Pass JavaScript Variables to Python in Flask

In this tutorial, we’ll look at using the Flask framework to leverage JavaScript variables in Python. In this section, we’ll talk about the many approaches and strategies used to combine the two languages, which is an essential step for many online applications. Everything from the fundamentals to more complex subjects, such as how to handle enormous data sets, will be covered. We’ll also examine the best ways to organize Python code for optimum effectiveness. You’ll have a thorough understanding of how to combine Python and JavaScript in Flask by the end of this lesson. You will be able to solve complicated problems with ease and speed once you have this information. then let’s get going!

Two of the most used programming languages worldwide are Python and JavaScript. They frequently work together in web development projects since they each have strengths in particular areas. Developers can produce sophisticated and potent web applications by integrating the two.

A Python library for creating web applications is the Flask framework. It is a simple framework that makes it easy for programmers to make dynamic web apps. It helps programmers to create applications rapidly and with little coding.

A crucial stage for many web applications is the integration of Python and JavaScript in Flask. The appropriate strategies and approaches for carrying out the integration must be carefully planned. Developers that are familiar with the Flask framework and how to use JavaScript variables in Python can build robust apps that are simple to maintain and expand.

You will learn the fundamentals of using JavaScript variables in Python with the Flask framework in this lesson. We’ll look at advanced subjects, handling massive data sets, and structuring Python programs for optimal efficiency. You’ll have a thorough understanding of how to combine Python and JavaScript in Flask by the end of this lesson. You will be able to solve complicated problems with ease and speed once you have this information.

Concepts:

Before we start, let’s briefly go over some of the concepts involved:

1. Flask – A Python web framework that allows you to build web applications.
2. JavaScript – A programming language that runs in the browser and can manipulate web pages.
3. AJAX – A technique for creating asynchronous web applications that can communicate with servers without refreshing the page.
4. JSON – A data format that is commonly used to exchange data between web applications.

Setting Up the Environment:

You must have Python and Flask installed on your computer in order to utilize Flask and provide JavaScript variables to Python. The steps to prepare your environment are as follows:

1. Install Flask: After installing Python, you can install Flask by typing the following command at a command line or terminal:

pip install flask

This command installs Flask and all of its dependencies.

        2. Install any additional libraries that are required: Depending on your use case, you might need to do this in order to process data, handle requests, or operate with particular file formats. For instance, you may use the following command to install the pandas library if you’re working with CSV files:

pip install pandas

       3. Organize your project: For your project, make a new directory and add a new Python file to it. Here is an illustration of how the structure of your project may appear:

myproject/
├── app.py
├── static/
│   ├── script.js
└── templates/
   └── index.html

        4. Launching the Flask development server Run the following command in your terminal or command prompt while in the root directory of your project:

Javascript

export FLASK_APP=app.py
export FLASK_ENV=development
flask run

This command starts the Flask development server, which you can access by visiting http://localhost:5000 in your web browser. If everything is set up correctly, you should see a “Hello, World!” message.

Steps for passing JavaScript variables to Python in Flask:

1. Create a Flask route that will receive the JavaScript variable.
2. Create a JavaScript function that will send the variable to the Flask route using an AJAX request.
3. In the Flask route, retrieve the variable using the request object.
4. Process the variable in Python code as needed.
5. Return the result to the JavaScript function as a response to the AJAX request.

Here is an example implementation:

Python

from flask import Flask,render_template, request

app = Flask(__name__,template_folder="templates")

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

@app.route('/process', methods=['POST'])
def process():
    data = request.get_json() # retrieve the data sent from JavaScript
    # process the data using Python code
    result = data['value'] * 2
    return jsonify(result=result) # return the result to JavaScript

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

HTML

<!DOCTYPE html>
<html>
<head>
    <title>JavaScript to Python Example</title>
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.5.1/jquery.min.js"></script>
</head>
<body>
    <input type="text" id="input">
    <button onclick="sendData()">Send Data</button>
    <div id="output"></div>
    <script>
        function sendData() {
            var value = document.getElementById('input').value;
            $.ajax({
                url: '/process',
                type: 'POST',
                contentType: 'application/json',
                data: JSON.stringify({ 'value': value }),
                success: function(response) {
                    document.getElementById('output').innerHTML = response.result;
                },
                error: function(error) {
                    console.log(error);
                }
            });
        }
    </script>
</body>
</html>

Here, we have a straightforward HTML form with a button and an input field. The JavaScript function `sendData()` is invoked when the button is pressed, and it retrieves the value from the input field and transmits it via an AJAX request to the Flask route `/process` in Flask.

