Question

I am using mock with Python and was wondering which of those two approaches is better (read: more pythonic).

Method one: Just create a mock object and use that. The code looks like:

def test_one (self):
    mock = Mock()
    mock.method.return_value = True 
    
    # This should call mock.method and check the result. 
    self.sut.something(mock) 

    self.assertTrue(mock.method.called)

Method two: Use patch to create a mock. The code looks like:

@patch("MyClass")
def test_two (self, mock):
    instance = mock.return_value
    instance.method.return_value = True
    
    # This should call mock.method and check the result.
    self.sut.something(instance) 

    self.assertTrue(instance.method.called)

Both methods do the same thing. I am unsure of the differences.

Could anyone enlighten me?

Answer 1

mock.patch is a very very different critter than mock.Mock. patch replaces the class with a mock object and lets you work with the mock instance. Take a look at this snippet:

>>> class MyClass(object):
...   def __init__(self):
...     print 'Created MyClass@{0}'.format(id(self))
... 
>>> def create_instance():
...   return MyClass()
... 
>>> x = create_instance()
Created MyClass@4299548304
>>> 
>>> @mock.patch('__main__.MyClass')
... def create_instance2(MyClass):
...   MyClass.return_value = 'foo'
...   return create_instance()
... 
>>> i = create_instance2()
>>> i
'foo'
>>> def create_instance():
...   print MyClass
...   return MyClass()
...
>>> create_instance2()
<mock.Mock object at 0x100505d90>
'foo'
>>> create_instance()
<class '__main__.MyClass'>
Created MyClass@4300234128
<__main__.MyClass object at 0x100505d90>

patch replaces MyClass in a way that allows you to control the usage of the class in functions that you call. Once you patch a class, references to the class are completely replaced by the mock instance.

mock.patch is usually used when you are testing something that creates a new instance of a class inside of the test. mock.Mock instances are clearer and are preferred. If your self.sut.something method created an instance of MyClass instead of receiving an instance as a parameter, then mock.patch would be appropriate here.