Introduction to Web development using Flask (original) (raw)

Flask is a lightweight and flexible web framework for Python. It’s designed to make getting started with web development quick and easy, while still being powerful enough to build complex web applications. It is an API of Python that allows us to build web applications.

It was developed by Armin Ronacher. Flask’s framework is more explicit than Django’s framework and is also easier to learn because it has less base code to implement a simple web application. Flask Python is based on the WSGI(Web Server Gateway Interface) toolkit and Jinja2 template engine.

Advantages of Flask

Getting Started With Flask

Python3 is required for the installation of the Python Web Framework Flask. You can start by importing Flask from the Flask Python package on any Python IDE. For installation on any environment, you can execute the command “**pip install flask” on your terminal. Let’s look at an example of a basic flask app.

Python `

from flask import Flask
app = Flask(name) # Flask constructor

A decorator used to tell the application

which URL is associated function

@app.route('/')
def hello(): return 'HELLO'

if name=='main': app.run(debug=True)

`

**Explanation:

Build Flask Routes in Python

Web frameworks provide routing technique so that user can remember the URLs. It is useful to access the web page directly without navigating from the Home page. It is done through the following route() decorator, to bind the URL to a function.

Python `

decorator to route URL

@app.route(‘/hello’)

binding to the function of route

def hello_world():
return ‘hello world’

`

**Explanation: If a user visits **http://localhost:5000/hello URL, the output of the **hello_world() function will be rendered in the browser.

One alternate way of doing this is by using “**add_url_rule()"** function of an application object, it can also be used to bind URL with the function similar to the above example.

Python `

def hello_world(): return ‘hello world’

app.add_url_rule(‘/’, ‘hello’, hello_world)

`

Variables in Flask

Variables in flask are used to build a URL dynamically by adding the variable parts to the rule parameter. It is passed as keyword argument. Here’s an example.

Python `

from flask import Flask app = Flask(name)

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

if name == 'main': app.run(debug = True)

`

**Output:

variable

Snapshot of a variable URL

Explanation: parameter of route() decorator contains the variable part attached to the URL ‘/hello‘ as an argument. Hence, if URL like “****http://localhost:5000/hello/GeeksforGeeks” is entered then “**GeeksforGeeks” will be passed to the **hello() function as an argument.

Besides the default string type, Flask also supports int, float, and path (which allows slashes for directories). Flask’s URL rules use **Werkzeug’s routing module, ensuring unique URLs following Apache’s conventions. Here’s an example.

Python `

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(debug=True)

`

Run the application and type the following URLs in a browser-

**http://127.0.0.1:5000/blog/555

blog

Snapshot of /blog URL

**http://127.0.0.1:5000/rev/1.1

rev

Snapshot of /rev URL

**Explanation:

Build a URL in Flask

Dynamic Building of the URL for a specific function is done using **url_for()**function. The function accepts the name of the function as first argument, and one or more keyword arguments. See this example

Python `

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

@app.route('/admin') # decorator for route(argument) function def hello_admin(): # binding to hello_admin call return 'Hello Admin'

@app.route('/guest/') def hello_guest(guest): # binding to hello_guest call return 'Hello %s as Guest' % guest

@app.route('/user/') def hello_user(name): if name == 'admin': # dynamic binding of URL to function return redirect(url_for('hello_admin')) else: return redirect(url_for('hello_guest', guest=name))

if name == 'main': app.run(debug=True)

`

To test this, save the above code and run through python shell and then open browser and enter the following URLs-

**http://localhost:5000/user/admin

admin

Admin

**http://localhost:5000/user/ABC

ABC

guest/ABC

**Explanation:

The above code has a function named user(name), accepts the value through input URL. It checks that the received argument matches the ‘admin’ argument or not. If it matches, then the function hello_admin() is called else the hello_guest() is called.

HTTP method are provided by Flask

Python Web Framework Flask support various HTTP protocols for data retrieval from the specified URL, these can be defined as:-

**Method **Description
**GET This is used to send the data in an without encryption of the form to the server.
**HEAD provides response body to the form
**POST Sends the form data to server. Data received by POST method is not cached by server.
**PUT Replaces current representation of target resource with URL.
**DELETE Deletes the target resource of a given URL

Serve HTML and Static Files in Flask

A web application often requires a static file such as javascript or a CSS file to render the display of the web page in browser. Usually, the web server is configured to set them, but during development, these files are served as static folder in your package or next to the module. See the example in JavaScript given below:

Python `

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)

`

HTML File (index.html)

Create a **Template folder in your project directory. This Template folder will contain all the **HTML files of the project and they will be rendered through our flask app using render_template method (we will learn about it in detail later).

html `

`

JavaScript file (hello.js)

Create a **static folder that will contain all the javascript and CSS files. It will be sibling of the templates folder.

javascript `

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

`

To serve this javascript code in a flask app, link it with a HTML file (index.html in this case) in the template folder using this line of code in the head section –

Run the application by executing “**python app.py” in the terminal and visit the development URL- **http://127.0.0.1:5000

**Output:

app

Home Route

flash

Rendering flask message using the js file

Cookies in Flask

A Cookie is a form of text file which is stored on a client’s computer, whose purpose is to remember and track data pertaining to client’s usage in order to improve the website according to the user’s experience and statistic of webpage.

