import weakref, gc

class O(object):
    pass

os = []
#od = weakref.WeakKeyDictionary()
od = weakref.WeakValueDictionary()
for i in range(10):
    o1 = O()
    o2 = O()
    os.append((o1, o2))
    od[o1] = o2

i = 0
for x in od:
    i += 1
    if i == 5:
        del os
        gc.collect()