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 investigate:
| | >>> 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 the absence of a value. We’ll see some uses for it later in the book.
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