Accessing query parameters in Flask - Mini Project: Build & Apply

Accessing query parameters in Flask

📖 Scenario: You are building a simple web app using Flask. You want to read values that users send in the URL after the question mark, called query parameters. For example, in /search?term=python, the query parameter is term with value python.

🎯 Goal: Create a Flask app that reads a query parameter called name from the URL and shows a greeting message using that name.

📋 What You'll Learn

Create a Flask app with one route /greet

Read the query parameter name from the URL

If name is missing, use Guest as default

Return a greeting message like Hello, <name>!

💡 Why This Matters

🌍 Real World

Web apps often need to read information from the URL to customize what they show. Query parameters are a common way to send small pieces of data from the browser to the server.

💼 Career

Knowing how to access query parameters is essential for backend web developers working with Flask or similar frameworks to build interactive web applications.

1

Set up the Flask app and route

Write code to import Flask and request from flask. Create a Flask app called app. Define a route for /greet using @app.route('/greet'). Inside the route function called greet, return the string 'Hello!'.

Flask

# Your code here to import Flask and request, create app, and define /greet route

Hint

Remember to import both Flask and request from the flask package. Use @app.route('/greet') to create the route.

2

Get the query parameter name

Inside the greet function, create a variable called name. Use request.args.get('name') to get the name query parameter from the URL.

Flask

from flask import Flask, request

app = Flask(__name__)

@app.route('/greet')
def greet():
    # Get the 'name' query parameter from the URL
    # Your code here
    return 'Hello!'

Hint

Use request.args.get('name') to read the query parameter named name.

3

Set a default value for name

Change the code to set name to request.args.get('name', 'Guest') so that if the name parameter is missing, it uses 'Guest' as the default value.

Flask

from flask import Flask, request

app = Flask(__name__)

@app.route('/greet')
def greet():
    # Get 'name' query parameter or use 'Guest' if missing
    # Your code here
    return 'Hello!'

Hint

Use the second argument of get to provide a default value.

4

Return a greeting message using the name

Change the return statement to return a string that says Hello, <name>! using an f-string with the variable name.

Flask

from flask import Flask, request

app = Flask(__name__)

@app.route('/greet')
def greet():
    name = request.args.get('name', 'Guest')
    # Return greeting message using name
    # Your code here

Hint

Use an f-string like f'Hello, {name}!' to include the variable in the string.

Practice

1. In Flask, which method is used to safely access a query parameter named user from the URL?

easy

A. request.form['user']

B. request.args.get('user')

C. request.query['user']

D. request.params.get('user')

Solution

1. Step 1: Understand Flask request object

   Flask's request object holds incoming request data, including query parameters in args.

2. Step 2: Use the correct method to access query parameters

   request.args.get('user') safely retrieves the value of the user parameter or returns None if missing.

3. Final Answer:

   request.args.get('user') -> Option B

4. Quick Check:

   Use args.get() for query parameters [OK]

Hint: Use request.args.get('param') to read query parameters [OK]

Common Mistakes:

• Using request.form for query parameters
• Trying to access request.query which doesn't exist
• Using request.params instead of request.args

2. Which of the following is the correct syntax to get a query parameter page with a default value of 1 in Flask?

easy

A. request.args.get('page', default=1)

B. request.args.get('page', 1)

C. request.args['page'] or 1

D. request.args.get('page', fallback=1)

Solution

1. Step 1: Check the get() method parameters

   The get() method accepts a default keyword argument to provide a fallback value if the parameter is missing.

2. Step 2: Identify correct syntax for default

   Using request.args.get('page', default=1) correctly sets 1 as default. Passing 1 as second positional argument is invalid.

3. Final Answer:

   request.args.get('page', default=1) -> Option A

4. Quick Check:

   Use default= for fallback in get() [OK]

Hint: Use default= in get() to set fallback value [OK]

Common Mistakes:

• Passing default value as second positional argument
• Using fallback= which is not valid in Flask
• Accessing with [] which raises error if missing

3. What will be the output of this Flask route when accessed with URL /search?term=flask?

from flask import Flask, request
app = Flask(__name__)

@app.route('/search')
def search():
    term = request.args.get('term', 'none')
    return f"Search term: {term}"

medium

A. Error: term parameter missing

B. Search term: none

C. Search term:

D. Search term: flask

Solution

1. Step 1: Understand the URL query parameter

   The URL contains ?term=flask, so the query parameter term has value 'flask'.

2. Step 2: Check how term is retrieved in code

   request.args.get('term', 'none') returns 'flask' because the parameter exists.

3. Final Answer:

   Search term: flask -> Option D

4. Quick Check:

   Parameter present returns its value, not default [OK]

Hint: If parameter exists, get() returns its value, else default [OK]

Common Mistakes:

• Assuming default is returned even if parameter exists
• Confusing query parameters with form data
• Expecting empty string instead of actual value

4. Identify the error in this Flask code snippet that tries to get a query parameter id:

from flask import Flask, request
app = Flask(__name__)

@app.route('/item')
def item():
    id = request.args['id']
    return f"Item ID: {id}"

medium

A. Using request.args['id'] raises KeyError if 'id' missing

B. Missing import of request module

C. Route decorator syntax is incorrect

D. Using f-string inside return is invalid

Solution

1. Step 1: Analyze how query parameter is accessed

   Using request.args['id'] accesses the parameter directly and raises KeyError if missing.

2. Step 2: Identify safer alternative

   Using request.args.get('id') avoids error by returning None if missing.

3. Final Answer:

   Using request.args['id'] raises KeyError if 'id' missing -> Option A

4. Quick Check:

   Direct dict access can cause KeyError [OK]

Hint: Use get() to avoid KeyError when parameter might be missing [OK]

Common Mistakes:

• Assuming request.args['id'] never fails
• Ignoring possibility of missing parameters
• Confusing import errors with runtime errors

5. You want to create a Flask route /filter that reads two optional query parameters: category (default 'all') and limit (default 10, integer). Which code correctly handles both parameters safely?

hard

A. category = request.args.get('category') or 'all'
limit = int(request.args.get('limit', 10))

B. category = request.args['category'] or 'all'
limit = int(request.args['limit'] or 10)

C. category = request.args.get('category', default='all')
limit = int(request.args.get('limit', default=10))

D. category = request.args.get('category', 'all')
limit = int(request.args.get('limit', '10'))

Solution

1. Step 1: Handle category parameter with default

   Using request.args.get('category', default='all') safely returns 'all' if missing.

2. Step 2: Handle limit parameter with default and convert to int

   request.args.get('limit', default=10) returns string or int 10; wrapping with int() converts string to integer safely.

3. Step 3: Check other options for errors

   category = request.args['category'] or 'all' limit = int(request.args['limit'] or 10) risks KeyError; category = request.args.get('category') or 'all' limit = int(request.args.get('limit', 10)) uses positional default incorrectly; category = request.args.get('category', 'all') limit = int(request.args.get('limit', '10')) converts default string '10' which works but less clear than int 10.

4. Final Answer:

   category = request.args.get('category', default='all')\nlimit = int(request.args.get('limit', default=10)) -> Option C

5. Quick Check:

   Use get() with default and convert types explicitly [OK]

Hint: Use get() with default and convert types explicitly [OK]

Common Mistakes:

• Using [] which raises error if missing
• Passing default as positional argument incorrectly
• Not converting string query param to int