Python3: command line interface and module import?

Question

So I have the following directory structure

• my_dir
  • my_mod
    • __init__.py
    • myfile.py

and in myfile.py

I have a nice command line interface via argparse.

But what if sometimes I want to import a function from this script and use it as if it were a module?

e.g. from myfile import myfunc

when I do this I get an error, mostly because I do some preprocess of the command line arguments which are all none in this case.

from this S.O. post, I know how I can tell if python is called from the command line:

import sys, os
if os.isatty(sys.stdin.fileno()):
    # do things

The inclusions of this line and corresponding replacement of my argument preprocessing does not solve this issue.

What should I do?