Parameter type converters (int, float, path) in Flask

Introduction

Parameter type converters help Flask understand what kind of data to expect in a URL. This makes your app more reliable and clear.

When you want to get a number from the URL and use it as an integer.

When you need to accept decimal numbers from the URL.

When you want to capture a full file path or folder path from the URL.

Syntax

Flask

@app.route('/item/<int:item_id>')
def function_name(item_id):
    pass

The part inside <> is the variable name and type.

Common types are int, float, and path.

Examples

This route only accepts integer user IDs.

Flask

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

This route accepts decimal numbers like 12.99.

Flask

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

This route accepts a full path, including slashes.

Flask

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

Sample Program

This Flask app has three routes. Each uses a different type converter to get data from the URL and show it back.

Flask

from flask import Flask
app = Flask(__name__)

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

@app.route('/cost/<float:cost>')
def cost_route(cost):
    return f"Cost is: {cost}"

@app.route('/path/<path:subpath>')
def path(subpath):
    return f"Subpath is: {subpath}"

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

Important Notes

If the URL part does not match the type, Flask returns a 404 error.

The path converter allows slashes, unlike string.

Use type converters to avoid manual data checks inside your functions.

Summary

Parameter type converters tell Flask what kind of data to expect in URLs.

Common converters are int, float, and path.

They help make your routes clear and your app safer.

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