This image shows pip being upgraded in the command line.

Now that we have the latest version ofpipinstalled, let’s install Flask. Run the following command:

pip install flask

Flask will be downloaded and installed as shown in the following screenshot:

This image shows Flask being installed in the command line.

Understanding the Code

Take a look at the following calculator application code. We must first import the Flask library into our application in order to use it later on.

from flask import Flaskapp = Flask(__name__)
@app.route('/')def welcome():  return "Welcome to the Codecademy Calculator!"
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from flask import Flaskapp = Flask(__name__)
@app.route('/')def welcome():  return "Welcome to the Codecademy Calculator!"

Next, we create a Flask instance and assign it to a variable namedapp. We can create several routes using the@app.route()decorator.

These routes correspond to the URL web requests made to access different pages of the web application. The function that follows the decorator executes upon the web request. The main URL of our application is denoted by/. If we want to add additional URL routes for/divisionor/multiplication, we can define routes in our Flask application for each.

# allows user to input two numbersnum1 = int(input('Enter your first number: '))num2 = int(input('Enter your second number: '))
@app.route('/division')def division():  return "Now dividing " + str(num1) + " and " + str(num2) + "! The result is: {result}".format(result=str(num1/num2))
@app.route('/multiplication')def multiplication():  return "Now multiplying " + str(num1) + " and " + str(num2) + "! The result is: {result}".format(result=str(num1*num2))
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# allows user to input two numbersnum1 = int(input('Enter your first number: '))num2 = int(input('Enter your second number: '))
@app.route('/division')def division():  return "Now dividing " + str(num1) + " and " + str(num2) + "! The result is: {result}".format(result=str(num1/num2))
@app.route('/multiplication')def multiplication():  return "Now multiplying " + str(num1) + " and " + str(num2) + "! The result is: {result}".format(result=str(num1*num2))

Let’s review the preceding code:

• num1andnum2allow users to input two numbers to multiply or divide.
• @app.route('/division')specifies the web request with/division.
• division()is a function that executes in the/divisionroute. This function divides the valuesnum1andnum2.
• @app.route('/multiplication')specifies the web request with/multiplication.
• multiplication()is a function that executes in the/multiplicationroute. This function multiplies the valuesnum1andnum2.

Finally, we need to add the last line to our program:

app.run()
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app.run()

Callingapp.run()will start the application server, allowing the application to load in a web browser.

Running the Code

Now, it’s time to run our program and see Flask running. You can either copy and paste the example code provided earlier into a Python file ordownload the code. Name your Python fileapp.py.

Once you have the code ready on your computer, run it usingflask --app app run. You should see the following:

Enter your first number:

After you enter two numbers, you should see something like this:

* Serving Flask app 'app' (lazy loading)
* Environment: production
   WARNING: This is a development server. Do not use it in a production deployment.
   Use a production WSGI server instead.
* Debug mode: off
* Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)

Visithttp://127.0.0.1:5000/in your browser, and the following page should appear:

This is a minimal webpage with the following text: "Welcome to the Codecademy Calculator!"

To go to the/divisionor/multiplicationroute, you can adddivisionormultiplicationafter the final/. If you want to see thedivision()function executed, go tohttp://127.0.0.1:5000/division, and you should see something like the following:

This is a minimal webpage that shows the division function.

Conclusion

Deploying allows us to quickly package our application for easy configuration and installation in environments other than where we developed it. This was a brief introduction to deployment with Flask. If you want to learn more, you can dive deeper into creating web applications in ourLearn Flaskcourse.

Happy coding!

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