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.