Mocking external services in Flask - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Flask route when the external API is mocked?

Consider this Flask route that calls an external API to get user data. The external API is mocked to return a fixed response.

import requests
from flask import Flask, jsonify
app = Flask(__name__)

@app.route('/user/')
def get_user(user_id):
    response = requests.get(f'https://api.example.com/users/{user_id}')
    data = response.json()
    return jsonify({'name': data['name'], 'id': data['id']})

# Mock setup:
# requests.get is mocked to always return a response with json() = {'id': 42, 'name': 'Alice'}

What will be the JSON output when accessing /user/10?

A {"name": "Unknown", "id": 10}

B {"name": "Alice", "id": 42}

C {"name": "Alice", "id": 10}

D 500 Internal Server Error

💡 Hint

Remember the mock always returns the same data regardless of the user_id.

The mocked requests.get always returns a response whose json() method returns {'id': 42, 'name': 'Alice'}. So the route returns this data wrapped in JSON, ignoring the URL user_id.

🔧 Debug

intermediate

Why does this Flask test fail when mocking an external service?

In this Flask test, the external API call is mocked but the test fails with a TypeError.

import pytest
from unittest.mock import patch

@patch('requests.get')
def test_user(mock_get, client):
    mock_get.return_value.json = {'id': 1, 'name': 'Bob'}
    response = client.get('/user/1')
    assert response.json == {'id': 1, 'name': 'Bob'}

What is the cause of the error?

A mock_get.return_value.json should be a callable, not a dict

B The patch decorator is applied to the wrong function

C The Flask client does not support .json attribute on response

D The test is missing a Flask app context

💡 Hint

Check what response.json expects to be in the mock.

The json attribute of the mocked response must be a callable method returning the dict, not the dict itself. Assigning a dict directly causes a TypeError when called.

📝 Syntax

advanced

Which option correctly mocks a Flask external API call using unittest.mock?

You want to mock requests.get in a Flask test to return a JSON response {'status': 'ok'}. Which code snippet is correct?

A

with patch('requests.get') as mock_get:
    mock_get.return_value.json = lambda: {'status': 'ok'}

B

with patch('requests.get') as mock_get:
    mock_get.return_value.json = {'status': 'ok'}

C

with patch('requests.get') as mock_get:
    mock_get.return_value = {'status': 'ok'}

D

with patch('requests.get') as mock_get:
    mock_get.json.return_value = {'status': 'ok'}

💡 Hint

The mocked json must be a callable method returning the dict.

Option A correctly sets mock_get.return_value.json to a lambda returning the dict. Option A tries to set json on the mock_get itself, which is wrong. Option A sets the return_value to a dict, not a mock response. Option A sets json to a dict, not a callable.

❓ state_output

advanced

What is the value of variable 'result' after mocking an external API in Flask?

Given this Flask function and test snippet:

def fetch_data():
    import requests
    response = requests.get('https://api.example.com/data')
    return response.json()['value']

# Test with mock:
from unittest.mock import patch

with patch('requests.get') as mock_get:
    mock_get.return_value.json = lambda: {'value': 123}
    result = fetch_data()

What is the value of result?

A Raises a TypeError

B {'value': 123}

C None

D 123

💡 Hint

Look at what fetch_data returns from the mocked JSON.

The mocked json() returns {'value': 123}. The function returns the value of the 'value' key, which is 123.

🧠 Conceptual

expert

Which option best explains why mocking external services is important in Flask testing?

Why do developers mock external API calls when testing Flask applications?

A To avoid writing any test code and rely on production data

B To increase the complexity of tests by adding real API calls

C To isolate tests from network issues and ensure consistent, fast, and reliable test results

D To make tests slower and dependent on external service uptime

💡 Hint

Think about test speed and reliability.

Mocking external services isolates tests from network failures and variability, making tests faster and more reliable. It avoids dependency on external service availability.