output

In [18]: %run semaphores.py

In [19]: working.size
Out[19]: 0

In [20]: len(working)
Out[20]: 0

In [21]: nonworking.size
Out[21]: 0

In [22]: len(nonworking)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-22-8b85c1cd9107> in <module>()
----> 1 len(nonworking)

TypeError: 'int' object is not callable

In [23]: test.size
Out[23]: 0

In [24]: len(test)
Out[24]: 0

In [25]: len(SemaphoreTest)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-25-db83fa0370cc> in <module>()
----> 1 len(SemaphoreTest)

TypeError: object of type 'type' has no len()

In [26]: SemaphoreTest.__len__
Out[26]: <function __main__.SemaphoreTest.size>

In [27]:

semaphore_size.py

from threading import BoundedSemaphore


class SemaphoreWorking(BoundedSemaphore):

    def _size(self):
        return self._initial_value - self._value

    size = property(_size)

    __len__ = _size


class SemaphoreNonWorking(BoundedSemaphore):

    @property
    def size(self):
        return self._initial_value - self._value

    __len__ = size


class SemaphoreTest(BoundedSemaphore):

    @property
    def size(self):
        return self._initial_value - self._value

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.__class__.__len__ = self.__class__.size.fget


N = 10
working = SemaphoreWorking(N)
nonworking = SemaphoreNonWorking(N)
test = SemaphoreTest(N)