Armin Rigo added the comment:
(B8) also discussed in connection with https://bugs.python.org/issue28427
weak dicts (both kinds) and weak sets have an implementation of
__len__ which doesn't give the "expected" result on PyPy, and in some
cases on CPython too. I'm not sure what is expected and what is not.
Here is an example on CPython 3.5.2+ (using a thread to run the weakref
callbacks only, not to explicitly inspect or modify 'd')::
import weakref, _thread
from queue import Queue
queue = Queue()
def subthread(queue):
while True:
queue.get()
_thread.start_new_thread(subthread, (queue,))
class X:
pass
d = weakref.WeakValueDictionary()
while True:
x = X()
d[52] = x
queue.put(x)
del x
while list(d) != []:
pass
assert len(d) == 0 # we've checked that list(d)==[], but this may fail
On CPython I've seen the assert fail only after editing the function
WeakValueDictionary.__init__.remove() to add ``time.sleep(0.01)`` as
the first line. Otherwise I guess the timings happen to make that test
pass.
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<http://bugs.python.org/issue28884>
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