Flask – Overview

What is Web Framework?

Web Application Framework or simply Web Framework represents a collection of libraries and modules that enables a web application developer to write applications without having to bother about low-level details such as protocols, thread management etc.

Flask – Environment

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.

Flask – Application

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’

Flask – Routing

Modern web frameworks use the routing technique to help a user remember application URLs. It is useful to access the desired page directly without having to navigate from the home page.
The route() decorator in Flask is used to bind URL to a function. For example −

Flask – Variable Rules

It is possible to build a URL dynamically, by adding variable parts to the rule parameter. This variable part is marked as <variable-name>. It is passed as a keyword argument to the function with which the rule is associated.

Flask – URL Building

The url_for() function is very useful for dynamically building a URL for a specific function. The function accepts the name of a function as first argument, and one or more keyword arguments, each corresponding to the variable part of URL.

Flask – HTTP methods

Http protocol is the foundation of data communication in world wide web. Different methods of data retrieval from specified URL are defined in this protocol.
The following table summarizes different http methods −

Flask – Templates

It is possible to return the output of a function bound to a certain URL in the form of HTML. For instance, in the following script, hello() function will render ‘Hello World’ with <h1> tag attached to it.

from flask import Flask
app = Flask(__name__)

Flask – Static Files

A web application often requires a static file such as a javascript file or a CSS file supporting the display of a web page. Usually, the web server is configured to serve them for you, but during the development, these files are served from static folder in your package or next to your module and it will be available at /static on the application.

Flask – Request Object

The data from a client’s web page is sent to the server as a global request object. In order to process the request data, it should be imported from the Flask module.
Important attributes of request object are listed below −

Flask – Sending Form Data to Template

We have already seen that the http method can be specified in URL rule. The Form data received by the triggered function can collect it in the form of a dictionary object and forward it to a template to render it on a corresponding web page.

Flask – Cookies

A cookie is stored on a client’s computer in the form of a text file. Its purpose is to remember and track data pertaining to a client’s usage for better visitor experience and site statistics.

Flask – Sessions

Unlike a Cookie, Session data is stored on server. Session is the time interval when a client logs into a server and logs out of it. The data, which is needed to be held across this session, is stored in a temporary directory on the server.

Flask – Redirect & Errors

Flask class has a redirect() function. When called, it returns a response object and redirects the user to another target location with specified status code.
Prototype of redirect() function is as below −

Flask.redirect(location, statuscode, response)

In the above function −

Flask – Message Flashing

A good GUI based application provides feedback to a user about the interaction. For example, the desktop applications use dialog or message box and JavaScript uses alerts for similar purpose.
Generating such informative messages is easy in Flask web application. Flashing system of Flask framework makes it possible to create a message in one view and render it in a view function called next.

Flask – File Uploading

Handling file upload in Flask is very easy. It needs an HTML form with its enctype attribute set to ‘multipart/form-data’, posting the file to a URL. The URL handler fetches file from request.files[] object and saves it to the desired location.

Flask – Extensions

Flask is often referred to as a micro framework, because a core functionality includes WSGI and routing based on Werkzeug and template engine based on Jinja2. In addition, Flask framework has support for cookie and sessions as well as web helpers like JSON, static files etc.

Flask – Mail

A web based application is often required to have a feature of sending mail to the users/clients. Flask-Mail extension makes it very easy to set up a simple interface with any email server.
At first, Flask-Mail extension should be installed with the help of pip utility.

Flask – WTF

One of the essential aspects of a web application is to present a user interface for the user. HTML provides a <form> tag, which is used to design an interface. A Form’s elements such as text input, radio, select etc. can be used appropriately.

Flask – SQLite

Python has an in-built support for SQlite. SQlite3 module is shipped with Python distribution. For a detailed tutorial on using SQLite database in Python, please refer to this link. In this section we shall see how a Flask application interacts with SQLite.