Flask框架详解：从入门到精通

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"Flask是Python的一个轻量级Web服务程序，它被设计为简单易用，适合构建小型到中型的应用。这份文档是Flask 1.1.x版本的官方指南，包含了从安装、快速入门到高级特性的详细说明。文档由Pallets项目维护，并在2021年3月16日更新。" Flask框架是Python Web开发中的一个流行选择，因其简洁和模块化的设计而受到欢迎。以下是一些主要的知识点： 1. **安装**：安装Flask通常通过Python的包管理器pip进行，命令是`pip install flask`。确保Python环境已经设置好，并且pip是最新的。 2. **快速启动**：快速启动部分介绍了如何创建一个基本的Flask应用。只需定义一个应用实例，然后使用装饰器将路由与处理函数关联起来。例如： ```python from flask import Flask app = Flask(__name__) @app.route('/') def hello_world(): return 'Hello, World!' ``` 3. **教程**：Flask的教程从基础开始，逐步引导开发者了解如何构建更复杂的应用，包括处理表单数据、使用模板、添加静态文件等。 4. **模板**：Flask使用Jinja2作为默认的模板引擎，允许开发者编写动态HTML页面。模板可以包含变量、控制结构（如if/else）以及继承和宏等特性。 5. **测试**：Flask提供了测试支持，可以通过`test_client()`方法创建一个测试客户端来模拟HTTP请求，以便对应用进行单元测试和集成测试。 6. **应用错误**：这部分讲解了如何处理和自定义Flask应用的错误页面，包括404 Not Found和500 Internal Server Error等。 7. **调试**：Flask提供了内置的调试器，可以帮助开发者在运行时追踪和解决错误。启用调试模式，应用会在遇到错误时暂停并显示调试信息。 8. **日志记录**：Flask提供了灵活的日志记录机制，可以与其他日志记录库集成，帮助记录应用运行时的信息。 9. **配置处理**：Flask允许通过环境变量或者配置文件来管理应用的配置，方便在不同环境下切换设置。 10. **信号（Signals）**：信号是Flask中的一个通信机制，允许扩展或应用的各个部分在特定事件发生时相互通知。 11. **插件视图（Pluggable Views）**：这允许视图功能作为可插入的组件，便于代码重用和模块化。 12. **应用上下文（ApplicationContext）**：每个Flask应用有一个全局的应用上下文，保存应用级别的信息，如配置。 13. **请求上下文（Request Context）**：每个请求都有一个请求上下文，包含当前请求的所有信息，如请求方法、URL、headers等。 14. **蓝图（Blueprints）**：蓝图是Flask用于实现模块化应用的方式，它允许你将大的应用分解为多个独立的部分。 15. **扩展（Extensions）**：Flask生态中有许多第三方扩展，如SQLAlchemy用于数据库操作，Flask-WTF用于表单处理等，它们能增强Flask的功能。 16. **命令行接口（CLI）**：Flask提供了一个命令行接口，用于执行各种任务，如初始化项目、运行服务器等。 17. **开发服务器**：Flask自带一个简单的开发服务器，方便本地开发和测试。 18. **Shell交互**：Flask提供了一种在Python shell中与应用交互的方式，方便调试和测试。 19. **模式和最佳实践**：文档中涵盖了在Flask中实现常见模式的建议，如URL构建、会话管理和错误处理。 20. **部署选项**：这部分讨论了如何将Flask应用部署到生产环境，包括WSGI服务器、反向代理和多进程/线程设置。 21. **性能优化**：随着应用规模的增长，文档也提供了如何优化性能和应对大型应用挑战的建议。 22. **API参考**：提供了Flask核心类和函数的详细API文档，包括`Flask`对象、`request`对象、`session`对象等。 23. **额外注意事项**：这部分涵盖了Flask的设计决策、HTML/XHTML FAQ、安全考虑、Unicode处理、Flask扩展开发指南以及Pocoo项目风格指南。 24. **升级指南**：如果你正在使用旧版本的Flask，这里提供了升级到新版本的步骤和注意事项。 25. **变更日志**：记录了每个版本的更改和改进。 26. **许可**：Flask遵循特定的开源许可协议，通常为BSD。 27. **贡献指南**：如果你想参与Flask的开发或贡献，这部分提供了如何开始的指导。这份文档是学习和掌握Flask框架的重要资源，覆盖了从初学者到进阶开发者的各种需求。无论你是刚接触Flask还是寻求更深入的理解，都能从中获益。

Flask Documentation (1.1.x), Release 1.1.2

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, url_for

from markupsafe import escape

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

/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 markupsafe 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

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

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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