Relative imports in Python 3

Question

In the same directory, I wish to import a function from another file.

One of the following usually works:

from .mymodule import myfunction

from mymodule import myfunction

The other, however, produces one of the following errors:

ImportError: attempted relative import with no known parent package

ModuleNotFoundError: No module named 'mymodule'

SystemError: Parent module '' not loaded, cannot perform relative import

Why is this?