isinstance and Mocking

Question

class HelloWorld(object):
    def say_it(self):
        return 'Hello I am Hello World'

def i_call_hello_world(hw_obj):
    print 'here... check type: %s' %type(HelloWorld)
    if isinstance(hw_obj, HelloWorld):
        print hw_obj.say_it()

from mock import patch, MagicMock
import unittest

class TestInstance(unittest.TestCase):
    @patch('__main__.HelloWorld', spec=HelloWorld)
    def test_mock(self,MK):
        print type(MK)
        MK.say_it.return_value = 'I am fake'
        v = i_call_hello_world(MK)
        print v

if __name__ == '__main__':
    c = HelloWorld()
    i_call_hello_world(c)
    print isinstance(c, HelloWorld)
    unittest.main()

Here is the traceback

here... check type: <type 'type'>
Hello I am Hello World
True
<class 'mock.MagicMock'>
here... check type: <class 'mock.MagicMock'>
E
======================================================================
ERROR: test_mock (__main__.TestInstance)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/mock.py", line 1224, in patched
    return func(*args, **keywargs)
  File "t.py", line 18, in test_mock
    v = i_call_hello_world(MK)
  File "t.py", line 7, in i_call_hello_world
    if isinstance(hw_obj, HelloWorld):
TypeError: isinstance() arg 2 must be a class, type, or tuple of classes and types

----------------------------------------------------------------------
Ran 1 test in 0.002s

Q1. Why is this error thrown? They are <class type='MagicMock>

Q2. How do I pause the mocking so that the first line will pass if the error is fixed?

From the docs :

Normally the __class__ attribute of an object will return its type. For a mock object with a spec, __class__ returns the spec class instead. This allows mock objects to pass isinstance() tests for the object they are replacing / masquerading as:

mock = Mock(spec=3)
isinstance(mock, int)
True

Answer 1

class HelloWorld(object):
    def say_it(self):
        return 'Hello I am Hello World'

def i_call_hello_world(hw_obj):
    print 'here... check type: %s' %type(HelloWorld)
    if isinstance(hw_obj, HelloWorld):
        print hw_obj.say_it()

from mock import patch, MagicMock
import unittest

class TestInstance(unittest.TestCase):
    @patch('__main__.HelloWorld', spec=HelloWorld)
    def test_mock(self,MK):
        print type(MK)
        MK.say_it.return_value = 'I am fake'
        v = i_call_hello_world(MK)
        print v

if __name__ == '__main__':
    c = HelloWorld()
    i_call_hello_world(c)
    print isinstance(c, HelloWorld)
    unittest.main()

Here is the traceback

here... check type: <type 'type'>
Hello I am Hello World
True
<class 'mock.MagicMock'>
here... check type: <class 'mock.MagicMock'>
E
======================================================================
ERROR: test_mock (__main__.TestInstance)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/mock.py", line 1224, in patched
    return func(*args, **keywargs)
  File "t.py", line 18, in test_mock
    v = i_call_hello_world(MK)
  File "t.py", line 7, in i_call_hello_world
    if isinstance(hw_obj, HelloWorld):
TypeError: isinstance() arg 2 must be a class, type, or tuple of classes and types

----------------------------------------------------------------------
Ran 1 test in 0.002s

Q1. Why is this error thrown? They are <class type='MagicMock>

Q2. How do I pause the mocking so that the first line will pass if the error is fixed?

From the docs :

Normally the __class__ attribute of an object will return its type. For a mock object with a spec, __class__ returns the spec class instead. This allows mock objects to pass isinstance() tests for the object they are replacing / masquerading as:

mock = Mock(spec=3)
isinstance(mock, int)
True

Answer 2

class HelloWorld(object):
    def say_it(self):
        return 'Hello I am Hello World'

def i_call_hello_world(hw_obj):
    print 'here... check type: %s' %type(HelloWorld)
    if isinstance(hw_obj, HelloWorld):
        print hw_obj.say_it()

from mock import patch, MagicMock
import unittest

class TestInstance(unittest.TestCase):
    @patch('__main__.HelloWorld', spec=HelloWorld)
    def test_mock(self,MK):
        print type(MK)
        MK.say_it.return_value = 'I am fake'
        v = i_call_hello_world(MK)
        print v

if __name__ == '__main__':
    c = HelloWorld()
    i_call_hello_world(c)
    print isinstance(c, HelloWorld)
    unittest.main()

Here is the traceback

here... check type: <type 'type'>
Hello I am Hello World
True
<class 'mock.MagicMock'>
here... check type: <class 'mock.MagicMock'>
E
======================================================================
ERROR: test_mock (__main__.TestInstance)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/mock.py", line 1224, in patched
    return func(*args, **keywargs)
  File "t.py", line 18, in test_mock
    v = i_call_hello_world(MK)
  File "t.py", line 7, in i_call_hello_world
    if isinstance(hw_obj, HelloWorld):
TypeError: isinstance() arg 2 must be a class, type, or tuple of classes and types

----------------------------------------------------------------------
Ran 1 test in 0.002s

Q1. Why is this error thrown? They are <class type='MagicMock>

Q2. How do I pause the mocking so that the first line will pass if the error is fixed?

