Flask使用

前言

所有Web框架的使用都有自己的套路，作为一个框架本质就是一个工具。

安装

pip install Flask

自动安装的依赖

Werkzeug 应用及服务间的标准接口
Jinja 模板
MarkupSafe Jinja附带，防止注入攻击
ItsDangerous 数据完整性，保护 session cookie 安全
Click 命令行应用框架

可选安装的依赖

Blinker provides support for Signals
SimpleJSON fast JSON
python-dotenv
Watchdog

Virtual environments

创建
$ mkdir myproject
$ cd myproject
$ python3 -m venv venv
激活
$ . venv/bin/activate

简单示例

from flask import Flask
app = Flask(__name__)

@app.route('/')
def hello_world():
return 'Hello, World!'

运行，设置环境变量 FLASK_APP
```
$ FLASK_APP=hello.py flask run
```
监听公共 IP ，确保 debugger 模式已关
```
flask run --host=0.0.0.0
```

Debug Mode

export FLASK_ENV=development
or FLASK_APP=hello.py FLASK_ENV=development flask run

Variable Rules

@app.route('/user/<username>')
def show_user_profile(username):
    # show the user profile for that user
    return 'User %s' % username

@app.route('/post/<int:post_id>')
def show_post(post_id):
    # show the post with the given id, the id is an integer
    return 'Post %d' % post_id

@app.route('/path/<path:subpath>')
def show_subpath(subpath):
    # show the subpath after /path/
    return 'Subpath %s' % subpath

URL 结尾斜线

不带斜线访问，会 redirects 到带斜线的 URL

@app.route('/projects/')
def projects():
    return 'The project page'

带斜线访问，会返回404，避免搜索引擎索引同一个页面两次
```
@app.route('/about')
def about():
    return 'The about page'
```

HTTP Methods

from flask import request

@app.route('/login', methods=['GET', 'POST'])
def login():
if request.method == 'POST':
    return do_the_login()
else:
    return show_the_login_form()

URL 构建

from flask import Flask, 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(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

Static Files

在项目下创建 static 目录，存放 static files ,对于 url_for ，static 是保留字，用于生成指向 static 目录下的 URL

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

Rendering Templates

Flask 自动配置Jinja2 template engine. Flask 自动去 templates 目录查找 template

from flask import render_template

@app.route('/hello/')
@app.route('/hello/<name>')
def hello(name=None):
    return render_template('hello.html', name=name)

The Request Object

from flask import request

@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)

获取 form data, 使用 form 属性
获取 parameters submitted in the URL (?key=value), request.args.get(‘key’, ‘’)

File Uploads

enctype=”multipart/form-data”

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')
...

使用客户端的文件名存储在服务端，可以使用 secure_filename() 获取文件名

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))

Cookies

读

from flask import request

@app.route('/')
def index():
    username = request.cookies.get('username')
    # use cookies.get(key) instead of cookies[key] to not get a
    # KeyError if the cookie is missing.

写

from flask import make_response

@app.route(‘/‘)
def index():

resp = make_response(render_template(...))
resp.set_cookie('username', 'the username')
return resp

通常 Flask 自动转换返回值为 response objects ，如果自己想明确的指定 response 需要使用 make_response()

redirect() abort()

from flask import abort, redirect, url_for

@app.route('/')
def index():
    return redirect(url_for('login'))

@app.route('/login')
def login():
    abort(401)
    this_is_never_executed()

使用 errorhandler() decorator 自定义 error page

from flask import render_template

@app.errorhandler(404)
def page_not_found(error):
    return render_template('page_not_found.html'), 404

Sessions

set a secret key

from flask import Flask, session, redirect, url_for, escape, request

app = Flask(__name__)

# Set the secret key to some random bytes. Keep this really secret!
app.secret_key = b'\_5#y2L"F4Q8z\n\xec]/'

@app.route('/')
def index():
    if 'username' in session:
        return 'Logged in as %s' % escape(session['username'])
    return 'You are not logged in'

@app.route('/login', methods=['GET', 'POST'])
def login():
    if request.method == 'POST':
        session['username'] = request.form['username']
        return redirect(url_for('index'))
    return '''
        <form method="post">
            <p><input type=text name=username>
            <p><input type=submit value=Login>
        </form>
    '''

@app.route('/logout')
def logout():
    # remove the username from the session if it's there
    session.pop('username', None)
    return redirect(url_for('index'))

escape(), 如果不使用模板引擎，escape 可转换 html 特殊字符
generate good secret keys, os.urandom(16)

Logging

app.logger.debug('A value for debugging')
app.logger.warning('A warning occurred (%d apples)', 42)
app.logger.error('An error occurred')

部署

Flask 自带 wsgi 扩展性能不好，仅适用于开发环境
WSGI HTTP Server 可选的有 Gunicorn 和 uWSGI
Nginx 作为静态文件访问容器和反向代理、分布式环境下作负载均衡

Gunicorn

安装
```
$ pip install gunicorn
```

Async Workers
Gunicorn 默认同步 Workers ，通常我们还需要安装 Eventlet 或则 Gevent ，实现异步 Workers 。greenlet 是默认安装的。eventlet 和 gevent 性能差不多，可以选一个安装。

$ pip install greenlet            # Required for both
$ pip install eventlet            # For eventlet workers
$ pip install gunicorn[eventlet]  # Or, using extra
$ pip install gevent              # For gevent workers
$ pip install gunicorn[gevent]    # Or, using extra

运行

gunicorn --workers=2 --worker-class=eventlet hello:app

其它参数

-c CONFIG, --config=CONFIG  Specify a config file in the form $(PATH), file:$(PATH), or python:$(MODULE_NAME).
-b BIND, --bind=BIND - Specify a server socket to bind. Server sockets can be any of $(HOST), $(HOST):$(PORT), or unix:$(PATH). An IP is a valid $(HOST).
-n APP_NAME, --name=APP_NAME - If setproctitle is installed you can adjust the name of Gunicorn process as they appear in the process system table (which affects tools like ps and top).

前言

安装