On Aug 11, 2:50 pm, Stefan Bellon <[EMAIL PROTECTED]> wrote:
> So why is the destructor not called when the generator is even
> explicitly 'del'ed? Does somebody else still hold a reference on it?
You ( we ) have produced a reference cycle. In that case __del__
doesn't work properly ( according to the docs ). The cycle is caused
by the following assignment in your code:
self.gen = self._value()
So we have to refactor the solution to eliminate the cycle. I spent
some time to create a generic decorator solution and a protocol for
handling / releasing the resources.
1) Create a FinalizerGenerator class that is cycle free ( of course
you can find a trick to shoot yourself in your foot ). Pass a
generator function together with its arguments into the constructor.
An additional callable attribute close is used together with __del__.
class FinalizerGenerator(object):
def __init__(self, gen, *args, **kwd):
print "gen init"
self._gen = gen # no cycle when passing the _value function
self.close = lambda: sys.stdout.write("gen del") # assign
cleanup function
def __del__(self):
self.close()
def __iter__(self):
print "gen iter"
return self
def next(self):
print "gen next"
return self.gen.next()
2) Define generators s.t. they yield the resource to be destroyed as
their first value.
def producer(resource):
print "gen value"
yield resource # yield resource before start iteration
for item in resource:
yield item
3) Define the destructor for the resource. The resource must be passed
as a first argument. The return value is a callable without arguments
that serves as a close() function within FinalizerGenerator.__del__
method.
def close_resource(resource):
return lambda: sys.stdout.write("close resource: %s"%resource)
4) The finalize_with decorator
def finalize_with(close_resource):
def closing(func_gen):
def fn(*args, **kwd):
# fg serves as a replacement for the original generator
func_def
fg = FinalizerGenerator(func_gen)(*args, **kwd)
# apply the closing protocol
resource = fg.next()
fg.close = close_resource(resource)
return fg
fn.__name__ = func_gen.__name__
return fn
return closing
5) decorate the generator and check it out
@finalize_with(close_resource)
def producer(resource):
print "gen value"
yield resource
for item in resource:
yield item
def test():
pg = producer([1,2,3])
pg.next()
>>> test()
gen init
gen next # request for resource in finalize_with
gen value
gen next
close resource: [1, 2, 3] # yep
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