How can I unit test when code throws an ImportError?

I have a Python module that tries to import a module, and if it fails, adds sets up a lightweight class to replicate some of the functionality as a fallback.

How can I write a unit test that covers both cases - when the module is present, and when the module is not present?

try:
    from foo import bar
except:
    class myFakeBar(object):
        def doAThing(self):
            return "blah"  

For example, if bar is present, the unit tests will report that only the 2nd line is covered, and the rest of the module isn't.