[docs]class CountedResource(object):
"""
This class is an attempt to combine the RAIA idiom with reference
counting semantics in order to model shared resources. RAIA stands
for "Resource Allocation Is Acquisition" (see 'Effective C++' for a
more in-depth explanation)
When a resource is needed, create an appropriate CountedResource
object. If the resource is already available (meaning another
CountedResource object of the same type already exists), no action
is taken. Otherwise, acquire() is invoked, and the resource is
allocated. The resource will remain valid until all matching
CountedResource objects have been deleted. When no objects of
a particular CountedResource type exist, the release() function for
that type is invoked and the managed resource is cleaned up.
Usage:
Define a subclass of CountedResource that defines the
@classmethods acquire() and release(). In these two
functions, define your resource allocation and cleanup code.
IMPORTANT:
If you define your own __init__ and __del__ methods, you
MUST be sure to call down to the ones defined in
CountedResource.
Notes:
Until we figure out a way to wrangle a bit more functionality
out of Python, you MUST NOT inherit from any class that has
CountedResource as its base class. In debug mode, this will
raise a runtime assertion during the invalid class's call to
__init__(). If you have more than one resource that you want to
manage/access with a single object, you should subclass
CountedResource again. See the example code at the bottom of
this file to see how to accomplish this (This is useful for
dependent resources).
"""
[docs] @classmethod
def incrementCounter(cls):
try:
cls.RESOURCE_COUNTER += 1
except AttributeError:
cls.RESOURCE_COUNTER = 1
if cls.RESOURCE_COUNTER == 1:
cls.acquire()
[docs] @classmethod
def decrementCounter(cls):
try:
cls.RESOURCE_COUNTER_INIT_FAILED
del cls.RESOURCE_COUNTER_INIT_FAILED
except AttributeError:
cls.RESOURCE_COUNTER -= 1
if cls.RESOURCE_COUNTER < 1:
cls.release()
[docs] @classmethod
def getCount(cls):
return cls.RESOURCE_COUNTER
[docs] @classmethod
def acquire(cls):
pass
[docs] @classmethod
def release(cls):
pass
[docs] def __init__(self):
cls = type(self)
cls.RESOURCE_COUNTER_INIT_FAILED = True
assert cls.mro()[1] == CountedResource, \
(lambda: \
'%s cannot be subclassed.' \
% cls.mro()[list(cls.mro()).index(CountedResource) - 1].__name__)()
del cls.RESOURCE_COUNTER_INIT_FAILED
self.incrementCounter()
def __del__(self):
self.decrementCounter()
