KlemensE KlemensE - 20 days ago 6
Python Question

Python mock class with function and property

I am having some trouble patching a class with a function and a property.

The project structure I am working with is as follows:

project
|- src
| |- logic
| | |- sub_logic
| | | | __init__.py
| | | | cache.py
| | | | manager.py
| | | __init__.py
|- test
| | test.py


My cache file looks like this

class Cache(object):
def __init__(self, val):
self._val = val

@property
def Val(self):
return self._val

def other_function(self):
return False


The manager file looks like this

from cache import Cache


class Manager(object):
def __init__(self):
self._cache = Cache(20)

def do_something(self):
if self._cache.Val != 20:
raise ValueError(u"Val is not 20")

return True

def do_something_else(self):
if self._cache.other_function():
raise ValueError(u"Something is True")


The tests I tried to make are the following:

from unittest import TestCase
from mock import PropertyMock, patch

from logic.sub_logic.manager import Manager
from logic.sub_logic.cache import Cache


class ManagerTestCase(TestCase):

def test_01_cache(self):
manager = Manager()
self.assertEqual(manager.do_something(), True)

@patch('logic.sub_logic.manager.Cache.Val', new_callable=PropertyMock)
def test_02_cache(self, property_mock):
property_mock.return_value = 20
manager = Manager()
self.assertEqual(manager.do_something(), True)

@patch('logic.sub_logic.manager.Cache', spec=Cache)
def test_03_cache(self, cache_mock):
cache_mock.other_function.return_value = True
manager = Manager()
with self.assertRaises(ValueError):
manager.do_something_else()

@patch('logic.sub_logic.manager.Cache', spec=Cache)
def test_04_cache(self, cache_mock):
cache_mock.other_function.return_value = True
cache_mock.Val = PropertyMock()
cache_mock.Val.return_value = 20

manager = Manager()
with self.assertRaises(ValueError):
manager.do_something_else()
self.assertEqual(manager.do_something(), True)

@patch('logic.sub_logic.manager.Cache.Val', new_callable=PropertyMock)
@patch('logic.sub_logic.manager.Cache', spec=Cache)
def test_05_cache(self, cache_mock, property_mock):
cache_mock.other_function.return_value = True
property_mock.return_value = 20
manager = Manager()
with self.assertRaises(ValueError):
manager.do_something_else()
self.assertEqual(manager.do_something(), True)

@patch('logic.sub_logic.manager.Cache', spec=Cache)
def test_06_cache(self, cache_mock):
cache_mock.other_function.return_value = True
cache_mock.Val = 20

manager = Manager()
with self.assertRaises(ValueError):
manager.do_something_else()
self.assertEqual(manager.do_something(), True)


The problem is that test_04_cache and test_05_cache are not working.
When debugging the the tests, the mock parameter provided is as I expected it to be. But the Manager creates a MagicMock where the property Val is not a PropertyMock but also a MagicMock.

I inspected test_06_cache in PyCharm Debugger which reports the following:


  • cache_mock.Val = {int}20

  • manager._cache.Val = {MagicMock}<MagicMock name='Cache().Val' id='61044848'>



Am I missing something? Or is it not possible?

Answer

When you use

@patch('logic.sub_logic.manager.Cache', spec=Cache)

the resulting mock is for the class. Your Manager then creates an instance by calling that class, in __init__. You should therefore be setting attributes and return values on mock_cache() (note parentheses), which is the "instance" that will be assigned to manager._cache, rather than on the "class" mock_cache.

Note that, as the manager doesn't know that the cache is using a @property, you can just set:

mock_cache().Val = 20
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