Why does unittest.mock fail when the production class constructor takes extra arguments?

Question

I have run into a problem which I think might be a bug with the libraries I am using. However, I am fairly new to python, unittest, and unittest.mock libraries so this may just be a hole in my understanding.

While adding tests to some production code I have run into an error, I have produced a minimal sample that reproduces the issue:

import unittest
import mock

class noCtorArg:
    def __init__(self):
        pass
    def okFunc(self):
        raise NotImplemented


class withCtorArg:
    def __init__(self,obj):
        pass
    def notOkFunc(self):
        raise NotImplemented
    def okWithArgFunc(self, anArgForMe):
        raise NotImplemented

class BasicTestSuite(unittest.TestCase):
    """Basic test Cases."""

    # passes
    def test_noCtorArg_okFunc(self):
        mockSUT = mock.MagicMock(spec=noCtorArg)
        mockSUT.okFunc()
        mockSUT.assert_has_calls([mock.call.okFunc()])

    # passes
    def test_withCtorArg_okWithArgFuncTest(self):
        mockSUT = mock.MagicMock(spec=withCtorArg)
        mockSUT.okWithArgFunc("testing")
        mockSUT.assert_has_calls([mock.call.okWithArgFunc("testing")])

    # fails
    def test_withCtorArg_doNotOkFuncTest(self):
        mockSUT = mock.MagicMock(spec=withCtorArg)
        mockSUT.notOkFunc()
        mockSUT.assert_has_calls([mock.call.notOkFunc()])


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

How I run the tests and the output is as follows:

E:\work>python -m unittest testCopyFuncWithMock
.F.
======================================================================
FAIL: test_withCtorArg_doNotOkFuncTest (testCopyFuncWithMock.BasicTestSuite)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "testCopyFuncWithMock.py", line 38, in test_withCtorArg_doNotOkFuncTest
    mockSUT.assert_has_calls([mock.call.notOkFunc()])
  File "C:\Python27\lib\site-packages\mock\mock.py", line 969, in assert_has_calls
    ), cause)
  File "C:\Python27\lib\site-packages\six.py", line 718, in raise_from
    raise value
AssertionError: Calls not found.
Expected: [call.notOkFunc()]
Actual: [call.notOkFunc()]

----------------------------------------------------------------------
Ran 3 tests in 0.004s

FAILED (failures=1)

I am using python 2.7.11, with mock version 2.0.0 installed via pip.

Any suggestions for what I am doing wrong? Or does this look like a bug in the library?

Answer 1

Interestingly, the way you chose to perform the assert has masked your issue.

Try, instead of this:

mockSUT.assert_has_calls(calls=[mock.call.notOkFunc()])

to do this:

mockSUT.assert_has_calls(calls=[mock.call.notOkFunc()], any_order=True)

You'll see the actual exception:

TypeError("'obj' parameter lacking default value")

This is because you tried to instantiate an instance of the class withCtorArg that has the parameter obj with no default value. If you had tried to actually instantiate it directly, you would've seen:

TypeError: __init__() takes exactly 2 arguments (1 given)

However, since you let the mock library handle the instantiation of a mock object, the error happens there - and you get the TypeError exception.

Modifying the relevant class:

class withCtorArg:
    def __init__(self, obj = None):
        pass
    def notOkFunc(self):
        pass
    def okWithArgFunc(self, anArgForMe):
        pass

and adding a default None value for obj solves the issue.