On 2014-02-13 20:03, Chris Angelico wrote:
On Fri, Feb 14, 2014 at 2:24 AM, Roy Smith <r...@panix.com> wrote:
In article <mailman.6834.1392292646.18130.python-l...@python.org>,
Chris Angelico <ros...@gmail.com> wrote:
On Thu, Feb 13, 2014 at 10:50 PM, Ned Batchelder <n...@nedbatchelder.com>
wrote:
I still don't see it. To convince me that a singleton class makes sense,
you'd have to explain why by virtue of the class's very nature, it never
makes sense for there ever to be more than one of them.
There's a huge difference, btw, between mutable and immutable
singletons. With immutables like None, True/False, integers, strings,
and tuples thereof, returning a preexisting object is just an
optimization. Do it if you want, don't if you don't, nobody's going to
hugely care.
People *depend* on None being a singleton (and are encouraged to do so),
when they use "is" as the test-for-Noneness.
Circular argument, though. If None weren't a singleton, people would
use == to test for Noneness. Since it's been guaranteed to be
optimized to a singleton, the comparison can also be optimized, but
it's still just an optimization, as can be seen with integers. In
CPython, you could test for small integer equality using 'is', but
since that optimization isn't guaranteed, neither is that code
pattern.
We don't use `is None` instead of `== None` for the speed. We use it for
robustness. We don't want arbitrary __eq__()s to interfere with our sentinel
tests. If None weren't a singleton that we could use as such a sentinel, we'd
make one.
--
Robert Kern
"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco
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