Flask 1.1.x 用户指南与API参考

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Flask文档1.1.x是Pallets项目的一部分，它提供了关于Flask Web框架的全面指南和参考。该文档适用于不同级别的开发者，包括初学者和经验丰富的程序员。文档结构清晰，涵盖了从安装到高级主题的广泛内容。 1. 用户指南： - **Foreword**：文档的引言部分，介绍了Flask的基本理念和目标。 - **快速入门** (Quickstart)：提供了一个简短的教程，让读者对Flask的基本用法有初步了解，包括创建第一个Flask应用。 2. 主要内容详解： - **模板系统** (Templates): 讨论了如何使用Flask的模板语言来渲染动态内容，以及模板继承、标签和其他高级功能。 - **测试Flask应用** (Testing Flask Applications): 提供了如何使用Flask的测试工具进行单元测试和集成测试的方法。 - **错误处理与调试** (Application Errors & Debugging): 阐述了如何捕获、处理应用中的错误，并使用调试工具调试Flask应用。 - **配置管理** (Configuration Handling): 介绍如何设置和管理Flask应用的配置选项。 - **信号系统** (Signals): 描述了如何使用Flask的信号系统来处理应用程序的特定事件。 - **插件视图** (Pluggable Views): 讲解如何利用Flask的蓝图（Blueprints）创建模块化和可重用的视图。 - **扩展支持** (Extensions): 提供了如何在Flask应用中集成第三方库和自定义扩展的信息。 - **命令行接口** (Command Line Interface): 展示了如何通过命令行工具管理和运行Flask应用。 - **开发服务器** (Development Server)：详细说明了Flask内置的开发服务器的使用方法。 - **Shell集成** (Working with the Shell): 讨论了如何在应用中使用Python shell进行交互式开发。 - **模式与最佳实践** (Patterns for Flask): 提供了一些常见的开发模式和优化建议，帮助开发者编写高效的应用。 - **部署选项** (Deployment Options): 讨论了将Flask应用部署到生产环境的各种策略和工具。 - **成长与扩展** (Becoming Big): 提供了对大型应用架构和性能优化的思考，以及应对复杂性的建议。 3. API参考： - **API文档** (API): 包含了Flask及其扩展的所有核心类和函数的详细说明。附加内容： - **设计决策** (Design Decisions): 解释了Flask框架的设计哲学和选择背后的理由。 - **常见问题解答** (HTML/XHTML FAQ): 关于HTML和XHTML使用的常见问题及解决方案。 - **安全注意事项** (Security Considerations): 提醒开发者注意Web应用的安全风险和防范措施。 - **Unicode支持** (Unicode in Flask): 说明Flask如何处理多语言和字符编码。 - **Flask扩展开发** (Flask Extension Development): 对开发者的指南，包括如何创建和发布自己的Flask插件。 - **升级指导** (Upgrading to Newer Releases): 如何平稳地升级到新版本的Flask。 - **变更日志** (Changelog): 记录了每个版本的主要变化和更新。 - **许可信息** (License): 提供了Flask文档和代码的开源许可协议。 - **贡献指南** (How to contribute to Flask): 为想要参与到Flask社区的开发者提供了参与贡献的步骤。 Flask文档1.1.x是一个全面的资源，无论是初次接触Flask还是深入学习，都能从中找到所需的信息，帮助开发者构建高效、模块化的Web应用。

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资源推荐

Flask Documentation (1.1.x), Release 1.1.2.dev0

URL Building

To build a URL to a speciﬁc function, use the url_for() function. It accepts the name of the function as its ﬁrst

argument and any number of keyword arguments, each corresponding to a variable part of the URL rule. Unknown

variable parts are appended to the URL as query parameters.

Why would you want to build URLs using the URL reversing function url_for() instead of hard-coding them into

your templates?

1. Reversing is often more descriptive than hard-coding the URLs.

2. You can change your URLs in one go instead of needing to remember to manually change hard-coded URLs.

3. URL building handles escaping of special characters and Unicode data transparently.

4. The generated paths are always absolute, avoiding unexpected behavior of relative paths in browsers.

5. If your application is placed outside the URL root, for example, in /myapplication instead of /,

url_for() properly handles that for you.

For example, here we use the test_request_context() method to try out url_for().

test_request_context() tells Flask to behave as though it’s handling a request even while we use a

Python shell. See Context Locals.

from flask import Flask, escape, url_for

app = Flask(__name__)

@app.route('/')

def index():

return 'index'

@app.route('/login')

def login():

return 'login'

@app.route('/user/<username>')

def profile(username):

return '{}\'s profile'.format(escape(username))

with app.test_request_context():

print(url_for('index'))

print(url_for('login'))

print(url_for('login', next='/'))

print(url_for('profile', username='John Doe'))

/login

/login?next=/

/user/John%20Doe

HTTP Methods

Web applications use different HTTP methods when accessing URLs. You should familiarize yourself with the HTTP

methods as you work with Flask. By default, a route only answers to GET requests. You can use the methods

argument of the route() decorator to handle different HTTP methods.

