern is the solution to the latter case, where we develop a special method inside a class, which will be responsible for initializing the objects dynamically, based on the input from the environment.

Now, let's see how we can implement the Factory pattern in Python in a simplistic way, and then we will see how we can use the dynamic nature of Python to make our Factory class more dynamic.

So, let&apos...

Let's start the discussion about the MVC pattern with a diagram:

The diagram shows the flow of a request in an Application using the MVC pattern. When a user makes a new Request, the Request is intercepted by the Application, which then forwards the Request to the appropriate Controller for that Request. Once the Request is received by the Controller, it then interacts with the model, which performs some business logic based on the Request received by it. This could involve the updating of databases, or fetching some data. Once the business logic is executed by the Model, the Controller executes the View with any data that needs to be passed to the View, which then displays the Response for the Request.

Although we will be implementing the MVC pattern later in the book, when we develop our BugZot application, let's take a look at the...

In this chapter, we went through the concept of design patterns and how they can help us solve some of the commonly encountered problems in designing our applications. We then went through how to decide upon which design pattern to use, and if it is compulsory to choose one of the patterns that has already been defined. Moving further in the chapter, we explored some of the object-oriented capabilities of Python as a language, and also explored some examples of implementing abstract classes and metaclasses in Python, and how we can use them to build other classes and modify their behavior.

Equipped with the knowledge of object-oriented Python, we moved on to implement some of the common design patterns, such as the Singleton and Factory patterns in Python, and also explored the MVC pattern, learning about what problems they try to solve.

Now with the knowledge of design...