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pythonbook. Contribute to shivam-jha/pythonbook development by creating an account on GitHub. to implementing design patterns in Python, which is available at Did you know that Packt offers eBook versions of every book published, with PDF and ePub. Learning Python Design Patterns. Python is a great programming language, elegant and concise, and at the same time, very powerful. It has all the essential.

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If you are a beginner to learning Python or design patterns, this book can prove to this offline PDF version at regular intervals to keep up with the updated. This tutorial explains the various types of design patterns and their implementation in. Python scripting . Python Design Patterns – Model View Controller Pattern. .. In this section, we will learn about the different features of Python language. Design Patterns in Python. Alex Martelli ([email protected]) sppn.info sppn.info classes customized to solve a general design problem in a.

Do a quick online search if you are not familiar with the GOF. Design patterns are a common way of solving well known problems.

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Two main principles are in the bases of the design patterns defined by the GOF: Program to an interface not an implementation. Favor object composition over inheritance. Program to an interface not an implementation Think about Duck Typing. But, listen to me! If it looks like a duck and quacks like a duck, it's a duck! Can the object quack? So, let it quack!

Did we program to the interface instead of the implementation? And, I find this so nice. We can restrict what methods of the wrapped class to expose. We can inject the persister instance in runtime! Composition is elegant and natural to Python. Behavioral Patterns Behavioural Patterns involve communication between objects, how objects interact and fulfil a given task. I find these patterns very useful, but this does not mean the other pattern groups are not.

Iterator Iterators are built into Python. This is one of the most powerful characteristics of the language. Years ago, I read somewhere that iterators make Python awesome, and I think this is still the case. Chain of responsibility This pattern gives us a way to treat a request using different methods, each one addressing a specific part of the request.

You know, one of the best principles for good code is the Single Responsibility principle. Every piece of code must do one, and only one, thing. This principle is deeply integrated in this design pattern. For example, if we want to filter some content we can implement different filters, each one doing one precise and clearly defined type of filtering. These filters could be used to filter offensive words, ads, unsuitable video content, and so on.

That reminds me: Patterns are not invented, they are discovered. They exist, we just need to find and put them to use. After using this pattern intensively in the code, I read more about it on some sites. The command pattern is handy in situations when, for some reason, we need to start by preparing what will be executed and then to execute it when needed.

Because of the dynamic nature of the language. Someone wiser than I once said that Factory is built into Python. It means that the language itself provides us with all the flexibility we need to create objects in a sufficiently elegant fashion; we rarely need to implement anything on top, like Singleton or Factory. Singleton The Singleton pattern is used when we want to guarantee that only one instance of a given class exists during runtime.

Do we really need this pattern in Python? But should you want to implement it, here is some good news: In Python, we can alter the instantiation process along with virtually anything else. These are the alternatives to using a Singleton in Python: Use a module. Create one instance somewhere at the top-level of your application, perhaps in the config file. Pass the instance to every object that needs it.

Combine it with Duck Typing and the Force will be with you. I listed it in the creational pattern section of this post because it deals with the question of when or even better: where the object is created. Better to say that the objects are not created at all where we use them, so the dependency is not created where it is consumed. The consumer code receives the externally created object and uses it. For further reference, please read the most upvoted answer to this Stackoverflow question.

Tell your parents that you need something to drink with lunch. Python offers us all we need to implement that easily. All the Command class needs is to execute them successfully without bothering with the implementation details. This way, we may use the Command class with whatever authentication and authorization mechanisms we decide to use in runtime.

But before you do that, read another Stackoverflow answer, the most upvoted one to this question. Again, we just demonstrated how implementing this wonderful design pattern in Python is just a matter of using the built-in functionalities of the language. Imagine an architecture where you can change data storing on-the-fly. You may also want to research Prototype, Builder and Factory design patterns. Structural Patterns Facade This may very well be the most famous Python design pattern.

Imagine you have a system with a considerable number of objects. Every object is offering a rich set of API methods. You can do a lot of things with this system, but how about simplifying the interface?

