Mock attributes in Python mock?

Question

I'm having a fairly difficult time using mock in Python:

def method_under_test():
    r = requests.post("http://localhost/post")

    print r.ok # prints "<MagicMock name='post().ok' id='11111111'>"

    if r.ok:
       return StartResult()
    else:
       raise Exception()

class MethodUnderTestTest(TestCase):

    def test_method_under_test(self):
        with patch('requests.post') as patched_post:
            patched_post.return_value.ok = True

            result = method_under_test()

            self.assertEqual(type(result), StartResult,
                "Failed to return a StartResult.")

The test actually returns the right value, but r.ok is a Mock object, not True. How do you mock attributes in Python's mock library?

Solution 1

You need to use return_value and PropertyMock:

with patch('requests.post') as patched_post:
    type(patched_post.return_value).ok = PropertyMock(return_value=True)

This means: when calling requests.post, on the return value of that call, set a PropertyMock for the property ok to return the value True.

Solution 2

A compact and simple way to do it is to use new_callable patch's attribute to force patch to use PropertyMock instead of MagicMock to create the mock object. The other arguments passed to patch will be used to create PropertyMock object.

with patch('requests.post.ok', new_callable=PropertyMock, return_value=True) as mock_post:
    """Your test"""

Solution 3

With mock version '1.0.1' the simpler syntax mentioned in the question is supported and works as is!

Example code updated (py.test is used instead of unittest):

import mock
import requests


def method_under_test():
    r = requests.post("http://localhost/post")

    print r.ok

    if r.ok:
        return r.ok
    else:
        raise Exception()


def test_method_under_test():
    with mock.patch('requests.post') as patched_post:
        patched_post.return_value.ok = True

        result = method_under_test()
        assert result is True, "mock ok failed"

Run this code with: (make sure you install pytest)

$ py.test -s -v mock_attributes.py 
======= test session starts =======================
platform linux2 -- Python 2.7.10 -- py-1.4.30 -- pytest-2.7.2 -- /home/developer/miniconda/bin/python
rootdir: /home/developer/projects/learn/scripts/misc, inifile: 
plugins: httpbin, cov
collected 1 items 

mock_attributes.py::test_method_under_test True
PASSED

======= 1 passed in 0.03 seconds =================

Comments

[Naftuli Kay]

If I print the value of r.ok from in the method_under_test, I see <MagicMock name='post().ok' id='57360464'>, not True.

[Simeon Visser]

@TKKocheran: I have updated my answer. You also need to use a PropertyMock.

[philant]

This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post - you can always comment on your own posts, and once you have sufficient reputation you will be able to comment on any post.

[Michele d'Amico]

FYI requests.post.ok is a property and not an attribute. If you try on simple object where ok is a simple attribute the syntax mentioned in the question works but for requests.post.ok object no: it will raise an AttributeError.

[howaryoo]

@philant thanks for your feedback, as proved by the example this is the up-to-date answer to the question and the syntax happens to be much simpler.

[howaryoo]

@Micheled'Amico thanks for your feedback, I tried please take a look ;-)

[lumbric]

Why not simpy using patched_post.return_value = mock.Mock(ok=True)?

[Bono]

@lumbric Because you can use PropertyMock to assert that it was accessed like any other Mock object. By just assigning a value to the property you can't do that.