Mocking external services in Flask - Mini Project: Build & Apply

Mocking External Services in Flask

📖 Scenario: You are building a Flask web app that fetches weather data from an external API. To test your app without calling the real API every time, you will mock the external service.

🎯 Goal: Create a Flask app that calls a mocked weather API service and returns the mocked data as JSON.

📋 What You'll Learn

Create a Flask app with a route /weather

Mock the external weather API call to return fixed data

Use the unittest.mock library to patch the external call

Return the mocked weather data as JSON from the route

💡 Why This Matters

🌍 Real World

Mocking external services is useful when you want to test your Flask app without relying on real APIs that may be slow, unreliable, or cost money.

💼 Career

Many jobs require writing tests for web apps that depend on external services. Knowing how to mock these services helps ensure your app works correctly and tests run fast.

1

Set up Flask app and route

Create a Flask app instance called app and define a route /weather with a function get_weather that returns an empty dictionary as JSON.

Flask

from flask import Flask, jsonify

app = Flask(__name__)

@app.route('/weather')
def get_weather():
    # Your code here

Need a hint?

Use Flask(__name__) to create the app and @app.route('/weather') to define the route.

2

Add external API call function

Create a function called fetch_weather_data that simulates calling an external API by returning a dictionary {'temp': 20, 'condition': 'Sunny'}. Update get_weather to call fetch_weather_data and return its result as JSON.

Flask

from flask import Flask, jsonify

app = Flask(__name__)

def fetch_weather_data():
    # Your code here

@app.route('/weather')
def get_weather():
    # Your code here

Need a hint?

Define fetch_weather_data to return the fixed dictionary. Call it inside get_weather.

3

Mock the external API call in a test

Write a test function called test_get_weather that uses unittest.mock.patch to replace fetch_weather_data with a mock returning {'temp': 10, 'condition': 'Cloudy'}. Inside the test, call get_weather and check that the returned JSON matches the mocked data.

Flask

from unittest.mock import patch

def test_get_weather():
    # Your code here

Need a hint?

Use @patch decorator to mock fetch_weather_data. Call get_weather() and assert the JSON matches the mock.

4

Run Flask app with debug mode

Add the standard Flask app runner code to run the app with debug=True when the script is executed directly.

Flask

from flask import Flask, jsonify

app = Flask(__name__)

def fetch_weather_data():
    return {'temp': 20, 'condition': 'Sunny'}

@app.route('/weather')
def get_weather():
    data = fetch_weather_data()
    return jsonify(data)

from unittest.mock import patch

@patch('__main__.fetch_weather_data', return_value={'temp': 10, 'condition': 'Cloudy'})
def test_get_weather(mock_fetch):
    response = get_weather()
    assert response.json == {'temp': 10, 'condition': 'Cloudy'}

if __name__ == '__main__':
    # Your code here

Need a hint?

Add the if __name__ == '__main__': block and call app.run(debug=True) inside it.