Keep track of instances properly (weakref)

---

examples/oop/keep_track_properly.py

import weakref

def prt():
    print(list(Thing.things.keys()))

class Thing:
    things = {}
    def __init__(self):
        Thing.things[id(self)] = weakref.ref(self)

    def __del__(self):
        print('__del__')
        del(Thing.things[id(self)])


def main():
    prt()
    t1 = Thing()
    prt()
    t2 = Thing()
    prt()
    t3 = Thing()
    prt()
    t3 = None
    prt()

main()
prt()

[]
[139793357111744]
[139793357111744, 139793355980224]
[139793357111744, 139793355980224, 139793355980080]
__del__
[139793357111744, 139793355980224]
__del__
__del__
[]