Mocking external services in Flask - Performance & Optimization

Performance: Mocking external services

MEDIUM IMPACT

This concept affects page load speed and interaction responsiveness by reducing delays caused by real external service calls during development and testing.

Testing a Flask app that calls an external API

Flask

from unittest.mock import patch

@patch('requests.get')
def test_get_data(mock_get):
    mock_get.return_value.json.return_value = {'key': 'value'}
    data = get_data()
    assert data == {'key': 'value'}

Mocks the external call to return instant, controlled data, avoiding network delays and failures.

📈 Performance GainRemoves network latency, making tests run instantly and improving INP during development.

Testing a Flask app that calls an external API

Flask

import requests

def get_data():
    response = requests.get('https://api.example.com/data')
    return response.json()

This makes a real network call every time, causing slow tests and possible flakiness if the external service is down.

📉 Performance CostBlocks test execution for network latency (100-500ms per call), increasing INP and slowing feedback.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Real external API calls during tests	Minimal	0	0	[X] Bad
Mocked external API calls during tests	Minimal	0	0	[OK] Good

Rendering Pipeline

Mocking external services bypasses the network request stage, so the browser or server does not wait for slow external responses, improving responsiveness.

→Network Request

→JavaScript Execution

→Rendering

⚠️ BottleneckNetwork Request stage is the most expensive due to latency and unpredictability.

Core Web Vital Affected

INP

This concept affects page load speed and interaction responsiveness by reducing delays caused by real external service calls during development and testing.

Optimization Tips

1Always mock slow or unreliable external services during development and testing.

2Use mocking to avoid network latency that blocks user interactions or test runs.

3Verify mocks prevent real network calls using browser DevTools Network panel.

Performance Quiz - 3 Questions

Why does mocking external services improve test performance in a Flask app?

A It reduces CSS complexity, speeding up style calculations.

B It avoids slow network calls, making tests faster and more reliable.

C It increases the number of DOM nodes, improving rendering.

D It decreases JavaScript bundle size significantly.

DevTools: Network

How to check: Open DevTools Network panel, run your app or tests, and observe if external API calls are made or blocked/mocked.

What to look for: No real external requests should appear during tests if mocking is correctly implemented, indicating faster response.

Practice

1. What is the main reason to mock external services in Flask testing?

easy

A. To change the Flask app configuration permanently

B. To speed up the Flask server in production

C. To add more routes to the Flask app

D. To avoid making real network calls during tests

Solution

1. Step 1: Understand the purpose of mocking

   Mocking replaces real external calls with fake ones to avoid delays and failures during tests.

2. Step 2: Identify the benefit in Flask testing

   This avoids real network calls, making tests faster and more reliable.

3. Final Answer:

   To avoid making real network calls during tests -> Option D

4. Quick Check:

   Mocking = Avoid real network calls [OK]

Hint: Mock to skip real network calls in tests [OK]

Common Mistakes:

• Thinking mocking speeds up production
• Confusing mocking with adding routes
• Believing mocking changes app config permanently

2. Which of the following is the correct way to patch an external function requests.get in a Flask test using unittest.mock?

easy

A. with patch('requests.get') as mock_get:

B. with patch(requests.get) as mock_get:

C. with patch('requests->get') as mock_get:

D. with patch('requests_get') as mock_get:

Solution

1. Step 1: Understand patch syntax

   Patch requires the full path as a string, e.g., 'requests.get'.

2. Step 2: Identify correct string format

   with patch('requests.get') as mock_get: uses the correct string 'requests.get'. Others use invalid syntax or missing quotes.

3. Final Answer:

   with patch('requests.get') as mock_get: -> Option A

4. Quick Check:

   Patch needs string path = with patch('requests.get') as mock_get: [OK]

Hint: Patch uses string path like 'module.function' [OK]

Common Mistakes:

• Passing function object instead of string
• Using wrong string format with arrows
• Missing quotes around patch target

3. Given this Flask test code snippet, what will be printed?

from unittest.mock import patch
import requests

def fetch_data():
    response = requests.get('http://example.com')
    return response.status_code

with patch('requests.get') as mock_get:
    mock_get.return_value.status_code = 404
    print(fetch_data())

medium

A. None

B. 200

C. 404

D. Raises an error

Solution

1. Step 1: Understand patch effect on requests.get

   The patch replaces requests.get with a mock whose return_value.status_code is set to 404.

2. Step 2: Trace fetch_data call inside patch

   fetch_data calls requests.get, which returns the mock with status_code 404, so fetch_data returns 404.

3. Final Answer:

   404 -> Option C

4. Quick Check:

   Mocked status_code = 404 [OK]

Hint: Mock return_value sets fake response attributes [OK]

Common Mistakes:

• Expecting real HTTP 200 response
• Confusing mock object with None
• Thinking patch causes error

4. What is wrong with this Flask test code that tries to mock requests.post?

from unittest.mock import patch
import requests

def send_data():
    response = requests.post('http://example.com', data={'key': 'value'})
    return response.ok

with patch('requests.post') as mock_post:
    mock_post.ok = True
    print(send_data())

medium

A. mock_post.ok should be set on return_value, not mock_post

B. patch target should be 'requests.get' not 'requests.post'

C. send_data should use requests.get instead of post

D. mock_post.ok is correct and code works fine

Solution

1. Step 1: Identify mock object usage

   mock_post is the mock for requests.post function, its return_value simulates the response object.

2. Step 2: Correct attribute assignment

   Setting mock_post.ok sets attribute on the function mock, not the response. It should be mock_post.return_value.ok = True.

3. Final Answer:

   mock_post.ok should be set on return_value, not mock_post -> Option A

4. Quick Check:

   Mock response attributes go on return_value [OK]

Hint: Set mock response attributes on return_value [OK]

Common Mistakes:

• Setting attributes on mock function instead of return_value
• Confusing post with get method
• Assuming code works without fixing mock

5. You want to test a Flask route that calls an external API and returns JSON data. Which approach correctly mocks the external API call and returns a custom JSON response {"success": true} during the test?

hard

A. Use patch on the Flask route function and return {"success": true} directly

B. Use patch on the external call, set return_value.json.return_value = {"success": true}

C. Call the real external API and check if it returns {"success": true}

D. Mock the Flask app config to include {"success": true}

Solution

1. Step 1: Identify what to mock

   The external API call (e.g., requests.get) should be mocked to avoid real network calls.

2. Step 2: Set mock return JSON data

   Set the mock's return_value.json.return_value to the desired JSON dict to simulate API response.

3. Final Answer:

   Use patch on the external call, set return_value.json.return_value = {"success": true} -> Option B

4. Quick Check:

   Mock external call's json() method = Use patch on the external call, set return_value.json.return_value = {"success": true} [OK]

Hint: Mock return_value.json.return_value for JSON responses [OK]

Common Mistakes:

• Mocking Flask route instead of external call
• Calling real API in tests
• Changing app config instead of mocking