class?A:
????def?__init__(self,?obj):
????????self.obj?=?obj
????def?__del__(self):
????????print("當實例對象被回收時,?會觸發(fā)我的執(zhí)行······")
#?顯然我們創(chuàng)建了一個對象?A(123),?然后讓變量?a?指向（引用）它
#?然后?b?=?a,?讓?b?也指向?a?指向的對象
a?=?A(123)
b?=?a
#?此時對象的引用計數(shù)為?2,?然后我們將?a?刪除掉
del?a
print("無事發(fā)生,?一切正常")
#?如果再?del?b,?那么?A(123)?的引用計數(shù)就變成了?0,?那么它就該被回收了
#?一旦被回收,?就會觸發(fā)析構(gòu)函數(shù)?__del__
del?b
print("觸發(fā)完析構(gòu)函數(shù),?這里會打印")
"""
無事發(fā)生,?一切正常
當實例對象被回收時,?會觸發(fā)我的執(zhí)行······
觸發(fā)完析構(gòu)函數(shù),?這里會打印
"""

class?A:
????def?__init__(self,?obj):
????????self.obj?=?obj
????def?__del__(self):
????????print("當實例對象被回收時,?會觸發(fā)我的執(zhí)行······")
#?創(chuàng)建兩個對象,?分別讓?a?和?b?引用
a?=?A(123)
b?=?A(123)
#?然后,?重點來了
a.obj?=?b
b.obj?=?a
#?此時?a?引用的實例對象被?b.obj?引用了
#?b?引用的實例對象被?a.obj?引用了
#?這個時候，兩個對象的引用計數(shù)都為?2
#?然后我們?del?a,?b，這個時候能把對象刪除掉嗎??顯然是不能的
#?因為它們的引用計數(shù)都變成了 1, 不是?0。只要不為?0, 就不會被回收
del?a,?b

import?gc
class?A:
????def?__init__(self,?obj):
????????self.obj?=?obj
????def?__del__(self):
????????print("當實例對象被回收時,?會觸發(fā)我的執(zhí)行······")
a?=?A(123)
b?=?A(123)
a.obj?=?b
b.obj?=?a
del?a,?b
print("析構(gòu)函數(shù)沒有被執(zhí)行,?因為引用計數(shù)不為零")
#?gc?觸發(fā)是需要條件的,?但是?Python?支持我們手動引發(fā)?gc
#?gc?發(fā)動之后會找出發(fā)生循環(huán)引用的對象
#?由于這里的兩個對象沒有外部的變量引用,?所以它們都是要被回收的
gc.collect()
print("兩個對象都被回收了")
"""
析構(gòu)函數(shù)沒有被執(zhí)行,?因為引用計數(shù)不為零
當實例對象被回收時,?會觸發(fā)我的執(zhí)行······
當實例對象被回收時,?會觸發(fā)我的執(zhí)行······
兩個對象都被回收了
"""

import?weakref
class?RefObject:
????def?__del__(self):
????????print("del?executed")
obj?=?RefObject()
#?對象的弱引用通過?weakref.ref?類來創(chuàng)建
r?=?weakref.ref(obj)
print(obj)?
"""
<__main__.RefObject?object?at?0x000001B7C573A5E0>
"""
#?顯示關(guān)聯(lián)?RefObject
print(r)
"""
<weakref?at?0x000001B7DCAE19A0;?to?'RefObject'?at?0x000001B7C573A5E0>
"""
#?對引用進行調(diào)用的話,?即可得到原對象
print(r()?is?obj)??
"""
True
"""
#?刪除?obj?會執(zhí)行析構(gòu)函數(shù)
del?obj??
"""
del?executed
"""
#?之前說過?r()?等價于?obj,?但是obj被刪除了,?所以返回?None
#?從這里返回?None?也能看出這個弱引用是不會增加引用計數(shù)的
print("r():",?r())?
"""
r():?None
"""
#?打印弱引用,?告訴我們狀態(tài)已經(jīng)變成了?dead
print(r)??
"""
<weakref?at?0x000001B7DCAE19A0;?dead>
"""

import?weakref
class?RefObject:
????def?__del__(self):
????????print("del?executed")
obj?=?RefObject()
r?=?weakref.ref(obj,?lambda?ref:?print("引用被刪除了",?ref))
del?obj??
print("r():",?r())?
"""
del?executed
引用被刪除了?<weakref?at?0x0000021A69681900;?dead>
r():?None
"""
#?回調(diào)函數(shù)會接收一個參數(shù),?也就是死亡之后的弱引用;?

