isinstance and Mocking

IMHO this is a good question and saying "don't use isinstance, use duck typing instead" is a bad answer. Duck typing is great, but not a silver bullet. Sometimes isinstance is necessary, even if it is not pythonic. For instance, if you work with some library or legacy code that isn't pythonic you must play with isinstance. It is just the real world and mock was designed to fit this kind of work.

In the code the big mistake is when you write:

@patch('__main__.HelloWorld', spec=HelloWorld)
def test_mock(self,MK):

From patch documentation we read (emphasize is mine):

Inside the body of the function or with statement, the target is patched with a new object.

That means when you patch the HelloWorld class object the reference to HelloWorld will be replaced by a MagicMock object for the context of the test_mock() function.

Then, when i_call_hello_world() is executed in if isinstance(hw_obj, HelloWorld): HelloWorld is a MagicMock() object and not a class (as the error suggests).

That behavior is because as a side effect of patching a class reference the 2nd argument of isinstance(hw_obj, HelloWorld) becomes an object (a MagicMock instance). This is neither a class or a type. A simple experiment to understand this behavior is to modify i_call_hello_world() as follows:

HelloWorld_cache = HelloWorld

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

The error will disappear because the original reference to HelloWorld class is saved in HelloWorld_cache when you load the module. When the patch is applied it will change just HelloWorld and not HelloWorld_cache.

Unfortunately, the previous experiment doesn't give us any way to play with cases like yours because you cannot change the library or legacy code to introduce a trick like this. Moreover, these are that kind of tricks that we would like to never see in our code.

The good news is that you can do something ,but you cannot just patch the HelloWord reference in the module where you have isinstace(o,HelloWord) code to test. The best way depends on the real case that you must solve. In your example you can just create a Mock to use as HelloWorld object, use spec argument to dress it as HelloWorld instance and pass the isinstance test. This is exactly one of the aims for which spec is designed. Your test would be written like this:

def test_mock(self):
    MK = MagicMock(spec=HelloWorld) #The hw_obj passed to i_call_hello_world
    print type(MK)
    MK.say_it.return_value = 'I am fake'
    v = i_call_hello_world(MK)
    print v

And the output of just unittest part is

<class 'mock.MagicMock'>
here... check type: <type 'type'>
I am fake
None

Michele d'Amico provides the correct answer in my view and I strongly recommend reading it. But it took me a while a grok and, as I'm sure I'll be coming back to this question in the future, I thought a minimal code example would help clarify the solution and provide a quick reference:

from mock import patch, mock

class Foo(object): pass

# Cache the Foo class so it will be available for isinstance assert.
FooCache = Foo

with patch('__main__.Foo', spec=Foo):
    foo = Foo()
    assert isinstance(foo, FooCache)
    assert isinstance(foo, mock.mock.NonCallableMagicMock)

    # This will cause error from question:
    # TypeError: isinstance() arg 2 must be a class, type, or tuple of classes and types
    assert isinstance(foo, Foo)

Tags:

Python

Unit Testing

Mocking

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