Omitting a return Statement: None

Книга: Practical Programming, Fourth Edition

Дальше: Dealing with Situations Tha t Your Code Doesn’t Handle

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 you get a NameError? Let’s investigate:

	>>> print(res)
	None
	>>> id(res)
	11330272

Variable res has a value: it’s None! And None has an identity. 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 the absence of a value. You’ll see some uses for it later in the book.

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