import?weakref
class?RefObject:
????def?__init__(self,?name):
????????self.name?=?name
????def?__del__(self):
????????print("del?executed")
obj?=?RefObject("my?obj")
r?=?weakref.ref(obj)
p?=?weakref.proxy(obj)
#?可以看到引用加上()才相當于原來的對象
#?而代理不需要，直接和原來的對象保持一致
print(obj.name)??#?my?obj
print(r().name)??#?my?obj
print(p.name)??#?my?obj
#?但是注意:?弱引用在調(diào)用之后就是原對象,?而代理不是
print(r()?is?obj)??#?True
print(p?is?obj)??#?False
del?obj??#?del?executed
try:
????#?刪除對象之后,?再調(diào)用引用,?打印為None
????print(r())??#?None
????#?如果是使用代理,?則會報錯
????print(p)
except?Exception?as?e:
????print(e)??#?weakly-referenced?object?no?longer?exists

class?A:
????def?__del__(self):
????????print("__del__")
a?=?A()
#?創(chuàng)建一個普通字典
d?=?{}
#?由于?a?作為了字典的?key,?那么?a?指向的對象引用計數(shù)會加?1,?變成?2
d[a]?=?"xxx"
#?刪除?a,?對對象無影響,?不會觸發(fā)析構(gòu)函數(shù)
del?a
print(d)
"""
{<__main__.A?object?at?0x000002092669A5E0>:?'xxx'}
__del__
"""
#?最后打印的?__del__?是程序結(jié)束時,?將對象回收時打印的

import?weakref
class?A:
????def?__del__(self):
????????print("__del__")
a?=?A()
#?創(chuàng)建一個弱引用字典,?它的?api?和普通字典一樣
d?=?weakref.WeakKeyDictionary()
print("d:",?d)??
"""
d:?<WeakKeyDictionary?at?0x7f8a581a0d30>
"""
#?此時?a?指向的對象的引用計數(shù)不會增加
d[a]?=?"xxx"
print("before?del?a:",?list(d.items()))
"""
before?del?a:?[(<__main__.A?object?at?0x7f8a581a0d60>,?'xxx')]
"""
#?刪除?a,?對象會被回收
del?a
"""
__del__
"""
print("after?del?a:",?list(d.items()))
"""
after?del?a:?[]
"""

import?weakref
class?A:
????def?__del__(self):
????????print("__del__")
a?=?A()
d?=?weakref.WeakValueDictionary()
#?value?為弱引用
d["xxx"]?=?a
print("before?del?a:",?list(d.items()))
"""
before?del?a:?[('xxx',?<__main__.A?object?at?0x7f89580a7d60>)]
"""
#?刪除?a,?對象會被回收
del?a
"""
__del__
"""
print("after?del?a:",?list(d.items()))
"""
after?del?a:?[]
"""

import?weakref
class?A:
????def?__del__(self):
????????print("__del__")
a?=?A()
s?=?weakref.WeakSet()
s.add(a)
print(len(s))
del?a
print(len(s))
"""
1
__del__
0
"""

import?weakref
class?A:
????__slots__?=?("name",?"age")
????def?__init__(self):
????????self.name?=?"古明地覺"
????????self.age?=?17
a?=?A()
try:
????weakref.ref(a)
except?Exception?as?e:
????print(e)??#?cannot?create?weak?reference?to?'A'?object
try:
????weakref.proxy(a)
except?Exception?as?e:
????print(e)??#?cannot?create?weak?reference?to?'A'?object
try:
????d?=?weakref.WeakSet()
????d.add(a)
except?Exception?as?e:
????print(e)??#?cannot?create?weak?reference?to?'A'?object

import?weakref
class?A:
????#?多指定一個__weakref__,?表示支持弱引用
????__slots__?=?("name",?"age",?"__weakref__")
????def?__init__(self):
????????self.name?=?"古明地覺"
????????self.age?=?17
a?=?A()
weakref.ref(a)
weakref.proxy(a)
d?=?weakref.WeakSet()
d.add(a)