Flask python module introduction for creating web servers

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Skill - Flask python module introduction for creating web servers

Skills Required

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flask is a python library for creating web applications.
It is easy and quick to learn flask

Use cases

Simple web interfaces for python applications can be build very easily with flask
flask can be used to create HTTP services or applications that require python modules like pandas, numpy etc

Installing Flask

Install flask using pip with the following command

python -m pip install flask

Simple Flask server

from flask import Flask

# create a flask server
app = Flask(__name__)

# just return 'Hello World!' text when the route '/' is called
@app.route('/')
def index():
    return 'Hello, World!'

# __name__ will be __main__ only if this file is the entry point
if __name__ == '__main__':
    # run the server on this ip and port 50100
    app.run(host='0.0.0.0', port=50100, debug=True)

In above example, the index function is assigned to serve the route at ‘/’ using the @app.route('/') decorator
The content returned by the function will be returned to the client or browser
In this example, only plain text Hello, World! was returned. Instead HTML like <html><body><h2>Hello World!!!</h2></body></html> can also be returned so that the browser can interpret the HTML and display content.

Multiple routes

# route for '/'
@app.route('/')
def index():
    return 'Hello, World!'

# route for /hello
@app.route('/hello')
def hello():
    return '''<html>
                <body>
                    <h2>Hi there!!!</h2>
                    <a target="_blank" href="https://google.com">Google</a>
                </body>
            </html>'''

In this example, two routes are handled by the server at ‘/’ and ‘/hello’ by two python functions

Rendering response from a template

The template files are to be stored in the ‘templates’ folder

from flask import Flask, render_template

# create a flask server
app = Flask(__name__)

@app.route('/')
def index():
    return render_template('home.html.j2')

# __name__ will be __main__ only if this file is the entry point
if __name__ == '__main__':
    # run the server on this ip and port 50100
    app.run(host='0.0.0.0', port=50100, debug=True)

home.html.j2 can be a HTML file with jinja2 templating for server side rendering of data in the template file

<html>
    <body>
        <h2>Hi there!!!</h2>
        <a target="_blank" href="https://google.com">Go to Google</a>
    </body>
</html>

Rendering python variables in template file

Python variables can be injected into the template as named parameters in the render_template function
The data from python variables can be rendered in the template using jinja2 templating

from flask import Flask, render_template

# create a flask server
app = Flask(__name__)

@app.route('/')
def index():
    return render_template('home.html.j2', user={'name':'Abcd', 'city':'XYZ'}, numViews=1234)

# __name__ will be __main__ only if this file is the entry point
if __name__ == '__main__':
    # run the server on this ip and port 50100
    app.run(host='0.0.0.0', port=50100, debug=True)

The home.html.j2 file can be as shown below

<html>
    <head><title>Jinja2 Template Example</title></head>
    <body>
        <h2>Hi {{ user.name }}!!!</h2>
        <h3>Your city is {{ user["city"] }}<h3>
        <span>number of views = {{ numViews }}</span>        
    </body>
</html>

Video

The video for this post can be seen here

References

Flask quickstart - https://flask.palletsprojects.com/en/2.1.x/quickstart/

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