How do I run the doctest for only a single function in python using the command line? I can python3 -m doctest -v main.py but this will run all the doctests in main.py. How do I specify one function to call the doctest on?
That depends on the code in main.py that runs the doctests. You can change that code to test a specific function by calling doctest.run_docstring_examples().
When that code runs doctest.testmod() however, you cannot limit testing to a single function from the command line.