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Flaskframework~5 mins

Variable substitution syntax in Flask

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Introduction

Variable substitution lets you show dynamic data inside your web pages easily. It replaces placeholders with real values when the page loads.

Showing a user's name on a welcome page.
Displaying a list of items from a database.
Updating page content based on user input.
Showing the current date or time dynamically.
Syntax
Flask
{{ variable_name }}

Use double curly braces {{ }} to insert variables.

Variables come from the Flask code you send to the template.

Examples
Shows the value of the variable username in the page.
Flask
{{ username }}
Shows the age property of the user object.
Flask
{{ user.age }}
Shows the result of a simple expression, here it will show 8.
Flask
{{ 5 + 3 }}
Concatenates strings and variables to show a greeting.
Flask
{{ 'Hello ' + username }}
Sample Program

This Flask app shows how to use variable substitution to greet a user by name. The variable name is replaced with the value of user_name when rendering the page.

Flask
from flask import Flask, render_template_string

app = Flask(__name__)

@app.route('/')
def home():
    user_name = 'Alice'
    html = '''
    <html lang="en">
    <head><title>Welcome</title></head>
    <body>
        <h1>Welcome, {{ name }}!</h1>
        <p>We are glad to see you.</p>
    </body>
    </html>
    '''
    return render_template_string(html, name=user_name)

if __name__ == '__main__':
    app.run(debug=False, port=5000)
OutputSuccess
Important Notes

Variable substitution is safe by default: Flask escapes HTML to prevent security issues.

You can use filters like {{ variable|upper }} to change output style.

Common mistake: forgetting to pass the variable from Flask to the template causes empty output.

Summary

Use {{ variable }} to insert dynamic data in Flask templates.

Variables come from your Flask code when rendering the template.

This makes your web pages interactive and personalized.