Documentation Strings (Docstrings)

Special attribute __doc__ in modules, classes and functions

Python libraries are well documented
>>> from random import randrange
>>> print randrange.__doc__
Choose a random item from range(start, stop[, step]).

        This fixes the problem with randint() which includes the
        endpoint; in Python this is usually not what you want.
        Do not supply the 'int' and 'default' arguments.
        
Used by interactive help utility
>>> help(randrange)
$ pydoc random.randrange

Docstrings are easily embedded into new code 
can provide testing framework