How in the world do you Mock a name attribute?

Published on March 14, 2013, 10:43 p.m.

mock python testing

Or... My adventures with Mock. Part 1.

I've been working a lot with Mock lately (and by lately, I meand for the last three months). Though it takes a while to wrap your head around it, it's an amazing and powerful testing tool.

To get started, let's look at some of the neat things you can do with Mock. Take this class, for example:

class Thing(object):
    shape = 'square'
    color = 'blue'

    def calculate(self):
        # ... do some stuff ...

If you were writing a test, and you wanted a Mocked Thing, but still wanted to ensure that your mock object had shape and color attributes, you'd do the following:

>>> from mock import Mock
>>> thing = Mock(shape='square', color='blue')
>>> thing.shape
'square'
>>> thing.color
'blue'

Cool! You get attributes whose values you can test against, and you still have a mock object on which you can call methods:

>>> from mock import Mock
>>> thing.calculate()  # pretend this calculates something
<Mock name='mock.calculate()' id='4338034768'>

I think Mock really shines when you're working with code that hits external APIs (I do this a lot with Work for Pie). Imagine for a minute that the Thing.calculate() sent some data up to an external API, then used the results to calculate and return a value. With a Mocked object, your tests can run without hitting the api. This is a Good Thing! In order to write that test, you'd do somethign like this

from mock import calls

def test_calculate():
    thing = Mock(shape='square', color='blue')
    thing.calculate()
    thing.assert_has_calls([call.calculate()])

So what was that thing about name attributes?

Now here's where things get tricky. For one reason or another, many of my API-wrapper classes have a name attribute. The Mock class also has a keyword argument name, that lets you give the Mock a name (the docs say this is useful for debugging).

So, how in the world am I supposed to write a Mock for something like this, and still be able to specify the value of an attribute?

class SomeAPIWrapper(object):
    name = 'brad'
    token = 'secret'

Well, this does not work:

>>> api = Mock(name='brad', token='secret')
>>> api.token  # Ok, this looks fine.
'secret'
>>> api.name  # not what I want
<Mock name='important-thing-here.name' id='4337316944'>

Luckily, there's this neat class called a PropertyMock. It took me a bit to figure out how to use it, but it's essentially used as a property or an attribute on another class (including another mock). I honestly don't know if this is supposed to work this way or if this is a nasty hack (feel free to let me know one way or another), but this is how I attached a name attribute to a Mock object:

>>> from mock import Mock, PropertyMock
>>> # Create a Mock for my ``SomeAPIWrapper`` class above
>>> api = Mock(token='secret')
>>> # Create a PropertyMock representing the ``name`` attribute
>>> p = PropertyMock(return_value='brad')
>>> # Replace Mock's name with our new property
>>> type(api).name = p
>>>
>>> # Now see what happens
>>> api.token
'secret'
>>> api.name
'brad'

Now, your Mock objects can have a name attribute with an expected return value.

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