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.

What is Flask? #

Flask is a web application framework written in Python. It is developed by Armin Ronacher, who leads an international group of Python enthusiasts named Pocco. Flask is based on the Werkzeug WSGI toolkit and Jinja2 template engine. Both are Pocco projects.

WSGI #

Web Server Gateway Interface (WSGI) has been adopted as a standard for Python web application development. WSGI is a specification for a universal interface between the web server and the web applications.

Werkzeug #

It is a WSGI toolkit, which implements requests, response objects, and other utility functions. This enables building a web framework on top of it. The Flask framework uses Werkzeug as one of its bases.

Jinja2 #

Jinja2 is a popular templating engine for Python. A web templating system combines a template with a certain data source to render dynamic web pages.

Flask is often referred to as a micro framework. It aims to keep the core of an application simple yet extensible. Flask does not have built-in abstraction layer for database handling, nor does it have form a validation support. Instead, Flask supports the extensions to add such functionality to the application. Some of the popular Flask extensions are discussed later in the tutorial.

Install virtualenv for development environment #

The following command installs virtualenv

pip install virtualenv

On Ubuntu virtualenv may be installed using its package manager.

Sudo apt-get install virtualenv

Once installed, new virtual environment is created in a folder.

mkdir newproj
cd newproj
virtualenv venv

To activate corresponding environment, on Linux/OS X, use the following −

venv/bin/activate

On Windows, following can be used

venv\scripts\activate

We are now ready to install Flask in this environment.

pip install Flask

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

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’

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

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)

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 −

@app.route(‘/hello’)
def hello_world():
   return ‘hello world’

Here, URL ‘/hello’ rule is bound to the hello_world() function. As a result, if a user visits http://localhost:5000/hello URL, the output of the hello_world() function will be rendered in the browser.

The add_url_rule() function of an application object is also available to bind a URL with a function as in the above example, route() is used.

A decorator’s purpose is also served by the following representation −

def hello_world():
   return ‘hello world’
app.add_url_rule(‘/’, ‘hello’, hello_world)

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 . It is passed as a keyword argument to the function with which the rule is associated.

In the following example, the rule parameter of route() decorator contains variable part attached to URL ‘/hello’. Hence, if the http://localhost:5000/hello/TutorialsPoint is entered as a URL in the browser, ‘TutorialPoint’ will be supplied to hello() function as argument.

from flask import Flask
app = Flask(__name__)

@app.route('/hello/<name>')
def hello_name(name):
   return 'Hello %s!' % name

if __name__ == '__main__':
   app.run(debug = True)

Save the above script as hello.py and run it from Python shell. Next, open the browser and enter URL http://localhost:5000/hello/Armoghan.

The following output will be displayed in the browser.

Hello Armoghan!

In addition to the default string variable part, rules can be constructed using the following converters −

In the following code, all these constructors are used.

from flask import Flask
app = Flask(__name__)

@app.route('/blog/<int:postID>')
def show_blog(postID):
   return 'Blog Number %d' % postID

@app.route('/rev/<float:revNo>')
def revision(revNo):
   return 'Revision Number %f' % revNo

if __name__ == '__main__':
   app.run()

Run the above code from Python Shell. Visit the URL http://localhost:5000/blog/11 in the browser.

The given number is used as argument to the show_blog() function. The browser displays the following output −

Blog Number 11

Enter this URL in the browser − http://localhost:5000/rev/1.1

The revision() function takes up the floating point number as argument. The following result appears in the browser window −

Revision Number 1.100000

The URL rules of Flask are based on Werkzeug’s routing module. This ensures that the URLs formed are unique and based on precedents laid down by Apache.

Consider the rules defined in the following script −

from flask import Flask
app = Flask(__name__)

@app.route('/flask')
def hello_flask():
   return 'Hello Flask'

@app.route('/python/')
def hello_python():
   return 'Hello Python'

if __name__ == '__main__':
   app.run()

Both the rules appear similar but in the second rule, trailing slash (/) is used. As a result, it becomes a canonical URL. Hence, using /python or /python/ returns the same output. However, in case of the first rule, /flask/ URL results in 404 Not Found page.

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.

