MagicMock

Learn magic-method mock with 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))  # 3

This 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.

Import MagicMock from unittest.mock. Create m = MagicMock(). Set m.__len__.return_value = 3; call print(len(m)).

Show example solution

from unittest.mock import MagicMock

m = MagicMock()
m.__len__.return_value = 3
print(len(m))

Import MagicMock

from unittest.mock import ____

m = MagicMock()
m.__len__.return_value = 3
print(len(m))

Show answer

from unittest.mock import MagicMock

m = MagicMock()
m.__len__.return_value = 3
print(len(m))

Next lesson: side_effect