CRUD operations in Flask - Mini Project: Build & Apply

CRUD Operations with Flask

📖 Scenario: You are building a simple web app to manage a list of books. Each book has a title and an author. You want to create a Flask app that can create, read, update, and delete books.

🎯 Goal: Build a Flask app with routes to add a book, view all books, update a book, and delete a book using an in-memory list as the data store.

📋 What You'll Learn

Create a list called books to store book dictionaries with keys title and author.

Add a configuration variable app as a Flask instance.

Create a route /books that returns the list of books in JSON format.

Add routes to add, update, and delete books using Flask route decorators.

💡 Why This Matters

🌍 Real World

Managing lists of items like books, tasks, or contacts is common in web apps. This project shows how to build basic CRUD operations with Flask, a popular Python web framework.

💼 Career

Understanding CRUD operations and Flask routing is essential for backend web development roles. This knowledge helps you build APIs and web services that handle data.

1

Data Setup: Create the books list

Create a list called books with these exact dictionaries: {'title': '1984', 'author': 'George Orwell'} and {'title': 'To Kill a Mockingbird', 'author': 'Harper Lee'}.

Flask

# Create the books list with two book dictionaries
# Your code here

Hint

Use a list with two dictionaries exactly as shown.

2

Configuration: Create the Flask app instance

Import Flask from flask and create a Flask app instance called app.

Flask

books = [
    {'title': '1984', 'author': 'George Orwell'},
    {'title': 'To Kill a Mockingbird', 'author': 'Harper Lee'}
]

# Import Flask and create app instance
# Your code here

Hint

Use app = Flask(__name__) to create the app.

3

Core Logic: Create a route to get all books

Use @app.route('/books') to create a function get_books() that returns the books list as JSON using from flask import jsonify.

Flask

from flask import Flask

books = [
    {'title': '1984', 'author': 'George Orwell'},
    {'title': 'To Kill a Mockingbird', 'author': 'Harper Lee'}
]

app = Flask(__name__)

# Create a route '/books' to return books as JSON
# Your code here

Hint

Use @app.route('/books') and return jsonify(books).

4

Completion: Add routes to create, update, and delete books

Add three routes: @app.route('/books', methods=['POST']) with function add_book() to add a book from JSON request; @app.route('/books/<int:index>', methods=['PUT']) with function update_book(index) to update a book at index; and @app.route('/books/<int:index>', methods=['DELETE']) with function delete_book(index) to remove a book at index. Use from flask import request and return updated books as JSON.

Flask

from flask import Flask, jsonify

books = [
    {'title': '1984', 'author': 'George Orwell'},
    {'title': 'To Kill a Mockingbird', 'author': 'Harper Lee'}
]

app = Flask(__name__)

@app.route('/books')
def get_books():
    return jsonify(books)

# Add routes to create, update, and delete books below
# Your code here

Hint

Use request.get_json() to get data and modify the books list accordingly.

Practice

1. What does the 'CRUD' acronym stand for in Flask web development?

easy

A. Cache, Route, User, Data

B. Connect, Run, Upload, Download

C. Code, Render, Use, Deploy

D. Create, Read, Update, Delete

Solution

1. Step 1: Understand CRUD basics

   CRUD is a common term in web development describing four basic operations on data.

2. Step 2: Match each letter to its meaning

   C stands for Create, R for Read, U for Update, and D for Delete.

3. Final Answer:

   Create, Read, Update, Delete -> Option D

4. Quick Check:

   CRUD = Create, Read, Update, Delete [OK]

Hint: Remember CRUD as the four main data actions [OK]

Common Mistakes:

• Confusing CRUD with unrelated terms
• Mixing up the order of operations
• Thinking CRUD refers to Flask-specific commands

2. Which HTTP method is typically used in Flask to update an existing resource in a CRUD API?

easy

A. PUT

B. POST

C. GET

D. DELETE

Solution

1. Step 1: Identify HTTP methods for CRUD

   GET is for reading, POST for creating, PUT for updating, DELETE for deleting.

2. Step 2: Match update operation

   Updating a resource uses the PUT method in RESTful APIs like Flask.

