How to patch a class in python unit test and get a handle on patched object's return value

Question

I am testing a class's method in Python 2.7 using Mock 2.0.0 Library

Here is how the method under test looks like:

from sklearn.externals import joblib class ClassUnderTest(): def MethodUnderTest(self, path, myDict): newDict= {} for key, val in myDict.iteritems(): retVal= (joblib.load(path + val)) newDict[key] = retVal return newDict

Now, my intention is to test MethodUnderTest but mock joblib.load and not call it in reality. So, to achieve this, I use the @patch decorator available in Mock library. My test looks as follows:

import unittest from mock import MagicMock, patch from sklearn.externals import joblib import ClassUnderTest

class TestClass(unittest.TestCase): @patch('ClassUnderTest.joblib') def test_MethodUnderTest(self, mockJoblibLoad): dict = {"v1": "p1.pkl"} retVal = ClassUnderTest.MethodUnderTest("whateverpath", dict)

Now if I have to assert on retVal's keys and values against something, that something is based on what the mocked return value of joblib.load is. If I knew that value somehow then, I would be able to know what MethodUnderTest returns.

The problem here is that I don't know what that mocked value of joblib.load is when it is mocked using @patch decorator.

Does someone know how to get around this problem ? Or if there is a better way to mock python libraries like joblib and their methods like load and get a handle of that mock object ?

Dan · Accepted Answer · 2017-06-15 04:31:01Z

class TestClass(unittest.TestCase): @patch('path.to.module.joblib.load') # You path is probably wrong here def test_MethodUnderTest(self, mockJoblibLoad): # Set the side_effect if you want to return different things on # each iteration e.g. mockJoblib.side_effect = [...] mockJoblibLoad.return_value = ... test_dict = {"v1": "p1.pkl"} expect = {"v1": mockJoblibLoad.return_value} actual = ClassUnderTest.MethodUnderTest("whateverpath", dict) self.assertEqual(expect, actual)

This worked fine. So what I had missing in my code was wrong path in the patch. I didn't have it all the way till .load i.e. the method name under joblib that I was trying to stub. And then, mockJoblibLoad.return_value = "somemockvalue" worked fine too. Thanks

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