Practical Programming, 2nd Edition

Omitting a Return Statement: None

If you don’t have a return statement in a function, nothing is produced:

>>> def f(x):
...     x = 2 * x
...
>>> res = f(3)
>>> res
>>>

Wait, that can’t be right—if res doesn’t have a value, shouldn’t we get a NameError? Let’s poke a little more:

>>> print(res)
None
>>> id(res)
1756120

Variable res has a value: it’s None! And None has a memory address. If you don’t have a return statement in your function, your function will return None. You can return None yourself if you like:

>>> def f(x):
...     x = 2 * x
...     return None
...
>>> print(f(3))
None

The value None is used to signal ...