On Thu, 13 Feb 2014 14:07:55 +1100, Ben Finney wrote:
> Steven D'Aprano <st...@pearwood.info> writes:
>
>> On Wed, 12 Feb 2014 23:04:32 +1300, Gregory Ewing wrote:
>> > If you really want to make sure nobody creates another instance by
>> > accident, delete the class out of the namespace after instantiating
>> > it.
>>
>> That does not work. It is trivial to get the type from an instance [by
>> calling ‘type(foo)’].
>
> Right, but surely you don't think people would do that “by accident”,
> which is what Gregory was addressing.
Of course it can happen by accident. It's happened to me, where I've
accidentally called NoneType() (which raises, rather than returning a new
instance).
The way it happened is I had a bunch of numeric values, all of the same
type, and wanted to test that summing them with a custom sum function
would give the same result whether I provided a start parameter or not:
for data in list_of_datas:
T = type(data[0])
zero = T()
assert sum(data) == sum(data, zero)
only somehow one of the data sets ends up containing None due to another
bug. Now in *this* case, the error was obvious: NoneType() raises an
exception. If the type happened to be bool(), I would have been okay too,
because bool() returns the appropriate singleton True instance.
Because I was testing a function with no side-effects, it wouldn't have
mattered even if I had accidentally created a extra instance of something
which nominally ought to have been a singleton. But if I were testing a
function with side-effects, say one which modifies the internal state of
the singleton, then I could have been in all sorts of strife.
If you're going to have a singleton, you ought to do one of two things:
* calling the constructor returns the same singleton each time; or
* calling the constructor (apart from the very first time) raises an
exception.
I prefer the first case.
--
Steven
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