`request.get_json()` is used to retrieve the data in the Flask route, and Python code is then used to process it. Just multiplying the value by 2 in this instance. A JSON object representing the outcome is then returned to the JavaScript function and shown in the output div.

Output:

Double String

Examples:

Here are a few more examples of passing JavaScript variables to Python in Flask:

Example 1:

Passing a String

Python

from flask import Flask,render_template, request

app = Flask(__name__,template_folder="templates")

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

@app.route('/process', methods=['POST'])
def process():
    data = request.form.get('data')
    # process the data using Python code
    result = data.upper()
    return result

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

HTML

<!DOCTYPE html>
<html>
<head>
    <title>JavaScript to Python Example</title>
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.5.1/jquery.min.js"></script>
</head>
<body>
    <input type="text" id="input">
    <button onclick="sendData()">Send Data</button>
    <div id="output"></div>
    <script>
        function sendData() {
            var value = document.getElementById('input').value;
            $.ajax({
                url: '/process',
                type: 'POST',
                data: { 'data': value },
                success: function(response) {
                    document.getElementById('output').innerHTML = response;
                },
                error: function(error) {
                    console.log(error);
                }
            });
        }
    </script>
</body>
</html>

In this example, we’re passing a string from the JavaScript function to the Flask route. The string is converted to uppercase in Python code and returned to the JavaScript function for display.

Output:

Web-app Output (Passing a String)

Example 2: 

Passing a Number

Python

from flask import Flask,render_template, request

app = Flask(__name__,template_folder="templates")

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

@app.route('/process', methods=['POST'])
def process():
    data = request.form.get('data')
    # process the data using Python code
    result = int(data) * 2
    return str(result)

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

HTML

<!DOCTYPE html>
<html>
<head>
    <title>JavaScript to Python Example</title>
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.5.1/jquery.min.js"></script>
</head>
<body>
    <input type="number" id="input">
    <button onclick="sendData()">Send Data</button>
    <div id="output"></div>
    <script>
        function sendData() {
            var value = document.getElementById('input').value;
            $.ajax({
                url: '/process',
                type: 'POST',
                data: { 'data': value },
                success: function(response) {
                    document.getElementById('output').innerHTML = response;
                },
                error: function(error) {
                    console.log(error);
                }
            });
        }
    </script>
</body>
</html>

In this example, we’re passing a number from the JavaScript function to the Flask route. The number is multiplied by 2 in Python code and returned to the JavaScript function for display.

Output:

Web-app Output (Passing a Number)

Example 3: 

Passing an Array

Javascript

const data = [1, 2, 3, 4, 5];

fetch('/process-data', {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json'
  },
  body: JSON.stringify({data: data})
})
.then(response => response.text())
.then(result => {
  console.log(result);
})
.catch(error => {
  console.error('Error:', error);
});

Python

from flask import Flask,render_template, request, jsonify

app = Flask(__name__,template_folder="templates")

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

@app.route('/process-data', methods=['POST'])
def process_data():
    data = request.json['data']
    result = sum(data)
    return jsonify({'result': result})

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

In this illustration, we used JavaScript to construct an array of integers using the Fetch API to transfer it to Python. We accessed the array in the Python code by using `request.json[‘data’]` and then computed the sum of the numbers. `jsonify()` was used to return the result as a JSON object.

Please take note that the Fetch API call used the `application/json` content type, and the Python access to the data used the `request.json` content type. This is because JSON is the format in which we send and receive data.

This function makes it simple to transfer arrays of data between Python and JavaScript in Flask.

Output:

Passing data of array