The following script demonstrates use of url_for() function.

from flask import Flask, redirect, url_for
app = Flask(__name__)

@app.route('/admin')
def hello_admin():
   return 'Hello Admin'

@app.route('/guest/<guest>')
def hello_guest(guest):
   return 'Hello %s as Guest' % guest

@app.route('/user/<name>')
def hello_user(name):
   if name =='admin':
      return redirect(url_for('hello_admin'))
   else:
      return redirect(url_for('hello_guest',guest = name))

if __name__ == '__main__':
   app.run(debug = True)

The above script has a function user(name) which accepts a value to its argument from the URL.

The User() function checks if an argument received matches ‘admin’ or not. If it matches, the application is redirected to the hello_admin() function using url_for(), otherwise to the hello_guest() function passing the received argument as guest parameter to it.

Save the above code and run from Python shell.

Open the browser and enter URL as − http://localhost:5000/user/admin

The application response in browser is −

Hello Admin

Enter the following URL in the browser − http://localhost:5000/user/mvl

The application response now changes to −

Hello mvl as Guest

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 −

By default, the Flask route responds to the GET requests. However, this preference can be altered by providing methods argument to route() decorator.

In order to demonstrate the use of POST method in URL routing, first let us create an HTML form and use the POST method to send form data to a URL.

Save the following script as login.html

<html>
   <body>
      <form action = "http://localhost:5000/login" method = "post">
         <p>Enter Name:</p>
         <p><input type = "text" name = "nm" /></p>
         <p><input type = "submit" value = "submit" /></p>
      </form>
   </body>
</html>

Now enter the following script in Python shell.

from flask import Flask, redirect, url_for, request
app = Flask(__name__)

@app.route('/success/<name>')
def success(name):
   return 'welcome %s' % name

@app.route('/login',methods = ['POST', 'GET'])
def login():
   if request.method == 'POST':
      user = request.form['nm']
      return redirect(url_for('success',name = user))
   else:
      user = request.args.get('nm')
      return redirect(url_for('success',name = user))

if __name__ == '__main__':
   app.run(debug = True)

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__)

@app.route('/')
def index():
   return '<html><body><h1>Hello World</h1></body></html>'

if __name__ == '__main__':
   app.run(debug = True)

However, generating HTML content from Python code is cumbersome, especially when variable data and Python language elements like conditionals or loops need to be put. This would require frequent escaping from HTML.

This is where one can take advantage of Jinja2 template engine, on which Flask is based. Instead of returning hardcode HTML from the function, a HTML file can be rendered by the render_template() function.

from flask import Flask
app = Flask(__name__)

@app.route('/')
def index():
   return render_template(‘hello.html’)

if __name__ == '__main__':
   app.run(debug = True)

Flask will try to find the HTML file in the templates folder, in the same folder in which this script is present.

* Application folder
* Hello.py
* templates
    * hello.html

The term ‘web templating system’ refers to designing an HTML script in which the variable data can be inserted dynamically. A web template system comprises of a template engine, some kind of data source and a template processor.

Flask uses jinja2 template engine. A web template contains HTML syntax interspersed placeholders for variables and expressions (in these case Python expressions) which are replaced values when the template is rendered.

The following code is saved as hello.html in the templates folder.

<!doctype html>
<html>
   <body>
   
      <h1>Hello {{ name }}!</h1>
      
   </body>
</html>

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.

A special endpoint ‘static’ is used to generate URL for static files.

In the following example, a javascript function defined in hello.js is called on OnClick event of HTML button in index.html, which is rendered on ‘/’ URL of the Flask application.

from flask import Flask, render_template
app = Flask(__name__)

@app.route("/")
def index():
   return render_template("index.html")

if __name__ == '__main__':
   app.run(debug = True)

The HTML script of index.html is given below.

<html>
   <head>
      <script type = "text/javascript" 
         src = "{{ url_for('static', filename = 'hello.js') }}" ></script>
   </head>
   
   <body>
      <input type = "button" onclick = "sayHello()" value = "Say Hello" />
   </body>
</html>

hello.js contains sayHello() function.

function sayHello() {
   alert("Hello World")
}

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 −

• Form − It is a dictionary object containing key and value pairs of form parameters and their values.

• args − parsed contents of query string which is part of URL after question mark (?).

• Cookies − dictionary object holding Cookie names and values.

• files − data pertaining to uploaded file.

• method − current request method.