我们为什么选择Django？ Django是工作原理 什么是模式？ 常见的模式合集 Django中的模式
Django的历史When you look at the Pyramids of Egypt, you would think that such a simple and minimal design must have been quite obvious. In truth, they are products of 4,000 years of architectural evolution. Step Pyramids, the initial (and clunky) design, had six rectangular blocks of decreasing size. It took several iterations of architectural and engineering improvements until the modern, glazing, and long-lasting limestone structures were invented.
Looking at Django you might get a similar feeling. So, elegantly built, it must have been awlessly conceived. n the contrary, it was the result of rewrites and rapid iterations in one of the most high-pressure environments imaginable—a newsroom!
In the fall of 2003, two programmers, Adrian Holovaty and Simon Willison, working at the Lawrence Journal-World newspaper, were working on creating several local news websites in Kansas. These sites, including LJWorld.com, Lawrence.com, and KUsports.com—like most news sites were not just content-driven portals chock- full of text, photos, and videos, but they also constantly tried to serve the needs of the local Lawrence community with applications, such as a local business directory, events calendar, classifieds, and so on.
This, of course, meant lots of work for Simon, Adrian, and later Jacob Kaplan Moss who had joined their team; with very short deadlines, sometimes with only a few hours' notice. Since it was the early days of web development in Python, they had to write web applications mostly from scratch. So, to save precious time, they gradually refactored out the common modules and tools into something called "The CMS."
Eventually, the content management parts were spun off into a separate project called the Ellington CMS, which went on to become a successful commercial CMS product. The rest of "The CMS" was a neat underlying framework that was general enough to be used to build web applications of any kind.
By July 2005, this web development framework was released as Django (pronounced Jang-Oh) under an open source Berkeley Software Distribution (BSD) license. It was named after the legendary jazz guitarist Django Reinhardt. And the rest, as they say, is history.
Due to its humble origins as an internal tool, Django had a lot of Lawrence ournalWorldspecific oddities. To ake Django truly general purpose, an effort dubbed "Removing the Lawrence" was already underway.
However, the ost significant refactoring effort that Django developers had to undertake was called "Removing the Magic." This ambitious project involved cleaning up all the warts Django had accumulated over the years, including a lot of magic (an informal term for implicit features) and replacing them with a more natural and explicit Pythonic code. For example, the model classes used to be imported from a magic module called django.models.*, rather than directly importing them from the models.py odule they were defined in.
At that time, Django had about a hundred thousand lines of code, and it was a significant rewrite of the PI. n May , , these changes, alost the sie of a small book, were integrated into Django's development version trunk and released as Django release .. This was a significant step toward the Django .
Every year, conferences called DjangoCons are held across the world for Django developers to meet and interact with each other. They have an adorable tradition of giving a semi-humorous keynote on "why Django sucks." This could be a member of the Django community, or someone who works on competing web frameworks or just any notable personality.
Over the years, it is amazing how Django developers took these criticisms positively and mitigated them in subsequent releases. Here is a short summary of the improvements corresponding to what once used to be a shortcoming in Django and the release they were resolved in:
- New form-handling library (Django 0.96)
- Decoupling admin from models (Django 1.0)
- Multiple database support (Django 1.2)
- Managing static files better Django .
- Better time zone support (Django 1.4)
- Customizable user model (Django 1.5)
- Better transaction handling (Django 1.6)
- Built-in database migrations (Django 1.7)
1. 浏览器发送请求（基本上是字节类型的字符串）到web服务器。 2. web服务器（比如，Nginx）把这个请求转交到一个WSGI（比如，uWSGI），或者直接地文件系统能够取出 一个文件（比如，一个CSS文件）。 3. 不像web服务器那样，WSGI服务器可以直接运行Python应用。请求生成一个被称为environ的Ptyhon字典， 而且，可以选择传递过去几个中间件的层，最终，达到Django应用。 4. URLconf中含有属于应用的urls.py选择一个视图处理基于请求的URL的那个请求，这个请求就已经变成了 HttpRequest——一个Python字典对象。 5. 被选择的那个视图通常要做下面所列出的一件或者更多件事情： 通过模型与数据库对话。 使用模板渲染HTML或者任何格式化过的响应。 返回一个纯文本响应（不被显示的）。 抛出一个异常。 6. HttpResponse对象离开Django后，被渲染为一个字符串。 7. 在浏览器见到一个美化的，渲染后的web页面。
What is coon between the words Blueprint, caffolding, and Maintenance? These software development terms have been borrowed from the world of building construction and architecture. However, one of the ost inuential ters coes from a treatise on architecture and urban planning written in 1977 by the leading Austrian architect Christopher Alexander and his team consisting of Murray Silverstein, Sara Ishikawa, and several others.