The Request object contains cookie’s attribute. It is the dictionary object of all the cookie variables and their corresponding values. It also contains expiry time of itself. In Flask, cookie are set on response object. See the example given below:

**app.py code

Python `

from flask import Flask, request, render_template, make_response

app = Flask(name) @app.route('/')

def index(): return render_template('index.html')

@app.route('/setcookie', methods = ['POST', 'GET']) def setcookie(): if request.method == 'POST': user = request.form['nm'] resp = make_response(render_template('cookie.html')) resp.set_cookie('userID', user) return resp

@app.route('/getcookie') def getcookie(): name = request.cookies.get('userID') return '

welcome '+name+'

' if name == "main": app.run()

`

**HTML code ( index.html )

html `

<form action = "/setcookie" method = "POST"> 
    <p><h3>Enter userID</h3></p> 
    <p><input type = 'text' name = 'nm'/></p> 
    <p><input type = 'submit' value = 'Login'/></p> 
</form>

`

**HTML code ( cookie.html )

html `

Click me to get Cookie

`

Run the application and visit http://127.0.0.1:5000 in your browser. The form submits to ‘/setcookie’, where the set function stores a cookie named userID. The cookie.html page includes a link to the getcookie() function, which retrieves and displays the cookie value in the browser. Output snippets are below:

Enter user ID into the form and click “Login”.

Enter-user-id

Home Route

Click on “Click me to get Cookie”

setcookie

cookie.html gets rendered

“/getcookie” route takes the User ID from cookies and renders welcome along with it.

getcookie

/getcookie route

Sessions in Flask

A session is the time between a user **logging in and **logging out of a server. Session data is stored on the server in a temporary folder, and each user is assigned a unique session ID.

**Session ID

The Session object is a dictionary that contains the **key-value pair of the variables associated with the session. A **SECRET_KEY is used to store the encrypted data on the cookies.

**Example:

Session[key] = value // stores the session value
Session.pop(key, None) // releases a session variable

Other Important Flask Functions

redirect

It is used to return the response of an object and redirects the user to another target location with specified status code.

**Syntax: Flask.redirect(location, statuscode, response)

**Parameters:

abort

It is used to handle the error in the code.

**Syntax: Flask.abort(code)

The code parameter can take the following values to handle the error accordingly:

File-Uploading in Flask

File Uploading in Flask needs an HTML form with enctype attribute and URL handler, that fetches file and saves the object to the desired location. Files are temporary stored on server and then on the desired location. The HTML Syntax that handle the uploading URL is :

form action=”http://localhost:5000/uploader” method=”POST” enctype=”multipart/form-data”

**Example:

Python `

from flask import Flask, render_template, request from werkzeug.utils import secure_filename

app = Flask(name)

@app.route('/upload') def upload_file(): return render_template('upload.html')

@app.route('/uploader', methods=['GET', 'POST']) def upload_file(): if request.method == 'POST': f = request.files['file'] f.save(secure_filename(f.filename)) return 'file uploaded successfully'

if name == 'main': app.run(debug = True)

`

Sending Form Data to the HTML File of Server

A Form in HTML is used to collect the information of required entries which are then forwarded and stored on the server. These can be requested to read or modify the form.

Python `

from flask import Flask, render_template, request

app = Flask(name)

@app.route('/') def student(): return render_template('student.html')

@app.route('/result', methods=['POST', 'GET']) def result(): if request.method == 'POST': result = request.form return render_template("result.html", result=result)

if name == 'main': app.run(debug=True)

`

**Explanation:

**HTML Code ( result.html )

HTML `

<table border = 1> 
    {% for key, value in result.items() %} 
    
        <tr> 
        <th> {{ key }} </th> 
        <td> {{ value }} </td> 
        </tr> 
        
    {% endfor %} 
</table>

`

**HTML Code ( student.html )

html `

<form action = "http://localhost:5000/result" method = "POST"> 
    <p>Name <input type = "text" name = "Name" /></p> 
    <p>Physics <input type = "text" name = "Physics" /></p> 
    <p>Chemistry <input type = "text" name = "chemistry" /></p> 
    <p>Maths <input type ="text" name = "Maths" /></p> 
    <p><input type = "submit" value = "submit" /></p> 
</form>

`

input_values

Input values to the HTML form

Template_output

Template as the output

Alert messages in Flask

Alert can be defined as a **pop-up or a dialog box that appears on the web-page or like alert in JavaScript, which are used to inform the user. This in flask can be done by using the method flash() in Flask. It passes the message to the next template.

**Syntax: flash(message, category)

**Prameters:

**Example

Python `

from flask import Flask app = Flask(name)

/login display login form

@app.route('/login', methods=['GET', 'POST'])

login function verify username and password

def login(): error = None

if request.method == 'POST':
    if request.form['username'] != 'admin' or \
            request.form['password'] != 'admin':
        error = 'Invalid username or password. Please try again !'
    else:

        # flashes on successful login
        flash('You were successfully logged in')
        return redirect(url_for('index'))
return render_template('login.html', error=error)

`