How do I replace a class variable for the duration of a single test in Python?

Question

I'm trying to test a cache class where old items should be automatically deleted. The amount of items the cache can hold is controlled by a "constant" class variable like this:

class Cache(dict):
    MAX_ITEMS = 100

For testing purposes I would like to change that constant for the duration of a single test case. Using unittest.mock.patch I could replace the class variable with a MagicMock but that's not really what I'm trying to do. I just need to replace it with a different number.

Is there an elegant way to do this in Python (like patch), or should I just reset the variable myself on tearDown?

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_{I'm using Python 3.4.3}

Answer 1

patch can do this already. try:

from unittest.mock import patch

class A:
    val = 1

assert A.val == 1
with patch.object(A, "val", "patched_value"):
    assert A.val == "patched_value"

assert A.val == 1

Of course you can use patch.object as a decorator as well.