From the docs :

Normally the __class__ attribute of an object will return its type. For a mock object with a spec, __class__ returns the spec class instead. This allows mock objects to pass isinstance() tests for the object they are replacing / masquerading as:

mock = Mock(spec=3)
isinstance(mock, int)
True

Answer 3

class HelloWorld(object):
    def say_it(self):
        return 'Hello I am Hello World'

def i_call_hello_world(hw_obj):
    print 'here... check type: %s' %type(HelloWorld)
    if isinstance(hw_obj, HelloWorld):
        print hw_obj.say_it()

from mock import patch, MagicMock
import unittest

class TestInstance(unittest.TestCase):
    @patch('__main__.HelloWorld', spec=HelloWorld)
    def test_mock(self,MK):
        print type(MK)
        MK.say_it.return_value = 'I am fake'
        v = i_call_hello_world(MK)
        print v

if __name__ == '__main__':
    c = HelloWorld()
    i_call_hello_world(c)
    print isinstance(c, HelloWorld)
    unittest.main()

Here is the traceback

here... check type: <type 'type'>
Hello I am Hello World
True
<class 'mock.MagicMock'>
here... check type: <class 'mock.MagicMock'>
E
======================================================================
ERROR: test_mock (__main__.TestInstance)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/mock.py", line 1224, in patched
    return func(*args, **keywargs)
  File "t.py", line 18, in test_mock
    v = i_call_hello_world(MK)
  File "t.py", line 7, in i_call_hello_world
    if isinstance(hw_obj, HelloWorld):
TypeError: isinstance() arg 2 must be a class, type, or tuple of classes and types

----------------------------------------------------------------------
Ran 1 test in 0.002s

Q1. Why is this error thrown? They are <class type='MagicMock>

Q2. How do I pause the mocking so that the first line will pass if the error is fixed?

From the docs :

Normally the __class__ attribute of an object will return its type. For a mock object with a spec, __class__ returns the spec class instead. This allows mock objects to pass isinstance() tests for the object they are replacing / masquerading as:

mock = Mock(spec=3)
isinstance(mock, int)
True

Answer 4

class HelloWorld(object):
    def say_it(self):
        return 'Hello I am Hello World'

def i_call_hello_world(hw_obj):
    print 'here... check type: %s' %type(HelloWorld)
    if isinstance(hw_obj, HelloWorld):
        print hw_obj.say_it()

from mock import patch, MagicMock
import unittest

class TestInstance(unittest.TestCase):
    @patch('__main__.HelloWorld', spec=HelloWorld)
    def test_mock(self,MK):
        print type(MK)
        MK.say_it.return_value = 'I am fake'
        v = i_call_hello_world(MK)
        print v

if __name__ == '__main__':
    c = HelloWorld()
    i_call_hello_world(c)
    print isinstance(c, HelloWorld)
    unittest.main()

Here is the traceback

here... check type: <type 'type'>
Hello I am Hello World
True
<class 'mock.MagicMock'>
here... check type: <class 'mock.MagicMock'>
E
======================================================================
ERROR: test_mock (__main__.TestInstance)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/mock.py", line 1224, in patched
    return func(*args, **keywargs)
  File "t.py", line 18, in test_mock
    v = i_call_hello_world(MK)
  File "t.py", line 7, in i_call_hello_world
    if isinstance(hw_obj, HelloWorld):
TypeError: isinstance() arg 2 must be a class, type, or tuple of classes and types

----------------------------------------------------------------------
Ran 1 test in 0.002s

Q1. Why is this error thrown? They are <class type='MagicMock>

Q2. How do I pause the mocking so that the first line will pass if the error is fixed?

