Flask Mega教程：一步步构建Web应用

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"这篇资源是关于‘The Flask Mega Tutorial’的系列文章索引，涵盖了从基础到高级的多个方面，适合初学者学习Flask web框架。作者是一名有多年经验的软件工程师，他认为Python与Flask的组合是最具灵活性的。教程内容包括：‘Hello, World!’、模板、Web表单、数据库、用户登录、个人资料页和头像、单元测试、关注者、联系人和朋友、分页、全文搜索、电子邮件支持、网站改版、日期和时间、国际化和本地化、Ajax、调试、测试和性能分析、在Linux（包括Raspberry Pi）上部署、在Heroku云平台上部署等。" 在这个详尽的Flask教程系列中，每个部分都旨在帮助开发者逐步掌握构建Web应用程序的关键技能。首先，"Part I: Hello, World!"引导读者了解基础，快速启动一个简单的Flask应用。这通常是学习任何编程或框架的第一步，让开发者对环境设置和基础API有一个初步认识。接下来，"Part II: Templates"介绍如何使用Flask的模板引擎 Jinja2，允许开发者创建动态HTML页面，从而实现数据和视图的分离。"Part III: Web Forms"则讲解如何处理用户输入，集成表单验证和提交，这对于任何交互式Web应用都是必不可少的。 "Part IV: Database"深入到数据管理，教读者如何使用SQLAlchemy这个强大的ORM（对象关系映射）库来连接和操作数据库。"Part V: User Logins"则涉及用户认证和授权，这是许多Web应用的核心功能。 "Part VI: Profile Page And Avatars"和"Part VII: Unit Testing"进一步扩展了应用的功能，包括创建用户个人资料页面和实现单元测试，以确保代码质量。"Part VIII to Part XIII"涉及社交功能、分页、全文搜索、电子邮件支持以及日期和时间处理，这些都是构建复杂Web应用时会遇到的实际问题。 "Part XIV: I18n and L10n"讨论了国际化和本地化，使应用能够适应不同语言和文化。"Part XV: Ajax"介绍了如何使用异步JavaScript和XML技术，提升用户体验。"Part XVI: Debugging, Testing and Profiling"则关注如何有效地调试、测试和优化代码性能。最后，"Part XVII and Part XVIII"提供了在Linux（包括Raspberry Pi小型计算机）和Heroku云平台上的部署指南，帮助开发者将完成的应用发布到实际环境中。这个系列教程覆盖了从基础到高级的完整Flask开发流程，对于想要学习或提升Flask技能的开发者来说，是一份宝贵的资源。作者的经验和对Python/Flask组合的高度评价，使得这个教程不仅具有实践性，也具有权威性。

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</form>

{% endblock %}

Note that in this template we are reusing the base.html template through the extends template

inheritance statement. We will actually do this with all our templates, to ensure a consistent layout

across all pages.

There are a few interesting differences between a regular HTML form and our template. This template

expects a form object instantiated from the form class we just defined stored in a template argument

named form. We will take care of sending this template argument to the template next, when we write

the view function that renders this template.

The form.hidden_tag() template argument will get replaced with a hidden field that implements

the CSRF prevention that we enabled in the configuration. This field needs to be in all your forms if

you have CSRF enabled. The good news is that Flask-WTF handles it for us, we just need to make sure

it is included in the form.

The actual fields of our form are rendered by the field objects, we just need to refer to a

{{form.field_name}} template argument in the place where each field should be inserted. Some

fields can take arguments. In our case, we are asking the text field to generate our openid field with a

width of 80 characters.

Since we have not defined the submit button in the form class we have to define it as a regular field.

The submit field does not carry any data so it doesn't need to be defined in the form class.

Form views

The final step before we can see our form is to code a view function that renders the template.

This is actually quite simple since we just need to pass a form object to the template. Here is our new

view function (file app/views.py):

from flask import render_template, flash, redirect

from app import app

from .forms import LoginForm

# index view function suppressed for brevity

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

def login():

form = LoginForm()

return render_template('login.html',

title='Sign In',

form=form)

So basically, we have imported our LoginForm class, instantiated an object from it, and sent it down

to the template. This is all that is required to get form fields rendered.

Let's ignore for now the flash and redirect imports. We'll use them a bit later.

The only other thing that is new here is the methods argument in the route decorator. This tells Flask

that this view function accepts GET and POST requests. Without this the view will only accept GET

requests. We will want to receive the POST requests, these are the ones that will bring in the form data

entered by the user.

At this point you can try the app and see the form in your web browser. After you start the application

you will want to open http://localhost:5000/login in your web browser, as this is the route

we have associated with the login view function.

We have not coded the part that accepts data yet, so pressing the submit button will not have any effect

at this time.

Receiving form data

Another area where Flask-WTF makes our job really easy is in the handling of the submitted form data.

