Python 弱引用

垃圾回收

Garbage Collector, 简写: GC

Python 垃圾回收是简单基于引用计数

弱引用

在计算机程序设计中，弱引用与强引用相对，是指不能确保其引用的对象不会被垃圾回收器回收的引用。一个对象若只被弱引用所引用，则被认为是不可访问（或弱可访问）的，并因此可能在任何时刻被回收。一些配有垃圾回收机制的语言，如Java、C#、Python、Perl、Lisp等都在不同程度上支持弱引用。

一句话：弱引用不增加计数，对引用计数型 GC 友好一些

垃圾回收与循环引用的问题

import gc
IDS = {}
class A:
def __del__(self):
_id = id(self)
print('A.__del__ %s: 0x%x' % (IDS[_id], _id))
OBJS = {i: A() for i in range(3)}
for i, obj in OBJS.items():
_id = id(obj)
IDS[_id] = f'OBJS[{i}]'
print('%s: 0x%x' % (IDS[_id], _id))
OBJS[1].attr = OBJS[1]
print('1' * 50)
print('====> del OBJS[0]')
del OBJS[0]
print('2' * 50)
print('====> del OBJS[1]')
del OBJS[1]
print('3' * 50)
print('====> del OBJS[2]')
del OBJS[2]
print('4' * 50)
gc.collect()

import weakref
print()
print('=' * 50)
class B:
def __init__(self, obj):
self.attrs = [obj]
def __del__(self):
_id = id(self)
print('B.__del__ %s: 0x%x' % (IDS[_id], _id))
a = A()
b = B(a)
a.xyz = b
IDS[id(a)] = 'a'
IDS[id(b)] = 'b'
del a, b  # do nothing
print('=' * 40)
gc.collect()  # will del a and b
print()
print('=' * 50)
class C:
def __init__(self, obj):
self.attrs = [weakref.ref(obj)]
def __del__(self):
_id = id(self)
print('C.__del__ %s: 0x%x' % (IDS[_id], _id))
a = A()
c = C(a)
a.xyz = c
IDS[id(a)] = 'a'
IDS[id(c)] = 'c'
del a, c
print('=' * 40)
gc.collect()

标准库：weakref

• class weakref.ref(object[, callback]) 回调
• weakref.proxy(object[, callback])
• weakref.getweakrefcount(object)
• weakref.getweakrefs(object)
• class weakref.WeakKeyDictionary([dict])
  • .keyrefs()
• class weakref.WeakValueDictionary([dict])
  • .valuerefs()
• class weakref.WeakSet([elements])
  > Set class that keeps weak references to its elements. An element will be discarded when no strong reference to it exists any more.
• class weakref.WeakMethod(method)
• class weakref.finalize(obj, func, /, *args, **kwargs)
• weakref.ReferenceType
• weakref.ProxyType
• weakref.CallableProxyType
• weakref.ProxyTypes

import weakref
class Klass:
pass
obj = Klass()
ref = weakref.ref(obj)
print(ref())
del obj
print(ref())  # None
obj = Klass()
p = weakref.proxy(obj)
print(p)
del obj
print(p)  # ReferenceError: weakly-referenced object no longer exists

参考资料与拓展阅读

• 维基百科， 弱引用
• https://docs.python.org/3/library/weakref.html
• 知乎专栏， 一文看懂弱引用！Python中的weakref有什么用？
• 博客园，爱编程的小灰灰， Python3标准库：weakref对象的非永久引用