From the docs :

Normally the __class__ attribute of an object will return its type. For a mock object with a spec, __class__ returns the spec class instead. This allows mock objects to pass isinstance() tests for the object they are replacing / masquerading as:

mock = Mock(spec=3)
isinstance(mock, int)
True

Answer 5

class HelloWorld(object):
    def say_it(self):
        return 'Hello I am Hello World'

def i_call_hello_world(hw_obj):
    print 'here... check type: %s' %type(HelloWorld)
    if isinstance(hw_obj, HelloWorld):
        print hw_obj.say_it()

from mock import patch, MagicMock
import unittest

class TestInstance(unittest.TestCase):
    @patch('__main__.HelloWorld', spec=HelloWorld)
    def test_mock(self,MK):
        print type(MK)
        MK.say_it.return_value = 'I am fake'
        v = i_call_hello_world(MK)
        print v

if __name__ == '__main__':
    c = HelloWorld()
    i_call_hello_world(c)
    print isinstance(c, HelloWorld)
    unittest.main()

Here is the traceback

here... check type: <type 'type'>
Hello I am Hello World
True
<class 'mock.MagicMock'>
here... check type: <class 'mock.MagicMock'>
E
======================================================================
ERROR: test_mock (__main__.TestInstance)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/mock.py", line 1224, in patched
    return func(*args, **keywargs)
  File "t.py", line 18, in test_mock
    v = i_call_hello_world(MK)
  File "t.py", line 7, in i_call_hello_world
    if isinstance(hw_obj, HelloWorld):
TypeError: isinstance() arg 2 must be a class, type, or tuple of classes and types

----------------------------------------------------------------------
Ran 1 test in 0.002s

Q1. Why is this error thrown? They are <class type='MagicMock>

Q2. How do I pause the mocking so that the first line will pass if the error is fixed?

From the docs :

Normally the __class__ attribute of an object will return its type. For a mock object with a spec, __class__ returns the spec class instead. This allows mock objects to pass isinstance() tests for the object they are replacing / masquerading as:

mock = Mock(spec=3)
isinstance(mock, int)
True

Answer 6

class HelloWorld(object):
    def say_it(self):
        return 'Hello I am Hello World'

def i_call_hello_world(hw_obj):
    print 'here... check type: %s' %type(HelloWorld)
    if isinstance(hw_obj, HelloWorld):
        print hw_obj.say_it()

from mock import patch, MagicMock
import unittest

class TestInstance(unittest.TestCase):
    @patch('__main__.HelloWorld', spec=HelloWorld)
    def test_mock(self,MK):
        print type(MK)
        MK.say_it.return_value = 'I am fake'
        v = i_call_hello_world(MK)
        print v

if __name__ == '__main__':
    c = HelloWorld()
    i_call_hello_world(c)
    print isinstance(c, HelloWorld)
    unittest.main()

Here is the traceback

here... check type: <type 'type'>
Hello I am Hello World
True
<class 'mock.MagicMock'>
here... check type: <class 'mock.MagicMock'>
E
======================================================================
ERROR: test_mock (__main__.TestInstance)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/mock.py", line 1224, in patched
    return func(*args, **keywargs)
  File "t.py", line 18, in test_mock
    v = i_call_hello_world(MK)
  File "t.py", line 7, in i_call_hello_world
    if isinstance(hw_obj, HelloWorld):
TypeError: isinstance() arg 2 must be a class, type, or tuple of classes and types

----------------------------------------------------------------------
Ran 1 test in 0.002s

Q1. Why is this error thrown? They are <class type='MagicMock>

Q2. How do I pause the mocking so that the first line will pass if the error is fixed?

From the docs :

Normally the __class__ attribute of an object will return its type. For a mock object with a spec, __class__ returns the spec class instead. This allows mock objects to pass isinstance() tests for the object they are replacing / masquerading as:

mock = Mock(spec=3)
isinstance(mock, int)
True

Answer 7

class HelloWorld(object):
    def say_it(self):
        return 'Hello I am Hello World'

def i_call_hello_world(hw_obj):
    print 'here... check type: %s' %type(HelloWorld)
    if isinstance(hw_obj, HelloWorld):
        print hw_obj.say_it()

from mock import patch, MagicMock
import unittest

class TestInstance(unittest.TestCase):
    @patch('__main__.HelloWorld', spec=HelloWorld)
    def test_mock(self,MK):
        print type(MK)
        MK.say_it.return_value = 'I am fake'
        v = i_call_hello_world(MK)
        print v

if __name__ == '__main__':
    c = HelloWorld()
    i_call_hello_world(c)
    print isinstance(c, HelloWorld)
    unittest.main()

Here is the traceback

here... check type: <type 'type'>
Hello I am Hello World
True
<class 'mock.MagicMock'>
here... check type: <class 'mock.MagicMock'>
E
======================================================================
ERROR: test_mock (__main__.TestInstance)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/mock.py", line 1224, in patched
    return func(*args, **keywargs)
  File "t.py", line 18, in test_mock
    v = i_call_hello_world(MK)
  File "t.py", line 7, in i_call_hello_world
    if isinstance(hw_obj, HelloWorld):
TypeError: isinstance() arg 2 must be a class, type, or tuple of classes and types

----------------------------------------------------------------------
Ran 1 test in 0.002s

Q1. Why is this error thrown? They are <class type='MagicMock>

Q2. How do I pause the mocking so that the first line will pass if the error is fixed?

