threading.RLock not subclassible?
tim.one at home.com
Sun Feb 4 20:22:23 EST 2001
[John Gault, or maybe George Young]
> I would like to make a subclass of threading.RLock so I can
> add a 'safe_acquire' method. Safe_acquire would use non-
> blocking acquires and sleeps over several iterations to try
> to acquire the lock, but would eventually return so my thread
> is not forever blocked.
> But the RLock class seems to be written in a way discouraging
> It looks like the author(not identified in the code) is explictly
> trying to avoid subclassing...
The author is Guido Himself, and he was indeed trying to avoid subclassing.
I didn't know why either, but had a chance ask him. The deal is that some
platforms may have a much more efficient way to implement one or more of
these abstractions, in which case threading.py would be happy to export the
platform-specific way when running on that platform.
There are no instances of that yet, although threading.py gets kinda close
Lock = _allocate_lock
You can't subclass from Lock because its *implementation*
(thread.allocate_lock) is not a class. The same may well be true of future
platform-specific implementations of synch objects. So threading.py exposes
factory functions instead of classes.
There's no immediate danger that you'll get in trouble by subclassing from
_RLock instead, but there's no guarantee that will remain safe (the leading
underscore means, as you know, "touch at your own risk").
BTW, an acquire with a timeout is a cool idea! Feel encouraged to submit a
patch to add such a thing directly to threading.py:
I suggest a name like timed_acquire, though -- if it's in the core, the name
safe_acquire reads more like a snotty value judgment about the other
safety-depends-more-on-how-it's-used-than-what-it-does-ly y'rs - tim
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