singleton ... again
roy at panix.com
Thu Feb 13 16:24:07 CET 2014
In article <mailman.6834.1392292646.18130.python-list at python.org>,
Chris Angelico <rosuav at gmail.com> wrote:
> On Thu, Feb 13, 2014 at 10:50 PM, Ned Batchelder <ned at nedbatchelder.com>
> > 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.
> With mutables, it's hugely different. A singleton
> "database connection" object would, imo, be hugely confusing; you'd
> think you created a separate connection object, but no, what you do on
> this one affects the other.
Except that if you do caching in the database connector, you would
certainly want the cache to be shared between all the users. There's no
right answer here. Each way has it's advantages and disadvantages. And
it would only be confusing if the documentation didn't spell out which
way it was doing it, leaving people to make (possibly wrong) assumptions.
> Better there to have module-level functions; at least Python
> programmers should understand that reimporting a module gives you
> back another reference to the same module.
Except when it doesn't. Singleton-ness of modules depends on the names
under which they were imported. Symlinks, for example, can fool the
$ ls -l s1 s2
lrwxrwxrwx 1 roy roy 1 Feb 13 10:13 s1 -> s
lrwxrwxrwx 1 roy roy 1 Feb 13 10:13 s2 -> s
$ ls -l s
-rw-rw-r-- 1 roy roy 0 Feb 13 10:13 __init__.py
-rw-rw-r-- 1 roy roy 101 Feb 13 10:14 __init__.pyc
-rw-rw-r-- 1 roy roy 9 Feb 13 10:12 singleton.py
-rw-rw-r-- 1 roy roy 123 Feb 13 10:14 singleton.pyc
>>> import s1.singleton
>>> import s2.singleton
>>> s1.singleton == s2.singleton
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