weak-ref-eg-2.py

import weakref
import gc
import ctypes


class PriceyObject:
    def __init__(self, name):
        self.name = name

    def __repr__(self):
        return f'PriceyObject({self.name})'

    def __del__(self):
        print(f'Deleting {self}')
        
def demo(cache_factory):
    print(f'cache type: {cache_factory}')
    
    strong_ref_obj_coll = []
    cache = cache_factory()
    for gem in ['opal', 'ruby', 'fluorite']:
        pobj = PriceyObject(gem)
        print(f'reference count after initialization gem {gem}: {ctypes.c_long.from_address(id(pobj)).value}')
        cache[gem] = pobj
        print(f'reference count after caching gem {gem}: {ctypes.c_long.from_address(id(pobj)).value}')
        strong_ref_obj_coll.append(pobj)
        print(f'reference count after appending to list gem {gem}: {ctypes.c_long.from_address(id(pobj)).value}')
        del pobj # decreasing reference count

    print(f'List length: {len(strong_ref_obj_coll)}, contents: {strong_ref_obj_coll}')
    print('cleaning list...')
    del strong_ref_obj_coll
    gc.collect()

    print(f'After cleaning list, cache contains: {cache.keys()}')

    for gem, gem_obj in cache.items():
        print(f'{gem}={gem_obj}')
    
    print('demo returning')
    return

demo(dict)
print('\n\n')
demo(weakref.WeakValueDictionary)