Flask — Web Framework

By: G M Nithin Sai

This is not the Flask we are going to know about now…..

Not a general Flask…..This is the flask we are going to know about

Flask is a micro Web Frame work written in Python. It is classified as a micro frame work because it does not require particular tools or libraries. It has no database abstraction layer, form validation, or any other components where pre-existing third-party libraries provide common functions. However, Flask supports extensions that can add application features as if they were implemented in Flask itself. Extensions exist for object-relational mappers, form validation, upload handling, various open authentication technologies and several common framework related tools.

Applications that use the Flask framework include Pinterest and Linkedin.

Prerequisite

Python 2.6 or higher is usually required for installation of Flask. Although Flask and its dependencies work well with Python 3 (Python 3.3 onwards), many Flask extensions do not support it properly. Hence, it is recommended that Flask should be installed on Python 2.7.

How to Install Flask?

pip install Flask

The above command can be run directly, without virtual environment for system-wide installation.

Let’s create a simple application using Flask,

In order to test Flask installation, type the following code in the editor as Hello.py

from flask import Flask
app = Flask(__name__)
@app.route('/')
def hello_world():
   return 'Hello World’
if __name__ == '__main__':
   app.run()

Importing flask module in the project is mandatory. An object of Flask class is our WSGI application.

Flask constructor takes the name of current module (__name__) as argument.

The route() function of the Flask class is a decorator, which tells the application which URL should call the associated function.

app.route(rule, options)

• The rule parameter represents URL binding with the function.
• The options is a list of parameters to be forwarded to the underlying Rule object.

In the above example, ‘/’ URL is bound with hello_world() function. Hence, when the home page of web server is opened in browser, the output of this function will be rendered.

Finally the run() method of Flask class runs the application on the local development server.

app.run(host, port, debug, options)

All parameters are optional in the above code, However I would like to present you a brief idea about this

host : Hostname to listen on. Defaults to 127.0.0.1 (localhost). Set to ‘0.0.0.0’ to have server available externally

port: Defaults to 5000

debug: Defaults to false. If set to true, provides a debug information

options: To be forwarded to underlying Werkzeug server.

The above given Python script is executed from Python shell.

Python Hello.py

A message in Python shell informs you that

* Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)

Open the above URL (localhost:5000) in the browser. ‘Hello World’ message will be displayed on it.

Debug mode

A Flask application is started by calling the run() method. However, while the application is under development, it should be restarted manually for each change in the code. To avoid this inconvenience, enable debug support. The server will then reload itself if the code changes. It will also provide a useful debugger to track the errors if any, in the application.

The Debug mode is enabled by setting the debug property of the application object to True before running or passing the debug parameter to the run() method.

app.debug = True
app.run()
app.run(debug = True)

References

Flask - Application