From the docs :

Normally the __class__ attribute of an object will return its type. For a mock object with a spec, __class__ returns the spec class instead. This allows mock objects to pass isinstance() tests for the object they are replacing / masquerading as:

mock = Mock(spec=3)
isinstance(mock, int)
True

Answer 8

I guess the possible solution can be the checking of the subclass of object.

issubclass(hw_obj.__class__, HelloWorld)

Example:

from unittest.mock import patch, MagicMock
import unittest


class HelloWorld(object):
    def say_it(self):
        return 'Hello I am Hello World'


def i_call_hello_world(hw_obj):
    print('here... check type: %s' % type(HelloWorld))
    if isinstance(hw_obj, HelloWorld) or issubclass(hw_obj.__class__, HelloWorld):
        print(hw_obj.say_it())


class TestInstance(unittest.TestCase):
    @patch('__main__.isinstance', return_value=True)
    def test_mock(self, MK):
        print(type(MK))
        MK.say_it.return_value = 'I am fake'
        v = i_call_hello_world(MK)
        print(v)
        self.assertTrue(MK.say_it.called)

    @patch('__main__.isinstance', return_value=False)
    def test_not_call(self, MK):
        print(type(MK))
        MK.say_it.return_value = 'I am fake'
        v = i_call_hello_world(MK)
        print(v)
        self.assertFalse(MK.say_it.called)


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

Answer 9

I came to the same problem. According to this answer https://stackoverflow.com/a/49718531/11277611

You can't mock the second argument of isinstance()

I don't know the solution but I know a good workaround:
It's good architecture point to hind all checks (like isintanse ) inside separate functions because it's not directly related dependency, in most cases you will change the checks but preserve the behavior.
Put isinstance check into separate function and check it separately.

def logic_checks() -> bool:
    ...
    return isinstance(mock, MyMock) 


def main_func()
    if logic_checks() is not True:
        ...
    ...


def test_main_func()
    logic_checks = Mock(return_value=True)
    main_func()
    logic_checks.assert_called_once_with()
    ...


def test_logic_checks()
    ...  # Here separate checks with another patches, mocks, etc.

Answer 10

class HelloWorld(object):
    def say_it(self):
        return 'Hello I am Hello World'

def i_call_hello_world(hw_obj):
    print 'here... check type: %s' %type(HelloWorld)
    if isinstance(hw_obj, HelloWorld):
        print hw_obj.say_it()

from mock import patch, MagicMock
import unittest

class TestInstance(unittest.TestCase):
    @patch('__main__.HelloWorld', spec=HelloWorld)
    def test_mock(self,MK):
        print type(MK)
        MK.say_it.return_value = 'I am fake'
        v = i_call_hello_world(MK)
        print v

if __name__ == '__main__':
    c = HelloWorld()
    i_call_hello_world(c)
    print isinstance(c, HelloWorld)
    unittest.main()

Here is the traceback

here... check type: <type 'type'>
Hello I am Hello World
True
<class 'mock.MagicMock'>
here... check type: <class 'mock.MagicMock'>
E
======================================================================
ERROR: test_mock (__main__.TestInstance)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/mock.py", line 1224, in patched
    return func(*args, **keywargs)
  File "t.py", line 18, in test_mock
    v = i_call_hello_world(MK)
  File "t.py", line 7, in i_call_hello_world
    if isinstance(hw_obj, HelloWorld):
TypeError: isinstance() arg 2 must be a class, type, or tuple of classes and types

----------------------------------------------------------------------
Ran 1 test in 0.002s

Q1. Why is this error thrown? They are <class type='MagicMock>

Q2. How do I pause the mocking so that the first line will pass if the error is fixed?

From the docs :

Normally the __class__ attribute of an object will return its type. For a mock object with a spec, __class__ returns the spec class instead. This allows mock objects to pass isinstance() tests for the object they are replacing / masquerading as:

mock = Mock(spec=3)
isinstance(mock, int)
True