None

https://docs.python.org/3/library/constants.html

INTRODUCTION

'None' is a special value representing 'nothing' (null object).

Use cases:
(a) a variable created but not processed yet [in graph coloring: a node without color],
(b) a way to reset a variable to an empty state.

answer = None
# ... some code ...
if answer is None:
    quit = True
elif answer == "quit":
    quit = True
else:
    quit = False

'None' is a singleton, there is only one instance of it in a program.
Testing 'answer is None' is better then 'answer == None' [or 'answer is not None'].

Functions without an explicit 'return' statement return 'None'. That way, programming with functions is much simplified; a function always returns 'something'.