Route with dynamic parameters in Flask

Introduction

Dynamic parameters let your web app handle different inputs in the URL. This helps show different pages or data without making many routes.

Showing user profiles by user ID in the URL

Displaying blog posts based on post titles or IDs

Filtering products by category or product ID in an online store

Accessing details of an event by event ID

Creating URLs that change based on user input or selections

Syntax

Flask

@app.route('/path/<parameter>')
def function_name(parameter):
    # use parameter inside function
    return f"Value is {parameter}"

Use in the route to capture part of the URL as a variable.

The function receives this variable as an argument to use inside.

Examples

This captures the username from the URL and shows it.

Flask

@app.route('/user/<username>')
def show_user(username):
    return f"User: {username}"

This captures a number as post_id and uses it inside the function.

Flask

@app.route('/post/<int:post_id>')
def show_post(post_id):
    return f"Post ID: {post_id}"

This captures a decimal number from the URL.

Flask

@app.route('/item/<float:price>')
def show_price(price):
    return f"Price: {price}"

Sample Program

This Flask app greets anyone by name from the URL. For example, visiting /hello/Alice shows "Hello, Alice!".

Flask

from flask import Flask

app = Flask(__name__)

@app.route('/hello/<name>')
def hello(name):
    return f"Hello, {name}!"

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

Important Notes

You can specify types like int, float, or path for parameters to control what is accepted.

If you don't specify a type, the parameter is treated as a string.

Dynamic routes help keep your app clean and flexible by reusing one route for many inputs.

Summary

Dynamic parameters let routes accept variable parts in the URL.

Use <parameter> in the route and receive it as a function argument.

Specify types like int or float to control input format.

Practice

1. What does the dynamic parameter in a Flask route do?
@app.route('/user/<username>')

easy

A. It captures the part of the URL as a variable passed to the function.

B. It creates a fixed URL that never changes.

C. It makes the route only accept numbers.

D. It disables the route from working.

Solution

1. Step 1: Understand dynamic parameters in Flask routes

   Dynamic parameters are parts of the URL wrapped in <> that Flask captures as variables.

2. Step 2: How Flask uses the captured variable

   The captured part is passed as an argument to the view function to use inside the code.

3. Final Answer:

   It captures the part of the URL as a variable passed to the function. -> Option A

4. Quick Check:

   Dynamic parameter = variable in function [OK]

Hint: Dynamic parts in <> become function arguments [OK]

Common Mistakes:

• Thinking dynamic means fixed URL
• Assuming only numbers can be captured
• Believing dynamic disables the route

2. Which of these is the correct syntax to define a route with an integer parameter named post_id?

easy

A. @app.route('/post/')

B. @app.route('/post/<post_id>')

C. @app.route('/post/')

D. @app.route('/post/')

Solution

1. Step 1: Identify the type syntax for dynamic parameters

   Flask uses <type:name> to specify the parameter type, like int for integers.

2. Step 2: Match the correct syntax for integer parameter

   The correct syntax is <int:post_id> to ensure only integers are accepted.

3. Final Answer:

   @app.route('/post/<int:post_id>') -> Option A

4. Quick Check:

   Integer type uses <int:name> [OK]

Hint: Use <int:name> for integer parameters in routes [OK]

Common Mistakes:

• Omitting the type for integer parameters
• Using str instead of int for numbers
• Using float when integer is needed

3. What will be the output if you visit /product/42 given this Flask route?

@app.route('/product/')
def product(id):
    return f"Product ID: {id}"

medium

A. 404 Not Found error

B. Product ID:

C. Product ID: id

D. Product ID: 42

Solution

1. Step 1: Understand the route and parameter type

   The route expects an integer parameter named id, which matches 42 in the URL.

2. Step 2: Check the function return value

   The function returns a string with the id value inserted, so it returns "Product ID: 42".

3. Final Answer:

   Product ID: 42 -> Option D

4. Quick Check:

   Dynamic int parameter outputs value [OK]

Hint: Dynamic int parameter passes number as function argument [OK]

Common Mistakes:

• Expecting the literal string instead of value
• Confusing parameter name with string
• Assuming route returns error for valid int

4. Identify the error in this Flask route definition:

@app.route('/item/')
def item(item):
    return f"Item {item_id}"

medium

A. Route parameter type int is invalid.

B. Function parameter name does not match route parameter name.

C. Missing return statement in function.

D. Route URL should not have dynamic parameters.

Solution

1. Step 1: Compare route parameter and function argument names

   The route defines <int:item_id> but the function uses parameter named item, which is inconsistent.

2. Step 2: Understand how Flask matches parameters

   Flask passes the URL part as argument named item_id, so function must accept item_id to receive it correctly.

3. Final Answer:

   Function parameter name does not match route parameter name. -> Option B

4. Quick Check:

   Parameter names must match [OK]

Hint: Function argument must match route parameter name exactly [OK]

Common Mistakes:

• Using different names for route and function parameters
• Thinking type int is invalid syntax
• Forgetting to return a value

5. You want a Flask route that accepts a username (string) and an optional integer page number with default 1. Which route and function definition is correct?

hard

A. @app.route('/profile/<username>/')
def profile(username, page=1):
return f"User: {username}, Page: {page}"

B. @app.route('/profile/<username>/<page>')
def profile(username, page=1):
return f"User: {username}, Page: {page}"

C. @app.route('/profile/<username>')
def profile(username, page=1):
return f"User: {username}, Page: {page}"

D. @app.route('/profile/<username>/')
def profile(username, page):
return f"User: {username}, Page: {page}"

Solution

1. Step 1: Understand optional parameters in Flask routes

   Flask routes do not support optional URL parts directly; optional parameters must be handled with defaults in function and route without that part.

2. Step 2: Check which route allows optional page parameter

   Route with only <username> works, and function uses default page=1 if not provided.

3. Final Answer:

   @app.route('/profile/<username>')\ndef profile(username, page=1):\n return f"User: {username}, Page: {page}" -> Option C

4. Quick Check:

   Optional params handled by function defaults, not route [OK]

Hint: Optional URL parts need separate routes or defaults in function [OK]

Common Mistakes:

• Assuming Flask supports optional URL parts in one route
• Using default values with required route parameters
• Mismatching parameter types in route and function