3. Final Answer:

   PUT -> Option A

4. Quick Check:

   Update = PUT method [OK]

Hint: Update uses PUT method in REST APIs [OK]

Common Mistakes:

• Using POST instead of PUT for updates
• Confusing GET with update actions
• Using DELETE for updating data

3. Given this Flask route code snippet, what will be the output when a DELETE request is sent to /item/5?

from flask import Flask, jsonify
app = Flask(__name__)
items = {1: 'apple', 5: 'banana'}

@app.route('/item/', methods=['DELETE'])
def delete_item(id):
    if id in items:
        del items[id]
        return jsonify({'message': 'Deleted'}), 200
    else:
        return jsonify({'error': 'Not found'}), 404

medium

A. {"error": "Not found"} with status 404

B. {"message": "Deleted"} with status 200

C. SyntaxError due to missing return statement

D. 500 Internal Server Error

Solution

1. Step 1: Check if item with id 5 exists

   The dictionary items contains key 5 with value 'banana'.

2. Step 2: Analyze DELETE request handling

   Since id 5 exists, it is deleted and a JSON message 'Deleted' with status 200 is returned.

3. Final Answer:

   {"message": "Deleted"} with status 200 -> Option B

4. Quick Check:

   Item 5 exists, so deletion returns success message [OK]

Hint: Check if key exists before delete to decide response [OK]

Common Mistakes:

• Assuming item does not exist and returning 404
• Forgetting to delete the item before returning
• Confusing status codes for success and error

4. Identify the error in this Flask route for creating a new item:

@app.route('/item', methods=['POST'])
def create_item():
    data = request.json
    items.append(data['name'])
    return jsonify({'message': 'Item added'}), 201

medium

A. Missing import of request from flask

B. Using append on a dictionary instead of a list

C. Incorrect HTTP method for creating resource

D. Returning wrong status code

Solution

1. Step 1: Check for necessary imports

   The code uses request.json but does not import request from flask.

2. Step 2: Verify data structure usage

   Assuming items is a list, append is valid; HTTP method POST is correct; status 201 is correct.

3. Final Answer:

   Missing import of request from flask -> Option A

4. Quick Check:

   request must be imported to access request.json [OK]

Hint: Always import request to access JSON data in Flask [OK]

Common Mistakes:

• Forgetting to import request
• Using append on a dict instead of list
• Using GET instead of POST for creation

5. You want to implement a Flask route to update an item only if it exists, otherwise return a 404 error. Which code snippet correctly performs this update using JSON data sent in a PUT request? A)

@app.route('/item/', methods=['PUT'])
def update_item(id):
    data = request.json
    items[id] = data['name']
    return jsonify({'message': 'Updated'}), 200

B)

@app.route('/item/', methods=['PUT'])
def update_item(id):
    if id not in items:
        return jsonify({'error': 'Not found'}), 404
    data = request.json
    items[id] = data['name']
    return jsonify({'message': 'Updated'}), 200

C)

@app.route('/item/', methods=['POST'])
def update_item(id):
    data = request.json
    if id in items:
        items[id] = data['name']
    return jsonify({'message': 'Updated'}), 200

D)

@app.route('/item/', methods=['PUT'])
def update_item(id):
    data = request.json
    if id in items:
        items[id] = data['name']
    else:
        items[id] = 'default'
    return jsonify({'message': 'Updated'}), 200

hard

A. Updates without checking existence, always returns 200

B. Uses POST method incorrectly for update

C. Checks existence, updates if found, else returns 404

D. Adds default if not found instead of 404 error

Solution

1. Step 1: Verify HTTP method and existence check

   Checks existence, updates if found, else returns 404 uses PUT method and checks if id exists before updating.

2. Step 2: Confirm correct response for missing item

   If id not found, it returns 404 error, which is correct behavior for update.

3. Final Answer:

   Checks existence, updates if found, else returns 404 -> Option C

4. Quick Check:

   Update with existence check returns 404 if missing [OK]

Hint: Always check if item exists before update to avoid errors [OK]

Common Mistakes:

• Not checking if item exists before update
• Using POST instead of PUT for update
• Adding default values instead of error on missing item