如何在Python中单个测试的持续时间内替换类变量？

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 ?

---

_{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.

Answer 2

Reading the docs more carefully, it turns out patch actually has an optional argument that controls what the variable is replaced by (instead of a mock object), so I can do this:

class CacheTest(TestCase):
    @patch("myproject.Cache.MAX_ITEMS", new=3)
    def testCacheTrim(self):
        # ...

Or, as Dunes pointed out:

class CacheTest(TestCase):
    @patch.object(Cache, "MAX_ITEMS", new=3)
    def testCacheTrim(self):
        # ...

Which arguably looks prettier.

Answer 3

You could consider providing a class constructor that sets MAX_ITEMS. Something like:

class Cache(dict):
    def __init__(self, *args, **kwargs):
        self.MAX_ITEMS = kwargs.get("MAX_ITEMS", 100)  # default value

    @classmethod
    def create_with_max(cls, MAX_ITEMS, *args, **kwargs):
        return cls(MAX_ITEMS=MAX_ITEMS, *args, **kwargs)

>>> c = Cache.create_with_max(40)
>>> c.MAX_ITEMS
40

This would allow you to set the cache size (during testing or otherwise)