物件引用、可變性和垃圾回收
摘要：
變數不是盒子，它是物件的標識
- 標識、相等性和別名
charles = {'name':'Chales L.Dogson','born':1832}
lewis = charles
alex = {'name':&#...
變數不是盒子，它是物件的標識
- 標識、相等性和別名
charles = {'name':'Chales L.Dogson','born':1832}
lewis = charles
alex = {'name':'Chales L.Dogson','born':1832}
# lewis是charles的別名，is比較的是兩個物件的id
print('charles is lewis:',charles is lewis)
# alex標識的物件和charles不同
print('charles is alex:',charles is alex)
# 但它們的值相等，'=='是語法糖，事實上呼叫的是__eq__
# dict對'=='進行了過載，比較的是物件的內容
print('charles == alex:',charles == alex)
輸出：
charles is lewis: True
charles is alex: False
charles == alex: True
複製
- 複製列表或多數內建可變集合，最簡單的方式是使用內建的型別構造方法
- 預設是淺複製
l1 = [3,[55,44],(7,8,9)]
l2 = list(l1)
# 複製之後,l2和l1內容相同
print('l2==l1:',l2 == l1)
# 但指向不同物件
print('l2 is l1:',l2 is l1)
# 但它們內部的元素仍然是同一個物件
print('l2[1] is l1[1]:',l2[1] is l1[1])
# 根據內部元素不同，將會產生不同表現
# 可變元素會相互影響
l1[1].append(33)
print('after l1[1] append:')
print('l1:',l1)
print('l2:',l2)
# 不可變元素不會互相影響
l1[2] += (10,)
print('after l1[2] append:')
print('l1:',l1)
print('l2:',l2)
輸出：
l2==l1: True
l2 is l1: False
l2[1] is l1[1]: True
after l1[1] append:
l1: [3, [55, 44, 33], (7, 8, 9)]
l2: [3, [55, 44, 33], (7, 8, 9)]
after l1[2] append:
l1: [3, [55, 44, 33], (7, 8, 9, 10)]
l2: [3, [55, 44, 33], (7, 8, 9)]
- 深複製需要用到copy.deepcopy
from copy import deepcopy
l1 = [3,[55,44],(7,8,9)]
l2 = deepcopy(l1)
# 內部的元素不再是同一個物件
print('l2[1] is l1[1]:',l2[1] is l1[1])
# 無論可變還是不可變元素，都不會相互影響
l1[1].append(33)
print('after l1[1] append:')
print('l1:',l1)
print('l2:',l2)
l1[2] += (10,)
print('after l1[2] append:')
print('l1:',l1)
print('l2:',l2)
輸出：
l2[1] is l1[1]: False
after l1[1] append:
l1: [3, [55, 44, 33], (7, 8, 9)]
l2: [3, [55, 44], (7, 8, 9)]
after l1[2] append:
l1: [3, [55, 44, 33], (7, 8, 9, 10)]
l2: [3, [55, 44], (7, 8, 9)]
WeakValueDictionary
import weakref
class Dog:
def __init__(self,name):
self.name = name
def __repr__(self):
return 'Dog(%s)' % self.name
# stock儲存一個弱引用的可變對映
stock = weakref.WeakValueDictionary()
catalog = [Dog('Alen'),Dog('Bob'),Dog('Wendy')]
for dog in catalog:
stock[dog.name] = dog
print(sorted(stock.keys()))
del catalog
# 其他Dog物件的引用都消失了，只剩下dog引用了Wendy
print(sorted(stock.keys()))
# 把dog刪除，Wendy也消失了
del dog
print(sorted(stock.keys()))
輸出：
['Alen', 'Bob', 'Wendy']
['Wendy']
[]
WeakKeyDictionary
import weakref
class Dog:
def __init__(self,name):
self.name = name
def __repr__(self):
return 'Dog(%s)' % self.name
# stock儲存一個弱引用的可變對映
stock = weakref.WeakKeyDictionary()
catalog = [Dog('Alen'),Dog('Bob'),Dog('Wendy')]
# 和WeakValueDictionary不同，WeakKeyDictionary的鍵是被引用的物件
for dog in catalog:
stock[dog] = dog.name
print([item.name for item in stock.keys()])
del catalog
# 其他Dog物件的引用都消失了，只剩下dog引用了Wendy
print([item.name for item in stock.keys()])
# 把dog刪除，Wendy也消失了
del dog
print([item.name for item in stock.keys()])
輸出：
['Alen', 'Bob', 'Wendy']
['Wendy']
[]