HTTP methods (GET, POST, PUT, DELETE) in Flask

Introduction

HTTP methods tell a web server what action you want to do with data. They help your app get, send, update, or delete information.

When you want to show data on a webpage (GET).

When you want to send new data to the server, like submitting a form (POST).

When you want to change existing data on the server (PUT).

When you want to remove data from the server (DELETE).

Syntax

Flask

from flask import Flask, request

app = Flask(__name__)

@app.route('/resource', methods=['GET', 'POST', 'PUT', 'DELETE'])
def resource():
    if request.method == 'GET':
        return 'Get data'
    elif request.method == 'POST':
        return 'Create data'
    elif request.method == 'PUT':
        return 'Update data'
    elif request.method == 'DELETE':
        return 'Delete data'

Use @app.route with methods to specify allowed HTTP methods.

Check request.method to handle each method differently.

Examples

This route only responds to GET requests to show items.

Flask

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

This route accepts POST requests to add a new item using JSON data.

Flask

@app.route('/items', methods=['POST'])
def add_item():
    data = request.json
    return f"Added item: {data['name']}"

This route updates an existing item by its ID using PUT.

Flask

@app.route('/items/<int:id>', methods=['PUT'])
def update_item(id):
    data = request.json
    return f"Updated item {id} with name {data['name']}"

This route deletes an item by its ID using DELETE.

Flask

@app.route('/items/<int:id>', methods=['DELETE'])
def delete_item(id):
    return f"Deleted item {id}"

Sample Program

This Flask app manages a simple list of items using all four HTTP methods. You can get all items, add a new item, update an existing item, or delete an item by its ID.

Flask

from flask import Flask, request

app = Flask(__name__)

items = {1: 'apple', 2: 'banana'}

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

@app.route('/items', methods=['POST'])
def add_item():
    new_id = max(items.keys()) + 1 if items else 1
    data = request.json
    items[new_id] = data['name']
    return {'id': new_id, 'name': items[new_id]}, 201

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

@app.route('/items/<int:id>', methods=['DELETE'])
def delete_item(id):
    if id in items:
        del items[id]
        return {'message': f'Item {id} deleted'}
    else:
        return {'error': 'Item not found'}, 404

Important Notes

GET requests should not change data; they only fetch it.

POST is for creating new data, PUT is for updating existing data.

DELETE removes data and should be used carefully.

Summary

HTTP methods tell the server what action to do with data.

Use GET to read, POST to create, PUT to update, and DELETE to remove data.

Flask lets you handle these methods easily with routes and request checks.

Practice

1. Which HTTP method is typically used in Flask to retrieve data without changing anything on the server?

easy

A. PUT

B. POST

C. GET

D. DELETE

Solution

1. Step 1: Understand HTTP methods purpose

   GET is used to request data without modifying it, so it is safe and idempotent.

2. Step 2: Match method to Flask usage

   In Flask, GET routes are used to send data back to the client without changing server state.

3. Final Answer:

   GET -> Option C

4. Quick Check:

   Retrieve data = GET [OK]

Hint: GET reads data, POST creates, PUT updates, DELETE removes [OK]

Common Mistakes:

• Confusing POST with GET for data retrieval
• Using DELETE to get data
• Thinking PUT is for reading data

2. Which of the following Flask route definitions correctly allows handling both GET and POST methods?

easy

A. @app.route('/submit', allow_methods=['GET', 'POST'])

B. @app.route('/submit', methods=['GET', 'POST'])

C. @app.route('/submit', methods='GET, POST')

D. @app.route('/submit', method=['GET', 'POST'])

Solution

1. Step 1: Recall Flask route syntax for methods

   Flask uses the 'methods' keyword argument as a list of strings to specify allowed HTTP methods.

2. Step 2: Check each option's syntax

   @app.route('/submit', methods=['GET', 'POST']) correctly uses methods=['GET', 'POST']. Others have wrong keywords or wrong data types.

3. Final Answer:

   @app.route('/submit', methods=['GET', 'POST']) -> Option B

4. Quick Check:

   Correct keyword and list format = @app.route('/submit', methods=['GET', 'POST']) [OK]

Hint: Use methods=['GET', 'POST'] exactly in route decorator [OK]

Common Mistakes:

• Using 'method' instead of 'methods'
• Passing methods as a string instead of list
• Using unknown keywords like 'allow_methods'

3. What will be the output when a client sends a DELETE request to this Flask route?

@app.route('/item/', methods=['DELETE'])
def delete_item(id):
    return f"Deleted item {id}", 200

medium

A. "Deleted item <id>" with status 200

B. 404 Not Found error

C. Method Not Allowed error

D. Empty response with status 204

Solution

1. Step 1: Analyze route and method

   The route accepts DELETE requests with an integer id parameter and returns a string with status 200.

2. Step 2: Understand response for DELETE request

   Since the method matches and function returns a message with 200, the client receives that message and status.

3. Final Answer:

   "Deleted item <id>" with status 200 -> Option A

4. Quick Check:

   DELETE request handled correctly = "Deleted item <id>" with status 200 [OK]

Hint: Route method matches request method, so response is returned [OK]

Common Mistakes:

• Expecting 404 if route exists
• Confusing DELETE with GET behavior
• Assuming empty response without checking code

4. Identify the error in this Flask route that should handle PUT requests:

@app.route('/update', methods=['PUT'])
def update():
    data = request.get_json()
    return 'Updated', 200

medium

A. Missing import of 'request' from Flask

B. Incorrect HTTP method name 'PUT'

C. Route decorator syntax is invalid

D. Function must return JSON, not string

Solution

1. Step 1: Check usage of 'request'

   The function uses 'request.get_json()' but 'request' is not imported from Flask, causing a NameError.

2. Step 2: Verify other parts

   Method 'PUT' is valid, route syntax is correct, and returning string with status 200 is allowed.

3. Final Answer:

   Missing import of 'request' from Flask -> Option A

4. Quick Check:

   Using 'request' without import causes error [OK]

Hint: Always import 'request' to access request data [OK]

Common Mistakes:

• Forgetting to import 'request'
• Thinking PUT is invalid method
• Assuming string return is wrong

5. You want to create a Flask API endpoint to update a user's email using PUT and delete the user using DELETE at the same URL '/user/'. Which is the best way to define this in Flask?

hard

A. Use GET method for both update and delete actions

B. Create two separate routes with different URLs for PUT and DELETE

C. Use only POST method and check a hidden field for action

D. Use one route with methods=['PUT', 'DELETE'] and check request.method inside the function

Solution

1. Step 1: Understand RESTful design

   PUT and DELETE are standard methods for update and delete at the same resource URL.

2. Step 2: Implement in Flask

   Flask allows one route to handle multiple methods; inside the function, you check request.method to run correct code.

3. Final Answer:

   Use one route with methods=['PUT', 'DELETE'] and check request.method inside the function -> Option D

4. Quick Check:

   Multiple methods on one route with method check = Use one route with methods=['PUT', 'DELETE'] and check request.method inside the function [OK]

Hint: One route, multiple methods, check request.method inside [OK]

Common Mistakes:

• Creating separate URLs for same resource actions
• Using POST or GET incorrectly for update/delete
• Not checking request.method inside route