Mock Return Values & Side Effects

Mock Return Values & Side Effects

Control what mocks return and how they behave using return_value, side_effect, and related attributes. This lets you simulate various scenarios in your tests.

return_value

from unittest.mock import Mock

mock_db = Mock()
mock_db.fetch_user.return_value = {"id": 1, "name": "Alice"}

result = mock_db.fetch_user(1)
print(result)  # {"id": 1, "name": "Alice"}

# For chained calls
mock_db.fetch_user.return_value.name = "Bob"
print(mock_db.fetch_user(1).name)  # "Bob"

side_effect for Exceptions

from unittest.mock import Mock, side_effect

# Simulate an exception
mock_api = Mock()
mock_api.get.side_effect = ConnectionError("Network timeout")

with self.assertRaises(ConnectionError):
    mock_api.get("/users")

# Verify the exception message
with self.assertRaises(ConnectionError) as ctx:
    mock_api.get("/users")
self.assertEqual(str(ctx.exception), "Network timeout")

side_effect for Multiple Values

mock_api = Mock()
mock_api.get.side_effect = [
    {"status": 200, "data": "first"},
    {"status": 200, "data": "second"},
    ConnectionError("timeout"),
]

print(mock_api.get())  # {"status": 200, "data": "first"}
print(mock_api.get())  # {"status": 200, "data": "second"}
# mock_api.get() raises ConnectionError

# Use a function for dynamic responses
def dynamic_response(url):
    if "users" in url:
        return {"users": []}
    return {"error": "not found"}

mock_api.get.side_effect = dynamic_response

call_count and call_args

mock_service = Mock()
mock_service.process()

self.assertEqual(mock_service.process.call_count, 1)
mock_service.process.assert_called_once()

mock_service.process("arg1", key="value")
mock_service.process.assert_called_with("arg1", key="value")

# Access call arguments
print(mock_service.process.call_args)
# call('arg1', key='value')

Key Takeaway

Use return_value for static responses, side_effect for exceptions or dynamic behavior, and call_args/call_count to verify how mocks were called.