The Flask Mega-Tutorial 之 Chapter 15： A Better Application Structure (Blueprint)

Current Limitations

1、当前 application 含有多个 subsystems，但是相关 code 交错分布，无明确界限，难以复用。

• User Authentication：

  • app/routes.py ，部分 view funcs，
  • app/forms.py ，部分 forms，
  • app/templates ，部分 templates，
  • app/email.py ， email 支持。

• Error Handling：

  • app/errors.py ， error handlers
  • app/templates ， 404.html & 500.html

• Core Functionality：

  • posts display & write
  • user profiles
  • follow & unfollow
  • live translations of posts

organization logic 是，不同 module 对应实现不同的 functionality，如 module for view func, module for web_forms, module for errors, module for emails, dir for HTML templates, etc..
当 project 规模增大时，这些 modules 的数量会越来越多，某些 module 会越来越大，难以维护。（譬如，复用 authentication 组块时，必须到不同的 module 中复制/粘贴，很不变；最理想的状况时，能够模块分离。）

解决方案： Blueprint （from Flask）

2、app/__init__.py 中的 application instance 是 global variable，不利于 testing

• 无法实例化出 两个 使用不同配置的 application instance，用于测试

  Imagine you want to test this application under different configurations. Because the application is defined as a global variable, there is really no way to instantiate two applications that use different configuration variables.

• 由于所有 tests 共用一个 application，前面的 test 有可能更改了 application，干扰后续的 test

  Another situation that is not ideal is that all the tests use the same application, so a test could be making changes to the application that affect another test that runs later. Ideally you want all tests to run on a pristine application instance.

解决方案： 创建 application factory function，接收 configuration 为 参数，返回不同配置的 application instance

---

Blueprints

• Flask blueprint，是一种逻辑结构（logical structure），代表 application 的一个子单元（subset）。
• Blueprint 可以包含多种元素，如 routes、view funcs、forms、templates 和 static files。

• 如果将 blueprint 写入 单独的 python package，则此包封装 application 某个功能特性相关的所有元素。

  In Flask, a blueprint is a logical structure that represents a subset of the application. A blueprint can include elements such as routes, view functions, forms, templates and static files. If you write your blueprint in a separate Python package, then you have a component that encapsulates the elements related to specific feature of the application.

• Blueprint 内含的元素内容最初处于休眠状态，为关联它们，必须将 blueprint 注册到 application。（注册过程中，blueprint 中已添加的内容将全部传到 application）。

  So you can think of a blueprint as a temporary storage for application functionality that helps in organizing your code.

1、Error Handling Blueprint

app/
    errors/                             <-- blueprint package
        __init__.py                     <-- blueprint creation
        handlers.py                     <-- error handlers
    templates/
        errors/                         <-- error templates
            404.html
            500.html
    __init__.py                         <-- blueprint registration

• 创建 app/errors/ ，移进原有的 app/errors.py ，并改名。
• 创建 templates/errors/，移进 app/templates error handling 相关的 templates。
• 创建生成蓝图的 app/errors/__init__.py

from flask import Blueprint

bp = Blueprint('errors', __name__)

from app.errors import handlers

注：若加参数 template_folder='templates'，则 app/errors/templates/ 亦可使用。
生成 bp 后，引入 handlers 模块，将模块注册到 bp 中。

so that the error handlers in it are registered with the blueprint. This import is at the bottom to avoid circular dependencies.

• handlers.py 中引入 bp，将 @app.errorhandler 替换为 @bp.app_errorhandler，并更改 template 路径。

from app.errors import bp

@bp.app_errorhandler(404)
def not_found_error(error):
    return render_template('errors/404.html'), 404

@bp 更加加 independent of application，。

• 注册 bp 到 app/__init__.py 中

app = Flask(__name__)

# ...

from app.errors import bp as errors_bp
app.register_blueprint(errors_bp)

# ...

from app import routes, models  # <-- remove errors from this import!

---

Authentication Blueprint

app/
    auth/                               <-- blueprint package
        __init__.py                     <-- blueprint creation
        email.py                        <-- authentication emails
        forms.py                        <-- authentication forms
        routes.py                       <-- authentication routes
    templates/
        auth/                           <-- blueprint templates
            login.html
            register.html
            reset_password_request.html
            reset_password.html
    __init__.py                         <-- blueprint registration

1、 每个 module，只保留 auth 相关部分

• email.py 中发密码重置邮件的函数

#..
from app.email import send_email

def send_password_reset_email(user):
    #...

