My Python tests passed. Production still broke.

#python #debugging #testing #webdev

Deployed my Python API to production last Friday. All tests green. Felt good about it.

Monday morning support messages me. API returning 500 errors for about 30% of requests. Tests still passing locally.

Great.

What broke

My code parsed JSON from an external API:

import requests

def get_user_data(user_id):
    response = requests.get(f"https://api.example.com/users/{user_id}")
    data = response.json()
    return {
        'name': data['profile']['name'],
        'email': data['contact']['email'],
        'age': data['profile']['age']
    }

Tests mocked the API response with perfect JSON. Every field present. Proper nesting. Exactly what I expected and nothing like what actually came back half the time.

Production API sometimes returned incomplete data. Missing profile object. Missing contact email. Still valid JSON, just structured differently.

My tests never checked for this because I wrote happy path mocks like an idiot.

def test_get_user_data():
    mock_response = {
        'profile': {'name': 'John', 'age': 30},
        'contact': {'email': 'john@example.com'}
    }
    # This never happens in real life
    result = get_user_data(123)
    assert result['name'] == 'John'

Real API had:

New users missing profile data
Deactivated accounts missing contact info
International users with different field names
Probably other stuff I still haven't found

My perfect mock data taught me nothing.

What actually worked

def get_user_data(user_id):
    response = requests.get(f"https://api.example.com/users/{user_id}")
    data = response.json()

    # Get with defaults instead of assuming structure
    profile = data.get('profile', {})
    contact = data.get('contact', {})

    return {
        'name': profile.get('name', 'Unknown'),
        'email': contact.get('email', None),
        'age': profile.get('age', None)
    }

Defensive access patterns everywhere. Also added tests with missing fields:

def test_get_user_data_missing_profile():
    mock_response = {'contact': {'email': 'test@example.com'}}
    # Entire profile object gone
    result = get_user_data(123)
    assert result['name'] == 'Unknown'

def test_get_user_data_empty_response():
    mock_response = {}
    # API returned but gave us nothing
    result = get_user_data(123)
    assert result['name'] == 'Unknown'