How to mock a fallback return value in Python

Test a function that returns a default value on failure by mocking requests.get and its side effects.

Easy Python 3.9+ Aug 9, 2026 Reliability & rate limiting 15 views 0 copies

unittest mocking requests error-handling fallback
Requires third-party packages — install first
pip install requests

Python code

20 lines
Python 3.9+ 
from unittest.mock import Mock, patch
import requests

def fetch_data(url, default=None):
    try:
        response = requests.get(url)
        response.raise_for_status()
        return response.json()
    except (requests.RequestException, ValueError):
        return default

with patch("requests.get") as mock_get:
    mock_get.return_value.raise_for_status.side_effect = requests.RequestException("Network error")
    result = fetch_data("https://api.example.com/data", default={"fallback": True})
    print(result)

    mock_get.return_value = Mock(status_code=200)
    mock_get.return_value.json.return_value = {"data": "real"}
    result = fetch_data("https://api.example.com/data", default={"fallback": True})
    print(result)

Output

stdout 
{'fallback': True}
{'data': 'real'}

How it works

The patch context manager replaces requests.get with a Mock. Setting raise_for_status.side_effect makes the mocked response raise an exception, triggering the fallback. Reassigning return_value creates a new mocked response for the successful path. The try/except in fetch_data catches both network errors and invalid JSON, returning the default when needed.

Common mistakes

  • Forgetting to set `raise_for_status.side_effect` so the error path is actually exercised
  • Reusing the same Mock return_value for both success and failure without reassigning
  • Not catching JSON decode errors, so invalid responses crash instead of falling back

Variations

  1. Use `mock_get.side_effect` with a list of responses to simulate sequential calls
  2. Use `responses` or `requests_mock` libraries for HTTP-level mocking rather than patching internals

Real-world use cases

  • Unit-testing an API client that must return cached data when the network is down.
  • Verifying a retry wrapper falls back to local defaults on service degradation.
  • Testing rate-limit handling where a 429 response must degrade to a safe fallback value.

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Run locally

This sample needs third-party packages, so it cannot run in the browser IDE. Copy the code above, install the packages shown at the top, then run it in your own Python environment.

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