The term "Pattern" came in vogue after their seminal work, A Pattern Language: Towns, Buildings, Construction volue in a fivebook series based on the astonishing insight that users know about their buildings more than any architect ever could. A pattern refers to an everyday problem and its proposed but time-tested solution.
In the book, Christopher Alexander states that "Each pattern describes a problem, which occurs over and over again in our environment, and then describes the core of the solution to that problem in such a way that you can use this solution a million times over, without ever doing it the same way twice."
For example, the Wings Of Light pattern describes how people prefer buildings with more natural lighting and suggests arranging the building so that it is composed of wings. These wings should be long and narrow, never more than 25 feet wide. Next time you enjoy a stroll through the long well-lit corridors of an old university, be grateful to this pattern.
Their book contained 253 such practical patterns, from the design of a room to the design of entire cities. Most importantly, each of these patterns gave a name to an abstract problem and together formed a pattern language.
Reeber when you first cae across the word déjà vu? You probably thought "Wow, I never knew that there was a word for that experience." Similarly, architects were not only able to identify patterns in their environent but could also, finally, name them in a way that their peers could understand.
In the world of software, the term design pattern refers to a general repeatable solution to a commonly occurring problem in software design. It is a formalization of best practices that a developer can use. Like in the world of architecture, the pattern language has proven to be extremely helpful to communicate a certain way of solving a design problem to other programmers.
There are several collections of design patterns but some have been considerably ore inuential than the others.
One of the earliest efforts to study and document design patterns was a book titled Design Patterns: Elements of Reusable Object-Oriented Software by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides, who later became known as the Gang of Four (GoF). This book is so inuential that any consider the design patterns in the book as fundamental to software engineering itself.
In reality, the patterns were written primarily for object-oriented programming languages, and it had code examples in C++ and Smalltalk. As we will see shortly, many of these patterns might not be even required in other programming languages with better higher-order abstractions such as Python.
The patterns have been broadly classified by their type as follows：
- Creational Patterns: These include Abstract Factory, Builder Pattern, Factory Method, Prototype Pattern, and Singleton Pattern
- Structural Patterns: These include Adapter Pattern, Bridge Pattern, Composite Pattern, Decorator Pattern, Facade Pattern, Flyweight Pattern, and Proxy Pattern
- Behavioral Patterns: These include Chain of Responsibility, Command Pattern, Interpreter Pattern, Iterator Pattern, Mediator Pattern, Memento Pattern, Observer Pattern, State Pattern, Strategy Pattern, Template Pattern, and Visitor Pattern
While a detailed explanation of each pattern would be beyond the scope of this book, it would be interesting to identify some of these patterns in Django itself:
Model-View-Controller (MVC) is an architectural pattern invented by Xerox PARC in the 70s. Being the framework used to build user interfaces in Smalltalk, it gets an early mention in the GoF book.
Today, MVC is a very popular pattern in web application frameworks. Beginners often ask the questionis Django an M fraework?
The answer is both yes and no. The MVC pattern advocates the decoupling of the presentation layer from the application logic. For instance, while designing an online game website API, you might present a game's high scores table as an HTML, ML, or coaseparated file. However, its underlying odel class would be designed independent of how the data would be finally presented.
MVC is very rigid about what models, views, and controllers do. However, Django takes a much more practical view to web applications. Due to the nature of the HTTP protocol, each request for a web page is independent of any other request. Django's framework is designed like a pipeline to process each request and prepare a response.
Django calls this the Model-Template-View (MTV) architecture. There is separation of concerns between the database interfacing classes (Model), request-processing classes iew, and a teplating language for the final presentation Teplate.
If you compare this with the classic MVC—"Model" is comparable to Django's Models, "View" is usually Django's Templates, and "Controller" is the framework itself that processes an incoming HTTP request and routes it to the correct view function.
If this has not confused you enough, Django prefers to name the callback function to handle each URL a "view" function. This is, unfortunately, not related to the MVC pattern's idea of a View.
In 2002, Martin Fowler wrote Patterns of Enterprise Application Architecture, which described 40 or so patterns he often encountered while building enterprise applications.
