bavaza bavaza - 4 months ago 22
Python Question

How do you mock patch a python class and get a new Mock object for each instantiation?

OK,

I know this is mentioned in the manual, and probably has to do with

side_effect
and/or
return_value
, but a simple, direct example will help me immensely.

I have:

class ClassToPatch():
def __init__(self, *args):
_do_some_init_stuff()

def some_func():
_do_stuff()


class UUT():
def __init__(self, *args)
resource_1 = ClassToPatch()
resource_2 = ClassToPatch()


Now, I want to unit test the
UUT
class, and mock the
ClassToPatch
. Knowing the
UUT
class will instantiate exactly two
ClassToPatch
objects, I want the Mock framework to return a new Mock object for each instantiation, so I can later assert calls on each separately.

How do I achieve this using the
@patch
decorator in a test case? Namely, how to fix the following code sample?

class TestCase1(unittest.TestCase):

@patch('classToPatch.ClassToPatch',autospec=True)
def test_1(self,mock1,mock2):
_assert_stuff()

Answer

Here's a quick'n'dirty example to get you going:

import mock
import unittest

class ClassToPatch():
   def __init__(self, *args):
       pass

   def some_func(self):
       return id(self)

class UUT():
    def __init__(self, *args):
        resource_1 = ClassToPatch()
        resource_2 = ClassToPatch()
        self.test_property = (resource_1.some_func(), resource_2.some_func())

class TestCase1(unittest.TestCase):
    @mock.patch('__main__.ClassToPatch', autospec = True)
    def test_1(self, mock1):
        ctpMocks = [mock.Mock(), mock.Mock()]
        ctpMocks[0].some_func.return_value = "funky"
        ctpMocks[1].some_func.return_value = "monkey"
        mock1.side_effect = ctpMocks

        u = UUT()
        self.assertEqual(u.test_property, ("funky", "monkey"))

if __name__ == '__main__':
    unittest.main()

I've added test_property to UUT so that the unit test does something useful. Now, without the mock test_property should be a tuple containing the ids of the two ClassToPatch instances. But with the mock it should be the tuple: ("funky", "monkey").

I've used the side_effect property of the mock object so that a different instance of ClassToPatch is returned on each call in the UUT initialiser.

Hope this helps.

Edit: Oh, by the way, when I run the unit test I get:

.
----------------------------------------------------------------------
Ran 1 test in 0.004s

OK
Comments