Accessing query parameters in Flask - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a query parameter in a URL?

A query parameter is a key-value pair added to the end of a URL after a question mark (?). It is used to send extra information to the server, like filters or options.

beginner

How do you access query parameters in a Flask route?

You use request.args to get query parameters in Flask. It works like a dictionary where keys are parameter names and values are parameter values.

beginner

What does request.args.get('name') do in Flask?

It retrieves the value of the query parameter named 'name'. If 'name' is not in the URL, it returns None by default.

intermediate

How can you provide a default value when accessing a query parameter in Flask?

Use request.args.get('param', default='value'). If 'param' is missing, it returns the default value instead of None.

intermediate

What type of object is request.args in Flask?

request.args is an ImmutableMultiDict. It behaves like a dictionary but can hold multiple values for the same key and cannot be changed.

In Flask, which object holds the query parameters from a URL?

A request.data

B request.form

C request.args

D request.json

request.args contains the query parameters sent in the URL after the question mark.

What will request.args.get('page', '1') return if the URL has no 'page' parameter?

A 1

B None

C An error

D Empty string

The second argument to get is the default value returned if the parameter is missing.

If the URL is /search?term=flask&term=python, what type of object is request.args.getlist('term')?

A A string with both terms joined

B None

C A dictionary

D A list of strings ['flask', 'python']

getlist returns all values for a key as a list.

Which symbol separates query parameters from the main URL?

A ?

B &

C #

D /

The question mark ? starts the query string in a URL.

What happens if you try to modify request.args directly?

A It updates the query parameters

B It raises an error because it's immutable

C It silently ignores changes

D It converts to a list

request.args is immutable and cannot be changed directly.

Explain how to access and use query parameters in a Flask route.

Think about how URLs send extra info and how Flask reads it.

Describe the difference between request.args and request.form in Flask.

Consider where the data comes from in each case.

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