python deep copy and shallow copy

Python中對於對象的賦值都是引用，而不是拷貝對象（Assignment statements in Python do not copy objects, they create bindings between a target and an object.）。對於可變對象來講，當一個改變的時候，另一個不用改變，由於是同時的，也就是說是保持同步的。

此時不想讓他們同步的話能夠使用copy模塊，copy.copy()淺拷貝，copy.deepcopy()深拷貝。

前者是對整個對象的拷貝，對子對象仍然是引用，後者是對對象以及子對象的拷貝。具體一些能夠下面例子：

In [26]: import copy                                                                                                                                                                       [6/165]

In [27]: a = [1, 2, [3, 4]]                                                                     

In [28]: b = a  # 引用

In [29]: c = copy.copy(a)　　# shallow copy, 對１，２，【３，４】拷貝，對【３，４】中的元素是引用

In [30]: d = copy.deepcopy(a)    # deep copy　對１，２，【３，４】都是拷貝，【３，４】中的元素也是拷貝

In [31]: b
Out[31]: [1, 2, [3, 4]]

In [32]: c
Out[32]: [1, 2, [3, 4]]

In [33]: d
Out[33]: [1, 2, [3, 4]]

In [34]: a.append(5)

In [35]: a
Out[35]: [1, 2, [3, 4], 5]

In [36]: b
Out[36]: [1, 2, [3, 4], 5]

In [37]: c
Out[37]: [1, 2, [3, 4]]

In [38]: d
Out[38]: [1, 2, [3, 4]]

In [39]: a[2].append(6)

In [40]: a
Out[40]: [1, 2, [3, 4, 6], 5]

In [41]: b
Out[41]: [1, 2, [3, 4, 6], 5]

In [42]: c
Out[42]: [1, 2, [3, 4, 6]]

In [43]: d
Out[43]: [1, 2, [3, 4]]

 

一些對象的淺拷貝能夠經過其餘方式實現，如字典能夠使用 dict.copy()，列表使用listb = lista[:]，以下:

In [46]: a = {'name': 'wang',}

In [47]: b = dict.copy(a)

In [48]: b
Out[48]: {'name': 'wang'}

In [49]: a.update({'age': 13})

In [50]: a
Out[50]: {'age': 13, 'name': 'wang'}

In [51]: b
Out[51]: {'name': 'wang'}

In [52]: a = {'name': {'first_name': 'wang'}}                                                                                                                                                     

In [53]: b = dict.copy(a)

In [54]: b
Out[54]: {'name': {'first_name': 'wang'}}

In [55]: a['name'].update({'last_name': 'Emma'})                                                                                                                                                  

In [56]: a
Out[56]: {'name': {'first_name': 'wang', 'last_name': 'Emma'}}

In [57]: b
Out[57]: {'name': {'first_name': 'wang', 'last_name': 'Emma'}}

In [58]: lista = [1, 2]

In [59]: listb = lista[:]

In [60]: lista.append(3)

In [61]: lista
Out[61]: [1, 2, 3]

In [62]: listb
Out[62]: [1, 2]

In [63]: lista = [1, 2, [3, 4]]

In [64]: listb = lista[:]

In [65]: lista[2].append(5)

In [66]: listb
Out[66]: [1, 2, [3, 4, 5]]

 

注意：

深拷貝淺拷貝都不能拷貝文件，模塊等類型（This module does not copy types like module, method, stack trace, stack frame, file, socket, window, array, or any similar types. It does 「copy」 functions and classes (shallow and deeply), by returning the original object unchanged;）

同時能夠自定義這兩個函數～　　

 

參考：

https://docs.python.org/2/library/copy.html

http://www.cnblogs.com/coderzh/archive/2008/05/17/1201506.html

http://blog.csdn.net/sharkw/article/details/1934090