CRUD operations in Flask

Introduction

CRUD operations let you create, read, update, and delete data in a web app. They help manage information easily.

Building a simple blog where users can add, edit, or remove posts.

Creating a contact list app to add, view, update, or delete contacts.

Making a to-do list where tasks can be added, marked done, or removed.

Developing a product catalog where items can be managed by an admin.

Syntax

Flask

from flask import Flask, request, jsonify

app = Flask(__name__)

# Create
@app.route('/items', methods=['POST'])
def create_item():
    # code to add item
    pass

# Read
@app.route('/items', methods=['GET'])
def get_items():
    # code to get items
    pass

# Update
@app.route('/items/<int:id>', methods=['PUT'])
def update_item(id):
    # code to update item
    pass

# Delete
@app.route('/items/<int:id>', methods=['DELETE'])
def delete_item(id):
    # code to delete item
    pass

Each operation uses a different HTTP method: POST for create, GET for read, PUT for update, DELETE for delete.

Routes define the URL paths and accept parameters like item IDs for update and delete.

Examples

This example shows how to add a new item to a list using POST.

Flask

from flask import Flask, request, jsonify

app = Flask(__name__)
items = []

@app.route('/items', methods=['POST'])
def create_item():
    data = request.json
    items.append(data)
    return jsonify(data), 201

This returns all items in JSON format using GET.

Flask

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

This updates an item by its index if it exists.

Flask

@app.route('/items/<int:id>', methods=['PUT'])
def update_item(id):
    data = request.json
    if 0 <= id < len(items):
        items[id] = data
        return jsonify(data)
    return jsonify({'error': 'Item not found'}), 404

This deletes an item by index and returns it.

Flask

@app.route('/items/<int:id>', methods=['DELETE'])
def delete_item(id):
    if 0 <= id < len(items):
        removed = items.pop(id)
        return jsonify(removed)
    return jsonify({'error': 'Item not found'}), 404

Sample Program

This Flask app lets you add, view, update, and delete items stored in a list. Each route handles one CRUD operation using JSON data.

Flask

from flask import Flask, request, jsonify

app = Flask(__name__)
items = []

@app.route('/items', methods=['POST'])
def create_item():
    data = request.json
    items.append(data)
    return jsonify(data), 201

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

@app.route('/items/<int:id>', methods=['PUT'])
def update_item(id):
    data = request.json
    if 0 <= id < len(items):
        items[id] = data
        return jsonify(data)
    return jsonify({'error': 'Item not found'}), 404

@app.route('/items/<int:id>', methods=['DELETE'])
def delete_item(id):
    if 0 <= id < len(items):
        removed = items.pop(id)
        return jsonify(removed)
    return jsonify({'error': 'Item not found'}), 404

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

Important Notes

Flask uses decorators (@app.route) to connect URLs to functions.

Use JSON format to send and receive data in APIs.

Remember to handle errors like missing items gracefully.

Summary

CRUD means Create, Read, Update, Delete - basic data actions.

Flask routes use HTTP methods to perform these actions.

JSON is commonly used to exchange data in Flask APIs.

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