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

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

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

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

Hint

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

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