Why not add an interface object exposing a well thought-out subset of all API methods? A Facade! Facade is an elegant Python design pattern. It's a perfect way of streamlining the interface. Adapter If Facades are used for interface simplification, Adapters are all about altering the interface. Like using a cow when the system is expecting a duck. Your method expects the destination to have a write method as every file object has, for example. Say you want to log to some UDP socket instead to a file,you know how to open this UDP socket but the only problem is that the socket object has no write method.

You need an Adapter! In brief, we will cover the following topics in this chapter:. Understanding Structural design patterns The following points will help us understand more about Structural patterns:.

Structural patterns describe how objects and classes can be combined to form larger structures. Structural patterns can be thought of as design patterns that ease the design by identifying simpler ways to realize or demonstrate relationships between entities. Entities mean objects or classes in the object-oriented world. While the Class patterns describe abstraction with the help of inheritance and provide a more useful program interface, Object patterns describe how objects can be associated and composed to form larger objects.

Structural patterns are a combination of Class and Object patterns. The following are a few examples of different Structural design patterns.

You'd notice how each of these involve interaction between objects or classes to achieve high-level design or architectural goals. Some of the examples of Structural design patterns are as follows:.

Adapter pattern: Adapting an interface to another one so that it meets the client's expectations. It tries to match interfaces of different classes based on the client's needs. Bridge pattern: This decouples an object's interface from its implementation so that both can work independently.

Decorator pattern: This defines additional responsibilities for an object at runtime or dynamically. We add certain attributes to objects with an interface. There are a few more Structural patterns that you will learn in this book. So, let's start by first taking up the Faade design pattern. Understanding the Faade design pattern The faade is generally referred to as the face of the building, especially an attractive one. It can be also referred to as a behavior or appearance that gives a false idea of someone's true feelings or situation.

When people walk past a faade, they can appreciate the exterior face but aren't aware of the complexities of the structure within. This is how a faade pattern is used. Faade hides the complexities of the internal system and provides an interface to the client that can access the system in a very simplified way.

Consider the example of a storekeeper. Now, when you, as a customer, visit a store to download certain items, you're not aware of the layout of the store. You typically approach the storekeeper, who is well aware of the store system. Based on your requirements, the storekeeper picks up items and hands them over to you. Isn't this easy? The Faade design pattern essentially does the following:. It provides a unified interface to a set of interfaces in a subsystem and defines a high-level interface that helps the client use the subsystem in an easy way.

Faade discusses representing a complex subsystem with a single interface object. It doesn't encapsulate the subsystem but actually combines the underlying subsystems.

As we observe the UML diagram, you'll realize that there are three main participants in this pattern:. The main responsibility of a faade is to wrap up a complex group of subsystems so that it can provide a pleasing look to the outside world.

This represents a set of varied subsystems that make the whole system compound and difficult to view or work with. The client interacts with the Faade so that it can easily communicate with the subsystem and get the work completed. It doesn't have to bother about the complex nature of the system. You will now learn a little more about the three main participants from the data structure's perspective.

It delegates the client's requests to the appropriate subsystem objects using composition.

Giridhar Chetan. Learning Python Design Patterns

For example, if the client is looking for some work to be accomplished, it need not have to go to individual subsystems but can simply contact the interface Faade that gets the work done. It implements subsystem functionality and is represented by a class. Ideally, a System is represented by a group of classes that are responsible for different operations.

It handles the work assigned by the Faade object but has no knowledge of the faade and keeps no reference to it. For instance, when the client requests the Faade for a certain service, Faade chooses the right subsystem that delivers the service based on the type of service.

Implementing the Faade pattern in the real world To demonstrate the applications of the Faade pattern, let's take an example that we'd have experienced in our lifetime.

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Consider that you have a marriage in your family and you are in charge of all the arrangements. That's a tough job on your hands. You have to book a hotel or place for marriage, talk to a caterer for food arrangements, organize a florist for all the decorations, and finally handle the musical arrangements expected for the event.

In yesteryears, you'd have done all this by yourself, for example by talking to the relevant folks, coordinating with them, negotiating on the pricing, but now life is simpler. You go and talk to an event manager who handles this for you. Putting it in the Faade pattern perspective:. It's you who need all the marriage preparations to be completed in time before the wedding. They should be top class and guests should love the celebrations.

