CRUD operations in Flask - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Flask route when a GET request is made?

Consider this Flask route that handles a GET request to fetch all items from a list.

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

items = ["apple", "banana", "cherry"]

@app.route('/items', methods=['GET'])
def get_items():
    return jsonify(items)

What will the client receive as a response body?

Flask

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

items = ["apple", "banana", "cherry"]

@app.route('/items', methods=['GET'])
def get_items():
    return jsonify(items)

A ["apple", "banana", "cherry"] as JSON array

B Plain text string: "apple, banana, cherry"

C Python list printed as string: "['apple', 'banana', 'cherry']"

D Empty JSON object: {}

💡 Hint

Remember that jsonify converts Python objects to JSON format.

The jsonify function converts the Python list items into a JSON array. So the client receives a JSON array with the strings.

📝 Syntax

intermediate

Which Flask route code correctly handles creating a new item with POST?

You want to add a new item sent in JSON format to the items list using a POST request.

Which code snippet correctly extracts the JSON data and appends the new item?

Flask

items = []

A

@app.route('/items', methods=['POST'])
def add_item():
    data = request.form
    items.append(data['item'])
    return jsonify({'message': 'Item added'})

B

@app.route('/items', methods=['POST'])
def add_item():
    data = request.json
    items.append(data.item)
    return jsonify({'message': 'Item added'})

C

@app.route('/items', methods=['POST'])
def add_item():
    data = request.get_json()
    items.append(data['item'])
    return jsonify({'message': 'Item added'})

D

@app.route('/items', methods=['POST'])
def add_item():
    data = request.get_data()
    items.append(data['item'])
    return jsonify({'message': 'Item added'})

💡 Hint

Use the method that parses JSON body from the request.

request.get_json() correctly parses JSON data sent in the POST body. Accessing data['item'] gets the new item string.

🔧 Debug

advanced

Why does this Flask PUT route raise a KeyError?

Given this Flask route to update an item by index, it raises a KeyError when called.

items = ["apple", "banana", "cherry"]

@app.route('/items/', methods=['PUT'])
def update_item(index):
    data = request.get_json()
    items[index] = data['value']
    return jsonify({'message': 'Item updated'})

What is the most likely cause of the KeyError?

Flask

items = ["apple", "banana", "cherry"]

A The JSON sent does not have the key 'value', causing KeyError

B Flask route decorator syntax is incorrect causing KeyError

C request.get_json() returns None causing KeyError

D The index is out of range causing KeyError

💡 Hint

Check the keys in the JSON data sent in the request body.

If the JSON body lacks the 'value' key, accessing data['value'] raises a KeyError.

❓ state_output

advanced

What is the state of the items list after these DELETE requests?

Starting with items = ['apple', 'banana', 'cherry', 'date'], these DELETE requests are made:

@app.route('/items/', methods=['DELETE'])
def delete_item(index):
    items.pop(index)
    return jsonify({'message': 'Deleted'})

# Requests:
# DELETE /items/1
# DELETE /items/2

What is the final content of items after both requests?

Flask

items = ['apple', 'banana', 'cherry', 'date']

A ['apple', 'banana']

B ['apple', 'cherry', 'date']

C ['apple', 'date']

D ['apple', 'cherry']

💡 Hint

Remember that list indexes shift after each pop.

First DELETE removes index 1 ('banana'), list becomes ['apple', 'cherry', 'date']. Second DELETE removes index 2 ('date'). Final list: ['apple', 'cherry'].

🧠 Conceptual

expert

Which statement best describes idempotency in RESTful CRUD operations?

In REST APIs, some HTTP methods are idempotent. Which statement correctly explains idempotency in the context of CRUD operations?

A Idempotency means the method always creates a new resource every time it is called.

B An idempotent method can be called multiple times with the same effect as a single call, like PUT updating a resource.

C POST is idempotent because it updates existing resources without creating duplicates.

D DELETE is not idempotent because it removes resources permanently.

💡 Hint

Think about what happens if you repeat the same request multiple times.

PUT is idempotent because updating a resource multiple times with the same data results in the same state. POST creates new resources and is not idempotent. DELETE is idempotent because deleting the same resource multiple times has the same effect as deleting once.