Here is an updated version of our login view function that validates and stores the form data (file

app/views.py):

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

def login():

form = LoginForm()

if form.validate_on_submit():

flash('Login requested for OpenID="%s", remember_me=%s' %

(form.openid.data, str(form.remember_me.data)))

return redirect('/index')

return render_template('login.html',

title='Sign In',

form=form)

The validate_on_submit method does all the form processing work. If you call it when the form

is being presented to the user (i.e. before the user got a chance to enter data on it) then it will return

False, so in that case you know that you have to render the template.

When validate_on_submit is called as part of a form submission request, it will gather all the

data, run all the validators attached to fields, and if everything is all right it will return True, indicating

that the data is valid and can be processed. This is your indication that this data is safe to incorporate

into the application.

If at least one field fails validation then the function will return False and that will cause the form to

be rendered back to the user, and this will give the user a chance to correct any mistakes. We will see

later how to show an error message when validation fails.

When validate_on_submit returns True our login view function calls two new functions,

imported from Flask. The flash function is a quick way to show a message on the next page

presented to the user. In this case we will use it for debugging, since we don't have all the infrastructure

necessary to log in users yet, we will instead just display a message that shows the submitted data. The

flash function is also extremely useful on production servers to provide feedback to the user

regarding an action.

The flashed messages will not appear automatically in our page, our templates need to display the

messages in a way that works for the site layout. We will add these messages to the base template, so

that all our templates inherit this functionality. This is the updated base template (file

app/templates/base.html):

<html>

<head>

{% if title %}

<title>{{ title }} - microblog</title>

{% else %}

<title>microblog</title>

{% endif %}

</head>

<body>

<div>Microblog: <a href="/index">Home</a></div>

<hr>

{% with messages = get_flashed_messages() %}

{% if messages %}

<ul>

{% for message in messages %}

<li>{{ message }} </li>

{% endfor %}

</ul>

{% endif %}

{% endwith %}

{% block content %}{% endblock %}

</body>

</html>

The technique to display the flashed message is hopefully self-explanatory. One interesting property of

flash messages is that once they are requested through the get_flashed_messages function they

are removed from the message list, so these messages appear in the first page requested by the user

after the flash function is called, and then they disappear.

The other new function we used in our login view is redirect. This function tells the client web

browser to navigate to a different page instead of the one requested. In our view function we use it to

redirect to the index page we developed in previous chapters. Note that flashed messages will display

even if a view function ends in a redirect.

This is a great time to start the app and test how the form works. Make sure you try submitting the form

with the openid field empty, to see how the DataRequired validator halts the submission process.

Improving field validation

With the app in its current state, forms that are submitted with invalid data will not be accepted.

Instead, the form will be presented back to the user to correct. This is exactly what we want.

What we are missing is an indication to the user of what is wrong with the form. Luckily, Flask-WTF

also makes this an easy task.

When a field fails validation Flask-WTF adds a descriptive error message to the form object. These

messages are available to the template, so we just need to add a bit of logic that renders them.

Here is our login template with field validation messages (file app/templates/login.html):

{% extends "base.html" %}

{% block content %}

<p>

Please enter your OpenID:<br>

{{ form.openid(size=80) }}<br>

{% for error in form.openid.errors %}

<span style="color: red;">[{{ error }}]</span>

{% endfor %}<br>

</p>

<p>{{ form.remember_me }} Remember Me</p>

</form>

{% endblock %}

The only change we've made is to add a for loop that renders any messages added by the validators

below the openid field. As a general rule, any fields that have validators attached will have errors

added under form.field_name.errors. In our case we use form.openid.errors. We

display these messages in a red style to call the user's attention.

Dealing with OpenIDs

In practice, we will find that a lot of people don't even know that they already have a few OpenIDs. It

isn't that well known that a number of major service providers on the Internet support OpenID

authentication for their members. For example, if you have an account with Google, you have an

OpenID with them. Likewise with Yahoo, AOL, Flickr and many other providers. (Update: Google is

shutting down their OpenID service on April 15 2015).

To make it easier for users to login to our site with one of these commonly available OpenIDs, we will

add links to a short list of them, so that the user does not have to type the OpenID by hand.

We will start by defining the list of OpenID providers that we want to present. We can do this in our

config file (file config.py):

WTF_CSRF_ENABLED = True

SECRET_KEY = 'you-will-never-guess'

OPENID_PROVIDERS = [

{'name': 'Google', 'url': 'https://www.google.com/accounts/o8/id'},

{'name': 'Yahoo', 'url': 'https://me.yahoo.com'},

{'name': 'AOL', 'url': 'http://openid.aol.com/<username>'},

{'name': 'Flickr', 'url': 'http://www.flickr.com/<username>'},

{'name': 'MyOpenID', 'url': 'https://www.myopenid.com'}]

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