I'm a big fan of code that doesn't need documentation. When you program correctly, choose the right names and take care of the details, your code should come out as self-explanatory and documentation should not be needed. Sometimes a comment is very useful though, and so is some documentation. You can find the guidelines for documenting Python in PEP 257 - Docstring conventions (https://www.python.org/dev/peps/pep-0257/), but I'll show you the basics here.
Python is documented with strings, which are aptly called docstrings. Any object can be documented, and you can use either one-line or multiline docstrings. One-liners are very simple. They should not provide another signature for the function, but clearly state its purpose:
# docstrings.py
def square(n):
"""Return the square of a number n. """
return n...