Unit Testing/Mocking in Python

Question

So let's say I have this bit of code:

import coolObject def doSomething(): x = coolObject() x.coolOperation()

Now it's a simple enough method, and as you can see we are using an external library(coolObject). In unit tests, I have to create a mock of this object that roughly replicates it. Let's call this mock object coolMock.

My question is how would I tell the code when to use coolMock or coolObject? I've looked it up online, and a few people have suggested dependency injection, but I'm not sure I understand it correctly.

Thanks in advance!

Did you try to search: stackoverflow.com/search?q=%5Bpython%5D+mock ? — user180100
– user180100, Commented Jul 11, 2017 at 20:48

Dan · Accepted Answer · 2017-07-11 20:57:18Z

def doSomething(cool_object=None): cool_object = cool_object or coolObject() ...

In you test:

def test_do_something(self): cool_mock = mock.create_autospec(coolObject, ...) cool_mock.coolOperation.side_effect = ... doSomthing(cool_object=cool_mock) ... self.assertEqual(cool_mock.coolOperation.call_count, ...)

Frank T · Accepted Answer · 2017-07-13 15:25:39Z

As Dan's answer says, one option is to use dependency injection: have the function accept an optional argument, if it's not passed in use the default class, so that a test can pass in a moc.

Another option is to use the mock library (here or here) to replace your coolObject.

Let's say you have a foo.py that looks like

from somewhere.else import coolObject def doSomething(): x = coolObject() x.coolOperation()

In your test_foo.py you can do:

import mock def test_thing(): path = 'foo.coolObject' # The fully-qualified path to the module, class, function, whatever you want to mock. with mock.patch('foo.coolObject') as m: doSomething() # Whatever you want to assert here. assert m.called

The path you use can include properties on objects, e.g. module1.module2.MyClass.my_class_method. A big gotcha is that you need to mock the object in the module being tested, not where it is defined. In the example above, that means using a path of foo.coolObject and not somwhere.else.coolObject.

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