Python：使用 side_effect 模拟实例变量

Question

we currently try to mock an instance variable of a class object with a side_effect. The first time we access the attribute we want to get a different value as the second time. We test an async method and the value can change during the two accesses.

Here is a simple example:

from unittest import TestCase
from unittest.mock import PropertyMock, patch


class MyObject:

    def __init__(self):
        self.variable = None

    def my_func(self):
        print(self.variable)
        print(self.variable)


class TestObject(TestCase):

    def test_my_func(self):
        object = MyObject()
        with patch.object(object, "variable", new_callable=PropertyMock(side_effect=["test", "test2"])):
            object.my_func()

We would expect that this print at first test and then test2. But the output is:

test
test
.
----------------------------------------------------------------------
Ran 1 test in 0.000s

Answer 1

Maybe you could simplify your use-case with something like this:-

class MyClass():
    def __init__(self):
        self.__var = 0

    @property
    def var(self):
        rv = self.__var
        self.__var += 1
        return rv


MC = MyClass()
print(MC.var)
print(MC.var)

Every time MC.var is acquired using this technique it will be incremented by 1. Obviously a variation on your theme but may give you some ideas