weakref — 对象的若引用

引用

# weakref_ref.py
import weakref


class ExpensiveObject:

    def __del__(self):
        print('(Deleting {})'.format(self))


obj = ExpensiveObject()
r = weakref.ref(obj)

print('obj:', obj)
print('ref:', r)
print('r():', r())

print('deleting obj')
del obj
print('r():', r())

引用回调

# weakref_ref_callback.py
import weakref


class ExpensiveObject:

    def __del__(self):
        print('(Deleting {})'.format(self))


def callback(reference):
    """Invoked when referenced object is deleted"""
    print('callback({!r})'.format(reference))


obj = ExpensiveObject()
r = weakref.ref(obj, callback)

print('obj:', obj)
print('ref:', r)
print('r():', r())

print('deleting obj')
del obj
print('r():', r())

Finalizing Objects

# weakref_finalize.py
import weakref


class ExpensiveObject:

    def __del__(self):
        print('(Deleting {})'.format(self))


def on_finalize(*args):
    print('on_finalize({!r})'.format(args))


obj = ExpensiveObject()
weakref.finalize(obj, on_finalize, 'extra argument')

del obj

# weakref_finalize_atexit.py
import sys
import weakref


class ExpensiveObject:

    def __del__(self):
        print('(Deleting {})'.format(self))


def on_finalize(*args):
    print('on_finalize({!r})'.format(args))


obj = ExpensiveObject()
f = weakref.finalize(obj, on_finalize, 'extra argument')
f.atexit = bool(int(sys.argv[1]))

# weakref_finalize_reference.py
import gc
import weakref


class ExpensiveObject:

    def __del__(self):
        print('(Deleting {})'.format(self))


def on_finalize(*args):
    print('on_finalize({!r})'.format(args))


obj = ExpensiveObject()
obj_id = id(obj)

f = weakref.finalize(obj, on_finalize, obj)
f.atexit = False

del obj

for o in gc.get_objects():
    if id(o) == obj_id:
        print('found uncollected object in gc')

# weakref_finalize_reference_method.py
import gc
import weakref


class ExpensiveObject:

    def __del__(self):
        print('(Deleting {})'.format(self))

    def do_finalize(self):
        print('do_finalize')


obj = ExpensiveObject()
obj_id = id(obj)

f = weakref.finalize(obj, obj.do_finalize)
f.atexit = False

del obj

for o in gc.get_objects():
    if id(o) == obj_id:
        print('found uncollected object in gc')

代理

# weakref_proxy.py
import weakref


class ExpensiveObject:

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

    def __del__(self):
        print('(Deleting {})'.format(self))


obj = ExpensiveObject('My Object')
r = weakref.ref(obj)
p = weakref.proxy(obj)

print('via obj:', obj.name)
print('via ref:', r().name)
print('via proxy:', p.name)
del obj
print('via proxy:', p.name)

缓存对象

# weakref_valuedict.py
import gc
from pprint import pprint
import weakref

gc.set_debug(gc.DEBUG_UNCOLLECTABLE)


class ExpensiveObject:

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

    def __repr__(self):
        return 'ExpensiveObject({})'.format(self.name)

    def __del__(self):
        print('    (Deleting {})'.format(self))


def demo(cache_factory):
    # hold objects so any weak references
    # are not removed immediately
    all_refs = {}
    # create the cache using the factory
    print('CACHE TYPE:', cache_factory)
    cache = cache_factory()
    for name in ['one', 'two', 'three']:
        o = ExpensiveObject(name)
        cache[name] = o
        all_refs[name] = o
        del o  # decref

    print('  all_refs =', end=' ')
    pprint(all_refs)
    print('\n  Before, cache contains:', list(cache.keys()))
    for name, value in cache.items():
        print('    {} = {}'.format(name, value))
        del value  # decref

    # remove all references to the objects except the cache
    print('\n  Cleanup:')
    del all_refs
    gc.collect()

    print('\n  After, cache contains:', list(cache.keys()))
    for name, value in cache.items():
        print('    {} = {}'.format(name, value))
    print('  demo returning')
    return


demo(dict)
print()

demo(weakref.WeakValueDictionary)