How does @mock.patch know which parameter to use for each mock object?

Question

Looking at this webpage: http://www.toptal.com/python/an-introduction-to-mocking-in-python -- The author talks about Mocking and Patching in Python and gives a pretty solid \"re

Answer 1

When applying a decorator, it is good to look at it like this

<wrapper1>
    <wrapper2>
        <wrapper3>
           **Your Function**
        </wrapper3>
    </wrapper2>
</wrapper1>

Basically your function is required to interact with the wrappers in this order:

wrapper3-> wrapper2->wrapper1

@wrapper1
@wrapper2
@wrapper3
def your_func(wrapper1.input, wrapper2.input, wrapper3.input):

NOTE wrapper1.input isn't how you would actually reference its input

To answer the second part of your question, how mymodule.os knows to refer to os. When Patching you are effectively intercepting calls to that specific name. When you call os in mymodule you are effectively calling mymodule.os. When patching you must refer to the class that is being mocked by the way it is being called in the actual code, not from the test modules perspective

Answer 2

it goes by the order of the execution of the decorators and that is also the order of the parameters passed on to your test method...

order of decorators execution is shown here: https://thadeusb.com/weblog/2010/08/23/python_multiple_decorators/

When you use patch the way you wrote it, a Mock instance it is automatically created for you and passed as a parameter to your test method. there is another version of it:

@mock.patch("subprocess.check_output", mock.MagicMock(return_value='True'))
def test_mockCheckOutput(self):
    self.assertTrue(subprocess.check_output(args=[])=='True')

in this case you pass your own Mock object and in this example, when you call subprocess.check_output(), it will return 'True'

you could however do:

def test_mockCheckOutput(self):
    m = mock.MagicMock(return_value='True')
    with mock.patch("subprocess.check_output", m):
        self.assertTrue(subprocess.check_output(args=[])=='True')

and in this case you can pass any mock item you want because it will be evaluated during runtime... :)