Run doctest with import

If I want to use side function to test my class in doctest how can I achieve this?

So here is what I want to get

class SomeItem(object):
"""
>>>side_function(2+2, SomeItem)
4
"""

I've tried to add code below. But it does not seem to have effect

if __name__ == "__main__":
    import doctest
    from somelib import side_function
    doctest.testmod(extraglobs={'side_function': side_function})