• forms.py 中：

LoginForm()
RegistrationForm()
ResetPasswordRequestForm()
ResetPasswordForm()

• routes.py 中：

login()
logout()
register()
reset_password_request()
reset_password(token)

(1) @app.route() 更新为 bp.route()
(2) render_template() 中的 path 更新
(3) url_for() 添加相应 bp 前缀 ，如 url_for(‘login’) → url_for(‘auth.login’)
注： url_for() 的更改，包括 templates 中设置重定向的 *.html

2、注册 authentication blueprint 到 app/__init__.py

# ...
from app.auth import bp as auth_bp
app.register_blueprint(auth_bp, url_prefix='/auth')
# ...

Flask gives you the option to attach a blueprint under a URL prefix, so any routes defined in the blueprint get this prefix in their URLs.
In many cases this is useful as a sort of “namespacing” that keeps all the routes in the blueprint separated from other routes in the application or other blueprints.

---

Main Application Blueprint

app/
    main/                               <-- blueprint package
        __init__.py                     <-- blueprint creation
        forms.py                        <-- main forms
        routes.py                       <-- main routes

    templates/
                                        <-- blueprint templates (位置不变）
            base.html
            index.html
            user.html
            _post.html
            edit_profile.html

    __init__.py                         <-- blueprint registration

• Blueprint 名字为bp，则所有调用 main 中 view func 的 url_for() 都须加 main. 前缀。

• main 相关的 templates 位置不变。

  Given that this is the core functionality of the application, we leave the templates in the same locations.
  This is not a problem because templates from the other two blueprints have been moved into sub-directories.

  -

---

Application Factory Function

• 引入 Blueprints 前，因为所有的 view funcs 和 error handlers 需要的 @app 装饰器都来自于 application，所以 application 必须是 global variable。
• 引入 Blueprints 后，所有的 view funcs 和 error handlers 已移至 blueprints 中，所以 application 无需为 global variable。

# ...
db = SQLAlchemy()
migrate = Migrate()
login = LoginManager()
login.login_view = 'auth.login'
login.login_message = _l('Please log in to access this page.')
mail = Mail()
bootstrap = Bootstrap()
moment = Moment()
babel = Babel()

def create_app(config_class=Config):
    app = Flask(__name__)
    app.config.from_object(config_class)

    db.init_app(app)
    migrate.init_app(app, db)
    login.init_app(app)
    mail.init_app(app)
    bootstrap.init_app(app)
    moment.init_app(app)
    babel.init_app(app)

    # ... no changes to blueprint registration

    if not app.debug and not app.testing:
        # ... no changes to logging setup

    return app

(1) 先创建各种 extension 实例（global）。
(2) 定义 create_app()，先创建 Flask app，后依次 extension.init_app(app)，将 extension 绑定到 application 上。
(3) not app.testing：如果 config 中 TESTING 为 True，则略过后续的 email & logging。

那么， app/__init__.py 中的 create_app() 的调用者是 ？

Microblog / microblog.py
Microblog / tests.py

只有这两处内创建的 application，才存在于 global scope。

---

current_app

1、替换 code 中 的 app 为 current_app

app/models.py：      token相关 app.config['SECRET_KEY']
app/translate.py：   MS Translator API相关 app.config['MS_TRANSLATOR_KEY']
app/main/routes.py： pagination相关 app.config['POSTS_PER_PAGE'] 
app/__init__.py：    get_locale 相关 app.config['LANGUAGES']

全部替换为 current_app （from Flask）

2、app/email.py 中涉及的 pass application instance to another thread.

from flask import current_app

def send_async_email(app, msg):
    with app.app_context():
        mail.send(msg)

def send_email(subject, sender, recipients, text_body, html_body):
    msg = Message(subject, sender=sender, recipients=recipients)
    msg.body = text_body
    msg.html = html_body
    Thread(target=send_async_email,
           args=(current_app._get_current_object(), msg)).start()

• 首先，由于 current_app 是 context-aware 的变量（tied to the thread that is handling the client request），所以将其传到 another thread无效。
• 其次，current_app 实质是个 proxy object，动态地映射到 application instance 上。真正需要的是 application instance。
• 用 current_app._get_current_object() 语句，提取 proxy object 内部真实的 application instance，

3、app / cli.py 中注册的 custom commands

• 首先，创建 register() 函数，然后将 custom commands 移入，将 app instance 作为参数传入。
• 然后，app / microblog.py 中，在创建 app 后，调用 register(app)。

为何不延续 current_app 的思路呢？

答： current_app 变量只局限于处理 request 时发挥作用，但 custom commands 是在 start up 时注册，而不是请求中。

import os
import click

def register(app):
    @app.cli.group()
    def translate():
        """Translation and localization commands."""
        pass

    @translate.command()
    @click.argument('lang')
    def init(lang):
        """Initialize a new language."""
        # ...

    @translate.command()
    def update():
        """Update all languages."""
        # ...

    @translate.command()
    def compile():
        """Compile all languages."""
        # ...