Unlike the GoF book, which described design patterns, Fowler's book was about architectural patterns. Hence, they describe patterns at a much higher level of abstraction and are largely programming language agnostic. Fowler's patterns are organized as follows:
• Domain Logic Patterns: These include Domain Model, Transaction Script, Service Layer , and Table Module • Data Source Architectural Patterns: These include Row Data Gateway, Table Data Gateway, Data Mapper, and Active Record • Object-Relational Behavioral Patterns: These include Identity Map, Unit of Work, and Lazy Load • Object-Relational Structural Patterns: These include Foreign Key Mapping, Mapping, Dependent Mapping, Association Table Mapping, Identity Field, Serialized LOB, Embedded Value, Inheritance Mappers, Single Table Inheritance, Concrete Table Inheritance, and Class Table Inheritance • Object-Relational Metadata Mapping Patterns: These include Query Object, Metadata Mapping, and Repository • Web Presentation Patterns: These include Page Controller, Front Controller, Model View Controller, Transform View, Template View, Application Controller, and Two-Step View • Distribution Patterns: These include Data Transfer Object and Remote Facade • Offline Concurrency Patterns: These include Coarse Grained Lock, Implicit Lock, Optimistic fine Lock, and Pessiistic fine Lock • Session State Patterns: These include Database Session State, Client Session State, and Server Session State • Base Patterns: These include Mapper, Gateway, Layer Supertype, Registry, Value Object, Separated Interface, Money, Plugin, Special Case, Service Stub, and Record Set
Almost all of these patterns would be useful to know while architecting a Django application. In fact, Fowler's website at http://martinfowler.com/eaaCatalog/ has an excellent catalog of these patterns. I highly recommend that you check them out.
Django also implements a number of these patterns. The following table lists a few of them:
Yes, of course. Patterns are discovered all the tie. Like living beings, soe mutate and form new patterns: take, for instance, MVC variants such as Model–view–presenter (MVP), Hierarchical model–view–controller (HMVC), or Model View ViewModel (MVVM).
Patterns also evolve with tie as better solutions to known probles are identified. For example, Singleton pattern was once considered to be a design pattern but now is considered to be an Anti-pattern due to the shared state it introduces, similar to using global variables. An Anti-pattern can be defined as coonly reinvented but a bad solution to a problem.
Some of the other well-known books which catalog patterns are Pattern-Oriented Software Architecture (known as POSA) by Buschmann, Meunier, Rohnert, Sommerlad, and Sta; Enterprise Integration Patterns by Hohpe and Woolf; and The Design of Sites: Patterns, Principles, and Processes for Crafting a Customer-Centered Web Experience by Duyne, Landay, and Hong.
This book will cover Djangospecific design and architecture patterns, which would be useful to a Django developer. The upcoming sections will describe how each pattern will be presented.
The heading is the pattern name. If it is a well-known pattern, the commonly used name is used; otherwise, a terse, self-descriptive name has been chosen. Names are important, as they help in building the pattern vocabulary. All patterns will have the following parts:
Problem: This briey entions the proble.
Solution: This summarizes the proposed solution(s).
Problem Details: This elaborates the context of the problem and possibly gives an example. Solution Details: This explains the solution(s) in general terms and provides a sample Django implementation.
Despite their near universal usage, Patterns have their share of criticism too. The most common arguments against them are as follows:
- Patterns compensate for the missing language features: Peter Norvig found that 16 of the 23 patterns in Design Patterns were 'invisible or simpler' in Lisp. Considering Python's introspective facilities and firstclass functions, this ight as well be the case for Python too.
- Patterns repeat best practices: Many patterns are essentially formalizations of best practices such as separation of concerns and could seem redundant.
- Patterns can lead to over-engineering: Implementing the pattern might be less efficient and excessive copared to a sipler solution.
While some of the previous criticisms are quite valid, they are based on how patterns are misused. Here is some advice that can help you understand how best to use design patterns:
- Don't implement a pattern if your language supports a direct solution
- Don't try to retrofit everything in ters of patterns
- Use a pattern only if it is the most elegant solution in your context
- Don't be afraid to create new patterns
In addition to design patterns, there might be a recommended approach to solving a problem. In Django, as with Python, there might be several ways to solve a problem but one idiomatic approach among those.
Generally, the Python community uses the term 'Pythonic' to describe a piece of idiomatic code. It typically refers to the principles laid out in 'The Zen of Python'. Written like a poem, it is extremely useful to describe such a vague concept.
Try entering import this in a Python prompt to view 'The Zen of Python'.
Furthermore, Django developers have crisply documented their design philosophies while designing the framework at https://docs.djangoproject.com/en/dev/ misc/design-philosophies/.
While the document describes the thought process behind how Django was designed, it is also useful for developers using Django to build applications. Certain principles such as Don't Repeat Yourself (DRY), loose coupling, and tight cohesion can help you write more maintainable and idiomatic Django applications.
Django or Python best practices suggested by this book would be formatted in the following manner:
In this chapter, we looked at why people choose Django over other web frameworks, its interesting history, and how it works. We also examined design patterns, popular pattern collections, and best practices.
In the next chapter, we will take a look at the first few steps in the beginning of a Django project such as gathering requirements, creating mockups, and setting up the project.