python中的深拷貝與淺拷貝解析

一、賦值，傳遞對象的引用

>>> a = [1, 2, 3, 4, ['a', 'b']]
>>> b = a
>>> b is a
True

b就是a ,兩者同步改變

二、淺拷貝

>>> a = [1,2,3,[4,5]]
>>> b = list(a)#######或者是b = copy.copy(a)
>>> b
[1, 2, 3, [4, 5]]
>>> a is b
False
>>> b[3][0] = 100#####嵌套的值改變時，影響到原值，兩者同步改變
>>> b
[1, 2, 3, [100, 5]]
>>> a
[1, 2, 3, [100, 5]]
>>> [id(x) for x in a]
[29982856, 29982844, 29982832, 39308384]
>>> [id(x) for x in b]
[29982856, 29982844, 29982832, 39308384]

>>> b[0] = 8#######第一層的值改變時，不影響原值。
>>> b
[8, 2, 3, [100, 5]]
>>> a
[1, 2, 3, [100, 5]]

複製了a的引用，只能做到頂層的複製，不能複製嵌套的數據。所以當b的頂層數據改變時，a不變，當嵌套的數據改變時，a也改變。

切片也屬於淺拷貝。

三、深拷貝

>>> b = copy.deepcopy(a)
>>> b is a
False
>>> b[3][0] = 100
>>> a
[1, 2, 3, [4, 5]]
>>> b
[1, 2, 3, [100, 5]]
>>> b[0] = 8
>>> a
[1, 2, 3, [4, 5]]
>>> b
[8, 2, 3, [100, 5]]
>>> 

Copy.deepcopy()可以拷貝完整的數據結構，b改變時不影響a