12 Chapter 1. User’s Guide

Flask Documentation (1.1.x), Release 1.1.2.dev0

/application

/__init__.py

/templates

/hello.html

For templates you can use the full power of Jinja2 templates. Head over to the ofﬁcial Jinja2 Template Documentation

for more information.

Here is an example template:

<!doctype html>

<title>Hello from Flask</title>

{% if name %}

<h1>Hello {{ name }}!</h1>

{% else %}

<h1>Hello, World!</h1>

{% endif %}

Inside templates you also have access to the request, session and g

objects as well as the

get_flashed_messages() function.

Templates are especially useful if inheritance is used. If you want to know how that works, head over to the Template

Inheritance pattern documentation. Basically template inheritance makes it possible to keep certain elements on each

page (like header, navigation and footer).

Automatic escaping is enabled, so if name contains HTML it will be escaped automatically. If you can trust a variable

and you know that it will be safe HTML (for example because it came from a module that converts wiki markup to

HTML) you can mark it as safe by using the Markup class or by using the |safe ﬁlter in the template. Head over to

the Jinja 2 documentation for more examples.

Here is a basic introduction to how the Markup class works:

>>> from flask import Markup

>>> Markup('<strong>Hello %s!</strong>') % '<blink>hacker</blink>'

Markup(u'<strong>Hello <blink>hacker</blink>!</strong>')

>>> Markup.escape('<blink>hacker</blink>')

Markup(u'<blink>hacker</blink>')

>>> Markup('<em>Marked up</em> » HTML').striptags()

u'Marked up \xbb HTML'

Changed in version 0.5: Autoescaping is no longer enabled for all templates. The following extensions for tem-

plates 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

Unsure what that g object is? It’s something in which you can store information for your own needs, check the documentation of that object

(g) and the Using SQLite 3 with Flask for more information.

14 Chapter 1. User’s Guide

Flask Documentation (1.1.x), Release 1.1.2.dev0

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 internal 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:

from flask import request

with app.request_context(environ):

assert request.method == 'POST'

The Request Object

The request object is documented in the API section and we will not cover it here in detail (see Request). Here is a

broad overview of some of the most common operations. First of all you have to import it from the flask module:

from flask import request

The current request method is available by using the method attribute. To access form data (data transmitted in a

POST or PUT request) you can use the form attribute. Here is a full example of the two attributes mentioned above:

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

def login():

error = None

if request.method == 'POST':

if valid_login(request.form['username'],

request.form['password']):

return log_the_user_in(request.form['username'])

else:

error = 'Invalid username/password'

# the code below is executed if the request method

# was GET or the credentials were invalid

return render_template('login.html', error=error)

1.4. Quickstart 15

Flask Documentation (1.1.x), Release 1.1.2.dev0

What happens if the key does not exist in the form attribute? In that case a special KeyError is raised. You can

catch it like a standard KeyError but if you don’t do that, a HTTP 400 Bad Request error page is shown instead. So

for many situations you don’t have to deal with that problem.

To access parameters submitted in the URL (?key=value) you can use the args attribute:

searchword = request.args.get('key', '')

We recommend accessing URL parameters with get or by catching the KeyError because users might change the

URL and presenting them a 400 bad request page in that case is not user friendly.

For a full list of methods and attributes of the request object, head over to the Request documentation.

File Uploads

You can handle uploaded ﬁles with Flask easily. Just make sure not to forget to set the enctype="multipart/

form-data" attribute on your HTML form, otherwise the browser will not transmit your ﬁles at all.

Uploaded ﬁles are stored in memory or at a temporary location on the ﬁlesystem. You can access those ﬁles by

looking at the files attribute on the request object. Each uploaded ﬁle is stored in that dictionary. It behaves just

like a standard Python file object, but it also has a save() method that allows you to store that ﬁle on the ﬁlesystem

of the server. Here is a simple example showing how that works:

from flask import request

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

def upload_file():

if request.method == 'POST':

f = request.files['the_file']

f.save('/var/www/uploads/uploaded_file.txt')

...

If you want to know how the ﬁle was named on the client before it was uploaded to your application, you can access

the filename attribute. However please keep in mind that this value can be forged so never ever trust that value. If

you want to use the ﬁlename of the client to store the ﬁle on the server, pass it through the secure_filename()

function that Werkzeug provides for you:

from flask import request

from werkzeug.utils import secure_filename

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

def upload_file():

if request.method == 'POST':

f = request.files['the_file']

f.save('/var/www/uploads/' + secure_filename(f.filename))

...

For some better examples, checkout the Uploading Files pattern.

To access cookies you can use the cookies attribute. To set cookies you can use the set_cookie method of

response objects. The cookies attribute of request objects is a dictionary with all the cookies the client transmits. If

you want to use sessions, do not use the cookies directly but instead use the Sessions in Flask that add some security

on top of cookies for you.

Reading cookies:

16 Chapter 1. User’s Guide

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