Flask - Basic Getting Started

Flask is called a micro-framework, which only provides a robust core, and all other functions are implemented by extending the library; that is to say, it can be tailored according to the needs of the project. He is suitable for introductory learning as well as master research.
Composition: WSGI, template engine (Jinja2)

B/S and C/S Architecture
How B/S Architecture Works
- Client (browser) <=> WEB server (nginx) <=> WSGI (uWSGI) <=> Python (flask) <=> database (MySQL)
- Note: The flask framework comes with a test server and does not consider performance deployment.

Install virtualenv:pip3 install virtualenv
- Create a virtual environment:virtualenv venv
- Start the virtual environment:source venv/bin/activate
- Exit the virtual environment:deactivate
Install virtualenvwrapper:
- Install pip:sudo apt-get install python-pip
- Install virtualenvwrapper:pip install virtualenvwrapper
- Configuration:
  - Common directories dedicated to storing virtual environments:~/.virtualenvs
  - Add the following at the end of the file :~/.bashrc
```
export WORKON_HOME=$HOME/.virtualenvs
source $HOME/.local/bin/virtualenvwrapper.sh
alias mkvirtualenv='mkvirtualenv -p /usr/bin/python'
alias mkvirtualenv3='mkvirtualenv -p /usr/bin/python3'
```
  Reload: source ~/.bashrc
View virtual environment
- lsvirtualenv or workon
Create a virtual environment
- python2：mkvirtualenv 虚拟环境名
- python3：mkvirtualenv3 虚拟环境名
Use a virtual environment:
- workon 虚拟环境名
Exit the virtual environment
- deactivate
delete virtual environment
- rmvirtualenv 虚拟环境名
install flask
- Create a virtual environment:mkvirtualenv3 flask
- Install flask:pip install flask

from flask import Flask

app = Flask(__name__)

@app.route('/')
def index():return '<h1>Hello world!</h1>'if __name__ == '__main__':app.run()

startup parameters

parameter	illustrate
debug	Whether to enable the debug mode, the default is False, the code modification will be automatically loaded after it is enabled
threaded	Whether to enable multithreading
port	designated port
	Specify the host, set to '0.0.0.0' to access by IP

app.run(debug=True, threaded=True, port=5050, host='0.0.0.0')

variable or object

name	context	illustrate
current_app	program context	The currently running application instance
g	program context	The global object, which handles the request's temporary data
request	request context	The request object, which contains all the HTTP request information of the client
session	request context	User session, for saving information that needs to be 'remembered'

request hook function

function	illustrate
before_first_request	before the first request
before_request	before each request
after_request	After each request, provided there is no exception
teardown_request	After each request, with or without exception

The so-called hook function is the backdoor reserved for the user by the framework

No parameter routing


@app.route('/test/')
def test():return 'for test yyy'

route with parameters

see code
illustrate
- After the '/' at the end of the route is added, it can be added or not during the test (in the browser)
- If parameters are required, the parameters need to be written in <>, and the parameters of the corresponding view function should be consistent with the routing parameters
- If you need to specify the parameter type, such as: int/float/path, etc., it should be written before the parameter name and separated from the parameter name by ':'
- If no parameter class is specified, the default is string, and path is actually a string, except that '/' is no longer a delimiter

Just return a string directly, the default status code is 200, indicating success
Returns a tuple, that is, specifying the status code directly after the string
You can also create a response object first, and then return the response object

Note: When the URL of the website is changed, it is necessary to support the old address
redirect: redirect function
url_for: Construct the routing address according to the view function name, the parameter is the view function name

abort: Throws an exception to the system, the system will catch and handle it uniformly
@app.errorhandler: custom error display

MVC: for decoupling
- M: model, model, that is, the data model
- V: view, view, responsible for display logic
- C: controller, controller, responsible for business logic processing
MTV: The function is the same as MVC
- M: model, model, that is, the data model
- T: template: template, responsible for display logic
- V: view function, view function, responsible for processing business logic

Getting Started with the Flask Framework