How to Write a Contract Test with Mock in Python

from unittest.mock import Mock

# Contract test: verify consumer expects data shape that provider delivers.
# We mock the provider and assert the consumer's calls match the agreed contract.

def fetch_user(provider_client, user_id):
    """Consumer code: expects provider to return {'id', 'name', 'email'}."""
    response = provider_client.get_user(user_id)
    if not response or not all(k in response for k in ('id', 'name', 'email')):
        raise ValueError(f"Provider contract violated for user {user_id}")
    return response

def test_consumer_contract():
    # Mock the provider with a valid contract-shaped response
    mock_provider = Mock()
    mock_provider.get_user.return_value = {
        'id': 42,
        'name': 'Alice',
        'email': 'alice@example.com'
    }

    result = fetch_user(mock_provider, 42)

    # Verify the consumer made exactly the expected call (contract on request)
    mock_provider.get_user.assert_called_once_with(42)
    assert result == {'id': 42, 'name': 'Alice', 'email': 'alice@example.com'}
    print("Consumer contract test passed: provider response matches expectation.")

if __name__ == "__main__":
    test_consumer_contract()