MagicMock
- Python course
- Advanced
- Lesson 21/83
- 5 min
Learn magic-method mock with MagicMock.
MagicMock
MagicMock extends Mock to also support magic (dunder) methods. It can mimic the behavior of __len__, __iter__, __getitem__ and the rest.
The plain Mock raises errors or returns odd results when these methods are called. MagicMock makes them controllable.
from unittest.mock import MagicMock
m = MagicMock()
m.__len__.return_value = 3
print(len(m)) # 3This is perfect for mimicking list-like or dict-like objects.
List mimicry with MagicMock
With MagicMock you easily mimic list-like behavior. You set length with __len__ and indexing with __getitem__.
Example: m.__len__.return_value = 2 and m.__getitem__.return_value = "a". Then len(m) returns 2; m[0] returns "a".
This lets you test list interfaces without a real data source. Control over magic methods makes the fake realistic.
Practice Length mimicry with MagicMock
Import MagicMock from unittest.mock. Create m = MagicMock(). Set m.__len__.return_value = 3; call print(len(m)).
Starter code
# import MagicMock from unittest.mock
# create MagicMock m; set __len__ return value to 3
# print len(m)Sample solution
from unittest.mock import MagicMock
m = MagicMock()
m.__len__.return_value = 3
print(len(m))Expected output
3
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match the MagicMock uses
| Term | Meaning |
|---|---|
MagicMock() |
Fake with magic methods |
m.__len__ |
len() behavior |
m.__iter__ |
for-loop behavior |
m.__getitem__ |
index behavior |
2 Import MagicMock
from unittest.mock import ____
m = MagicMock()
m.__len__.return_value = 3
print(len(m))from unittest.mock import MagicMock
m = MagicMock()
m.__len__.return_value = 3
print(len(m))Explanation: The MagicMock class comes from the unittest.mock module.
3 Which method call causes TROUBLE with the plain Mock?
- len(m) Correct answer
- m.attr
- m.normal_method()
- m()
Explanation: len() calls the __len__ magic method; the plain Mock does not handle it well. MagicMock is needed.