The event manager who's responsible for talking to all the folks that need to work on specific arrangements such as food, and flower decorations, among others. They represent the systems that provide services such as catering, hotel management, and flower decorations. Let's develop an application in Python v3. We start with the client first. It's you! Remember, you're the one who has been given the responsibility to make sure that the marriage preparations are done and the event goes fine!

Let's now move ahead and talk about the Faade class. As discussed earlier, the Faade class simplifies the interface for the client. In this case, EventManager acts as a faade and simplifies the work for You. Faade talks to the subsystems and does all the booking and preparations for the marriage on your behalf. Here is the Python code for the EventManager class: Now that we're done with the Faade and client, let's dive into the subsystems.

We have developed the following classes for this scenario:. Hotelier is for the hotel bookings.

The Florist class is responsible for flower decorations. Florist has the setFlowerRequirements method to be used to set the expectations on the kind of flowers needed for the marriage decoration. The Caterer class is used to deal with the caterer and is responsible for the food arrangements. Caterer exposes the setCuisine method to accept the type of cuisine to be served at the marriage.

The Musician class is designed for musical arrangements at the marriage. It uses the setMusicType method to understand the music requirements for the event. Let us now look at the Hotelier object, followed by Florist object and their methods: However, you're being clever here and passing on the responsibility to the event manager, aren't you?

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Let's now look at the You class. In this example, you create an object of the EventManager class so that the manager can work with the relevant folks on marriage preparations while you relax. Marriage Arrangements??!!! Thanks to Event Manager, all preparations done!

EventManager uses composition to create objects of the subsystems such as Hotelier, Caterer, and others. The principle of least knowledge As you have learned in the initial parts of the chapter, the Faade provides a unified system that makes subsystems easy to use.

It also decouples the client from the subsystem of components. The design principle that is employed behind the Faade pattern is the principle of least knowledge. The principle of least knowledge guides us to reduce the interactions between objects to just a few friends that are close enough to you.

In real terms, it means the following:. When designing a system, for every object created, one should look at the number of classes that it interacts with and the way in which the interaction happens. Following the principle, make sure that we avoid situations where there are many classes created that are tightly coupled to each other. If there are a lot of dependencies between classes, the system becomes hard to maintain.

Any changes in one part of the system can lead to unintentional changes to other parts of the system, which means that the system is exposed to regressions and this should be avoided. Frequently asked questions Q1. What is the Law of Demeter and how is it related to the Factory pattern?

The Law of Demeter is a design guideline that talks about the following: Each unit should have only limited knowledge of other units in the system 2. A unit should talk to its friends only 3.

A unit should not know about the internal details of the object that it manipulates. The principle of least knowledge and Law of Demeter are the same and both point to the philosophy of loose coupling.

The principle of least knowledge fits the use case of the Faade pattern as the name is intuitive and the word principle acts as a guideline, not being strict, and being useful only when needed. Can there be multiple Faades for a subsystem? Yes, one could implement more than one faade for a group of subsystem components. What are the disadvantages of the principle of least knowledge? A Faade provides a simplified interface for the clients to interact with subsystems. In the spirit of providing a simplified interface, an application can have multiple unnecessary interfaces that add to the complexity of the system and reduce runtime performance.

Can the client access the subsystems independently? Yes, in fact, the Faade pattern provides simplified interfaces so that the client need not be bothered about the complexity of the subsystems.

Does the Faade add any functionality of its own? A Faade can add its "thinking" to the subsystems, such as making sure that the order of innovation for subsystems can be decided by the Faade.

Summary We began the chapter by first understanding the Structural design patterns. You then learned about the Faade design pattern and the context in which it's used.

We understood the basis of Faade and how it is effectively used in software architecture.They represent the systems that provide services such as catering, hotel management, and flower decorations. Chapter 11, AntiPatterns, tells you about AntiPatternswhat we shouldn't do as architects or software engineers. Do we really need this pattern in Python? Each topic is explained and placed in context, and for the more inquisitive, there are more details on the concepts used.

They exist, we just need to find and put them to use. You also learned the pattern with a UML diagram and sample code implementation in Python v3. This makes sure that the cost of software maintenance is low and code can be easily reused or is adaptable to change. By the end of the book, you will have enhanced your professional abilities in software architecture, design, and development.