using "private" parameters as static storage?
Steven D'Aprano
steve at REMOVE-THIS-cybersource.com.au
Thu Nov 13 22:26:04 EST 2008
On Thu, 13 Nov 2008 10:16:59 -0700, Joe Strout wrote:
> One thing I miss as I move from REALbasic to Python is the ability to
> have static storage within a method -- i.e. storage that is persistent
> between calls, but not visible outside the method. I frequently use
> this for such things as caching, or for keeping track of how many
> objects a factory function has created, and so on.
>
> Today it occurred to me to use a mutable object as the default value of
> a parameter. A simple example:
>
> def spam(_count=[0]):
> _count[0] += 1
> return "spam " * _count[0]
This is a common trick, often used for things like caching. One major
advantage is that you are exposing the cache as an *optional* part of the
interface, which makes testing easier. For example, instead of a test
that looks something like this:
cache = get_access_to_secret_cache() # somehow
modify(cache)
result = function(arg)
restore(cache)
assert something_about(result)
you can simple do this:
result = function(arg, _cache=mycache)
assert something_about(result)
Periodically people complain that Python's mutable default argument
behaviour is a problem, and ask for it to be removed. I agree that it is
a Gotcha that trips up newbies, but it is far to useful to give up, and
simple caching is one such reason.
> Ooh, for a change I had another thought BEFORE hitting Send rather than
> after. Here's another trick:
>
> def spam2():
> if not hasattr(spam2,'count'):spam2.count=0 spam2.count += 1
> return "spam2 " * spam2.count
>
> This doesn't expose any uncleanliness outside the function at all. The
> drawback is that the name of the function has to appear several times
> within itself, so if I rename the function, I have to remember to change
> those references too. But then, if I renamed a function, I'd have to
> change all the callers anyway. So maybe this is better. What do y'all
> think?
I've used this myself, but to me it feels more icky than the semi-private
argument trick above.
--
Steven
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