4、重构 Microblog / microblog.py

from app import create_app, db, cli
from app.models import User, Post

app = create_app()
cli.register(app)

@app.shell_context_processor
def make_shell_context():
    return {'db': db, 'User': User, 'Post' :Post}

---

Unit Testing Improvements

1、定制 TestConfig （Microblog/tests.py）

from config import Config

class TestConfig(Config):
    TESTING = True
    SQLALCHEMY_DATABASE_URI = 'sqlite://'

2、为 each test 创建各自的 app instance （Microblog/tests.py）

from app import create_app, db
#...

class UserModelCase(unittest.TestCase):
    def setUp(self):
        self.app = create_app(TestConfig)
        self.app_context = self.app.app_context()
        self.app_context.push()
        db.create_all()

    def tearDown(self):
        db.session.remove()
        db.drop_all()
        self.app_context.pop()

在调用 db.create_all() 之前，db 必须先从 app.config 获得 db URI，但 app factory func 有可能已生成多个 app。

那如何使 db 使用 self.app=create_app() 刚刚生成的 app 呢？
答：利用 application context

• current_app 变量，作为 app proxy，会在 current thread 中寻找 活跃的 application context，一旦找到，就可从中获取到 app；

• 若 current_app 未找到 application context，则无法知晓活跃的 app，最终抛出异常。

  Remember the current_app variable, which somehow acts as a proxy for the application when there is no global application to import? This variable looks for an active application context in the current thread, and if it finds one, it gets the application from it. If there is no context, then there is no way to know what application is active, so current_app raises an exception.

• 拿 python 测试（勿用 flaskshell，因为会自动激活 application context）
  
  可以发现，直接运行 current_app 时，抛出 RuntimeError: Working outside of application context.

3、app_context().push() 和 app_context().pop()

• Flask 在调用 view func 处理一个 request 之前，会先 push 一个application context ，从而激活 current_app 和 g 变量
• 当 request 处理结束，context 会被 pop 移掉 （连同 context 激活的种种变量）。
• -

Before invoking your view functions, Flask pushes an application context, which brings current_app and g to life.
When the request is complete, the context is removed, along with these variables.

For the db.create_all() call to work in the unit testing setUp() method, we push an application context for the application instance just created, and in that way, db.create_all() can use current_app.config to know where is the database.
Then in the tearDown() method I pop the context to reset everything to a clean state.

联想到 app/emai.py 中，send_email() 开启一个新 thread，然后将 app instance 作为参数传入。
传入的 app instance，在 send_async_email() 中首先激活 app_context()，然后在此内执行任务。

def send_async_email(app, msg):
    with app.app_context():
        mail.send(msg)

def send_email(subject, sender, recipients, text_body, html_body):
    msg = Message(subject, sender=sender, recipients=recipients)
    msg.body = text_body
    msg.html = html_body
    Thread(target=send_async_email,
           args=(current_app._get_current_object(), msg)).start()

4、Flask 有两类 context，

• 
  
• application context： current_app 、 g
  
  
• request context：request 、session、current_user(flask-login’s)
  

request context is more specific and applies to a request.
When a request context is activated right before a request is handled, Flask’s request and session variables become available, as well as Flask-Login’s current_user.

5、tests.py 测试结果

---

Environment Variables (使用 .env)

• 每次打开（重开）一个 terminal session时，都须将 microblog / config.py 中的环境变量设置一番…..

  • 优化方案：在 root application dir中，创建 。env 文件，写入环境变量。

  
  

• 支持.env 的第三方库：python-dotenv

  (venv) $ pip install python-dotenv
  
  

• microblog/config.py 调用 .env

import os
from dotenv import load_dotenv

basedir = os.path.abspath(os.path.dirname(__file__))
load_dotenv(os.path.join(basedir, '.env'))

class Config(object):
    # ...

• 在 root application dir 下创建 .env

  注：勿加入 source control

注意，.env 无法用于环境变量 FLASK_APP 及 FLASK_DEBUG ，因为这两者在 application 引导过程时即需要使用，当时 app instance 和 config obj 尚不存在。

The .env file can be used for all the configuration-time variables, but it cannot be used for Flask’s FLASK_APP and FLASK_DEBUG environment variables, because these are needed very early in the application bootstrap process, before the application instance and its configuration object exist.

-

---

Requirements File

• 生成

(venv) $ pip freeze > requirements.txt

requirments.txt format

alembic==0.9.9
async-timeout==3.0.0
attrs==18.1.0
Babel==2.6.0

• 导入安装
  To create a same virtual environment on another machine, instead of installing packages one by one, just:

(venv) $ pip install -r requirements.txt

若使用 pipenv，

• 在pipenv install 时，可利用已有的 requirments.txt 。参看 Importing from requirements.txt

pipenv install -r path/to/requirements.tx

If your requirements file has version numbers pinned, you’ll likely want to edit the new Pipfile to remove those, and let pipenv keep track of pinning.
If you want to keep the pinned versions in your Pipfile.lock for now, run pipenv lock --keep-outdated. Make sure to upgrade soon!

• pipenv lock -r to generate a requirements.txt
  Generating a requirements.txt