Flask官方教程：快速入门与Web开发指南

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Flask官方文档提供了一个全面的指南，旨在帮助开发者深入了解和使用Flask框架进行Web开发。该文档分为用户手册和API参考两大部分。首先，用户手册部分从背景介绍开始，包括对Flask框架的简述，目标受众（新老程序员）的引导，以及安装步骤。接着，通过快速入门和教程，逐步教授如何设置和构建基础的Flask应用，包括模板的使用、测试应用、错误处理和调试方法。核心知识点包括： 1. **应用程序结构** - 介绍了Flask的应用上下文（Application Context）和请求上下文（Request Context），这对于理解Flask如何管理应用程序状态至关重要。 2. **蓝图（Blueprints）** - 提供了模块化应用程序的设计方法，使得大型项目组织更加清晰和可维护。 3. **插件与扩展（Flask Extensions）** - 指出如何利用现有的扩展来增强Flask的功能，如数据库集成、认证和会话管理等。 4. **命令行接口（Command Line Interface, CLI）** 和开发服务器，这些工具有助于开发过程中的快速迭代和本地测试。 5. **调试和部署** - 文档详细介绍了如何处理应用错误，使用调试工具，并讨论了不同的部署选项，确保应用能够顺利运行在生产环境中。 6. **设计决策和最佳实践** - 提供了关于Flask设计理念的深入解析，以及推荐的编码风格（Pocoo Style Guide）和Python 3支持。 API参考部分则详尽地列出了Flask及其扩展的所有功能和函数的使用方法，便于开发者查找和调用具体功能。此外，文档还涵盖了额外的注意事项，如设计决策背后的思考、HTML/XHTML常见问题、安全考虑、Unicode支持，以及如何开发和升级Flask扩展。最后，文档附有Python模块索引，方便读者快速定位所需的信息。总体来说，Flask官方文档为Flask开发者提供了一个详尽的资源库，无论你是初学者还是经验丰富的开发者，都能从中找到构建高效、可扩展的Web应用所需的工具和知识。

Flask Documentation, Release 0.12.2

2. URL building will handle escaping of special characters and Unicode data trans-

parently for you, so you don’t have to deal with them.

3. If your application is placed outside the URL root - say, in /myapplication in-

stead of / - url_for() will handle that properly for you.

HTTP Methods

HTTP (the protocol web applications are speaking) knows different methods for ac-

cessing URLs. By default, a route only answers to GET requests, but that can be changed

by providing the methods argument to the route() decorator. Here are some examples:

from flask import request

@app.route('/login', methods=['GET', 'POST'])

def login():

if request.method == 'POST':

do_the_login()

else:

show_the_login_form()

If GET is present, HEAD will be added automatically for you. You don’t have to deal

with that. It will also make sure that HEAD requests are handled as the HTTP RFC (the

document describing the HTTP protocol) demands, so you can completely ignore that

part of the HTTP speciﬁcation. Likewise, as of Flask 0.6, OPTIONS is implemented for

you automatically as well.

You have no idea what an HTTP method is? Worry not, here is a quick introduction to

HTTP methods and why they matter:

The HTTP method (also often called “the verb”) tells the server what the client wants

to do with the requested page. The following methods are very common:

GET The browser tells the server to just get the information stored on that page and

send it. This is probably the most common method.

HEAD The browser tells the server to get the information, but it is only interested in the

headers, not the content of the page. An application is supposed to handle that as

if a GET request was received but to not deliver the actual content. In Flask you

don’t have to deal with that at all, the underlying Werkzeug library handles that

for you.

POST The browser tells the server that it wants to post some new information to that

URL and that the server must ensure the data is stored and only stored once.

This is how HTML forms usually transmit data to the server.

PUT Similar to POST but the server might trigger the store procedure multiple times

by overwriting the old values more than once. Now you might be asking why

this is useful, but there are some good reasons to do it this way. Consider that

the connection is lost during transmission: in this situation a system between the

browser and the server might receive the request safely a second time without

12 Chapter 1. User’s Guide

Flask Documentation, Release 0.12.2

breaking things. With POST that would not be possible because it must only be

triggered once.

DELETE Remove the information at the given location.

OPTIONS Provides a quick way for a client to ﬁgure out which methods are supported

by this URL. Starting with Flask 0.6, this is implemented for you automatically.

Now the interesting part is that in HTML4 and XHTML1, the only methods a form can

submit to the server are GET and POST. But with JavaScript and future HTML standards

you can use the other methods as well. Furthermore HTTP has become quite popular

lately and browsers are no longer the only clients that are using HTTP. For instance,

many revision control systems use it.

1.4.5 Static Files

Dynamic web applications also need static ﬁles. That’s usually where the CSS and

JavaScript ﬁles are coming from. Ideally your web server is conﬁgured to serve them

for you, but during development Flask can do that as well. Just create a folder called

static in your package or next to your module and it will be available at /static on

the application.

To generate URLs for static ﬁles, use the special 'static' endpoint name:

url_for('static', filename='style.css')

The ﬁle has to be stored on the ﬁlesystem as static/style.css.

1.4.6 Rendering Templates

Generating HTML from within Python is not fun, and actually pretty cumbersome be-

cause you have to do the HTML escaping on your own to keep the application secure.

Because of that Flask conﬁgures the Jinja2 template engine for you automatically.

To render a template you can use the render_template() method. All you have to do

is provide the name of the template and the variables you want to pass to the template

engine as keyword arguments. Here’s a simple example of how to render a template:

from flask import render_template

@app.route('/hello/')

@app.route('/hello/<name>')

def hello(name=None):

return render_template('hello.html', name=name)

Flask will look for templates in the templates folder. So if your application is a module,

this folder is next to that module, if it’s a package it’s actually inside your package:

Case 1: a module:

1.4. Quickstart 13

Flask Documentation, Release 0.12.2

following extensions for templates trigger autoescaping: .html, .htm, .xml, .xhtml.

Templates loaded from a string will have autoescaping disabled.

1.4.7 Accessing Request Data

For web applications it’s crucial to react to the data a client sends to the server. In

Flask this information is provided by the global request object. If you have some

experience with Python you might be wondering how that object can be global and

how Flask manages to still be threadsafe. The answer is context locals:

Context Locals

Insider Information

If you want to understand how that works and how you can implement tests with

context locals, read this section, otherwise just skip it.

Certain objects in Flask are global objects, but not of the usual kind. These objects are

actually proxies to objects that are local to a speciﬁc context. What a mouthful. But

that is actually quite easy to understand.

Imagine the context being the handling thread. A request comes in and the web server

decides to spawn a new thread (or something else, the underlying object is capable

of dealing with concurrency systems other than threads). When Flask starts its inter-

nal request handling it ﬁgures out that the current thread is the active context and

binds the current application and the WSGI environments to that context (thread). It

does that in an intelligent way so that one application can invoke another application

without breaking.

So what does this mean to you? Basically you can completely ignore that this is the

case unless you are doing something like unit testing. You will notice that code which

depends on a request object will suddenly break because there is no request object. The

solution is creating a request object yourself and binding it to the context. The easiest

solution for unit testing is to use the test_request_context() context manager. In

combination with the with statement it will bind a test request so that you can interact

with it. Here is an example:

from flask import request

with app.test_request_context('/hello', method='POST'):

# now you can do something with the request until the

# end of the with block, such as basic assertions:

assert request.path == '/hello'

assert request.method == 'POST'

The other possibility is passing a whole WSGI environment to the request_context()

method:

1.4. Quickstart 15

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