Parameter type converters (int, float, path) in Flask - Mini Project: Build & Apply

Using Parameter Type Converters in Flask Routes

📖 Scenario: You are building a simple Flask web app that shows different pages based on URL parameters. You want to make sure the parameters are the right type so your app works smoothly.

🎯 Goal: Create Flask routes that use parameter type converters int, float, and path to handle URL parameters correctly.

📋 What You'll Learn

Create a Flask app instance named app

Add a route /item/<int:item_id> that accepts only integer item_id

Add a route /price/<float:price> that accepts only float price

Add a route /file/<path:filepath> that accepts a path string filepath

💡 Why This Matters

🌍 Real World

Web developers often need to get data from URLs and ensure it is the right type before using it. Flask's parameter type converters make this easy and reliable.

💼 Career

Understanding how to use Flask routes with typed parameters is essential for backend web development jobs that use Python and Flask frameworks.

1

Set up the Flask app and integer route

Import Flask from flask and create a Flask app instance called app. Then add a route /item/<int:item_id> that accepts an integer parameter called item_id. Inside the route function show_item, return a string showing the item ID using an f-string.

Flask

from flask import Flask

app = Flask(__name__)

# Create route /item/ below
# Your code here

Hint

Use @app.route('/item/<int:item_id>') to specify the integer parameter in the URL.

2

Add a route with a float parameter

Add a new route /price/<float:price> that accepts a float parameter called price. Define a function show_price that returns a string showing the price with two decimal places using an f-string.

Flask

from flask import Flask

app = Flask(__name__)

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

# Add route /price/ below
# Your code here

Hint

Use @app.route('/price/<float:price>') and format the float with {price:.2f} in the f-string.

3

Add a route with a path parameter

Add a route /file/<path:filepath> that accepts a path parameter called filepath. Define a function show_file that returns a string showing the file path.

Flask

from flask import Flask

app = Flask(__name__)

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

@app.route('/price/')
def show_price(price):
    return f"Price is ${price:.2f}"

# Add route /file/ below
# Your code here

Hint

Use @app.route('/file/<path:filepath>') to accept a path parameter.

4

Add the app run command

Add the code to run the Flask app only if the script is run directly. Use app.run(debug=True) inside the if __name__ == '__main__': block.

Flask

from flask import Flask

app = Flask(__name__)

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

@app.route('/price/')
def show_price(price):
    return f"Price is ${price:.2f}"

@app.route('/file/')
def show_file(filepath):
    return f"File path is {filepath}"

# Add the app run command below
# Your code here

Hint

Use the standard Python check if __name__ == '__main__': to run the app.

Practice

1. What does the int converter do in a Flask route parameter like <int:id>?

easy

A. It matches any text including slashes.

B. It ensures the parameter is an integer and converts it automatically.

C. It converts the parameter to a string.

D. It converts the parameter to a floating-point number.

Solution

1. Step 1: Understand the role of int converter

   The int converter tells Flask to accept only integer values for that URL part.

2. Step 2: Check what happens with the parameter

   Flask automatically converts the matched URL part to an integer type before passing it to the view function.

3. Final Answer:

   It ensures the parameter is an integer and converts it automatically. -> Option B

4. Quick Check:

   int converter = integer parameter [OK]

Hint: int converter means only whole numbers allowed [OK]

Common Mistakes:

• Thinking int converts to string
• Confusing int with float converter
• Assuming int matches any text

2. Which of the following is the correct syntax to define a Flask route that accepts a floating-point number parameter named price?

easy

A. @app.route('/item/<path:price>')

B. @app.route('/item/<int:price>')

C. @app.route('/item/<float:price>')

D. @app.route('/item/<str:price>')

Solution

1. Step 1: Identify the correct converter for floating-point numbers

   The float converter is used to accept decimal numbers in the URL.

2. Step 2: Match the syntax for route parameter

   The correct syntax is <float:price> inside the route decorator.

3. Final Answer:

   @app.route('/item/<float:price>') -> Option C

4. Quick Check:

   float converter = decimal number parameter [OK]

Hint: Use for decimal numbers in route parameters [OK]

Common Mistakes:

• Using int instead of float for decimals
• Using str which is not a Flask converter
• Using path which allows slashes, not numbers

3. Given this Flask route:

@app.route('/files/<path:filename>')
def show_file(filename):
    return f"File requested: {filename}"

What will be the output if the URL is /files/docs/manual.pdf?

medium

A. File requested: docs/manual.pdf

B. File requested: manual.pdf

C. 404 Not Found error

D. File requested: docs

Solution

1. Step 1: Understand the path converter behavior

   The path converter matches the entire remaining path including slashes.

2. Step 2: Analyze the URL and parameter value

   The URL /files/docs/manual.pdf matches filename as docs/manual.pdf.

3. Final Answer:

   File requested: docs/manual.pdf -> Option A

4. Quick Check:

   path converter = matches full path with slashes [OK]

Hint: path converter captures slashes in URL parts [OK]

Common Mistakes:

• Assuming path stops at first slash
• Expecting 404 error for slashes in parameter
• Confusing path with string converter

4. What is wrong with this Flask route definition?

@app.route('/product/<int:product_id>/<float:price>')
def show_product(product_id, price):
    return f"Product {product_id} costs {price}"

When accessing /product/abc/19.99, what error occurs?

medium

A. ValueError because 'abc' is not an integer

B. TypeError because price is not a float

C. No error, it works fine

D. 404 Not Found error due to URL mismatch

Solution

1. Step 1: Check the int converter requirement

   The first parameter expects an integer, but 'abc' is not an integer.

2. Step 2: Understand Flask routing behavior on type mismatch

   Flask does not raise ValueError but returns 404 Not Found if the URL does not match the route pattern.

3. Final Answer:

   404 Not Found error due to URL mismatch -> Option D

4. Quick Check:

   Type mismatch in route = 404 error [OK]

Hint: Wrong type in URL parameter causes 404, not exception [OK]

Common Mistakes:

• Expecting Python exceptions instead of 404
• Confusing ValueError with Flask routing errors
• Assuming route matches any string

5. You want to create a Flask route that accepts a file path parameter which may include slashes, and an integer version number after it, like /download/docs/manual/2. Which route definition correctly uses converters to achieve this?

hard

A. @app.route('/download/<path:filepath>/<int:version>')

B. @app.route('/download/<int:version>/<path:filepath>')

C. @app.route('/download/<path:filepath><int:version>')

D. @app.route('/download/<path:filepath>/<float:version>')

Solution

1. Step 1: Identify the order of parameters in the URL

   The URL pattern is /download/ followed by a path (which may include slashes), then a slash, then an integer version.

2. Step 2: Use path for filepath and int for version in correct order

   The route must be /download/<path:filepath>/<int:version> to match the URL structure.

3. Final Answer:

   @app.route('/download/<path:filepath>/<int:version>') -> Option A

4. Quick Check:

   path then int in route matches URL structure [OK]

Hint: Use path converter before int for multi-part paths [OK]

Common Mistakes:

• Swapping parameter order
• Missing slash between parameters
• Using float instead of int for version