unittest.mock
- Python course
- Advanced
- Lesson 19/83
- 5 min
Learn mock objects with unittest.mock.Mock.
unittest.mock
unittest.mock is Python's built-in mocking library. It produces fake objects that replace real ones; tests then run free of external dependencies.
The most basic class is Mock. You create a fake object with Mock(); any attribute or method call on it returns another Mock by default.
from unittest.mock import Mock
m = Mock()
m.method.return_value = 42
print(m.method()) # 42With return_value you decide what a call returns. This is the simplest way to mimic behavior without hitting a real service.
Behavior with return_value
You set what a Mock returns for a call with return_value. It can be a fixed value, a function or another Mock.
Example: m = Mock(); m.return_value = 5. Then calling m() returns 5. For methods, write m.fetch.return_value = data; calling m.fetch() gives data.
Calling it multiple times returns the same value each time. You check the call count with m.call_count and the call arguments with m.call_args.
Practice Mock with a fake method
Import Mock from unittest.mock. Create m = Mock(); set m.fetch.return_value = 42. Call print(m.fetch()).
Starter code
# import Mock from unittest.mock
# create Mock m; set fetch method return value to 42
# print the result of fetch()Sample solution
from unittest.mock import Mock
m = Mock()
m.fetch.return_value = 42
print(m.fetch())Expected output
42
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does Mock().method() return by default?
- It raises an error
- None
- A new Mock object Correct answer
- The called method name
Explanation: A call on a Mock returns a new Mock by default.
2 Import Mock
from unittest.mock import ____
m = Mock()
m.method.return_value = 42from unittest.mock import Mock
m = Mock()
m.method.return_value = 42Explanation: The Mock class comes from the unittest.mock module.
3 Match the Mock pieces
| Term | Meaning |
|---|---|
Mock() |
Fake object |
return_value |
Return value |
call_count |
Call count |
call_args |
Call arguments |