Python —— 深拷贝和浅拷贝

浅拷贝 —— Shallow Copy

浅拷贝是指复制对象本身，但不会复制对象内部的嵌套对象。换句话说，浅拷贝会创建一个新的对象，但是新对象中的元素是原始对象元素的引用（即指向同一块内存地址）。因此，若原始对象中的元素是可变对象，修改这些元素会影响到原始对象和拷贝对象。

s = "abcdef123456"
l1 = list(s) # 把字符串变成列表
l2 = l1.copy() # list 数据类型自带的浅拷贝
print(f"l1 = {l1}\nl2 = {l2}")
print(f"id(l1) = {id(l1)}\nid(l2) = {id(l2)}") # 打印两者内存地址
print(f"id(l1[3]) = {id(l1[3])}\nid(l2[3]) = {id(l2[3])}") # 每一个元素扔指向原始字符d，所以id一致。
l1[0] = 'x' # l1、l2中的元素都是不可变对象，故改变拷贝对象，不会影响原对象。
print(f"l1 = {l1}")
print(f"l2 = {l2}")
l2[0] = 'y'
print(f"l1 = {l1}")
print(f"l2 = {l2}")

l2是l1的浅拷贝，是两个不同的对象，两者内存地址不同。
注意：拷贝的列表l2中每个元素仍然指向原始字符（它们是字符串，字符串是不可变的，所以不会受到影响）。运行结果如下：

l1 = [‘a’, ‘b’, ‘c’, ‘d’, ‘e’, ‘f’, ‘1’, ‘2’, ‘3’, ‘4’, ‘5’, ‘6’]
l2 = [‘a’, ‘b’, ‘c’, ‘d’, ‘e’, ‘f’, ‘1’, ‘2’, ‘3’, ‘4’, ‘5’, ‘6’]
id(l1) = 140376061742848
id(l2) = 140376061990144
id(l1[3]) = 140376062729520
id(l2[3]) = 140376062729520
l1 = [‘x’, ‘b’, ‘c’, ‘d’, ‘e’, ‘f’, ‘1’, ‘2’, ‘3’, ‘4’, ‘5’, ‘6’]
l2 = [‘a’, ‘b’, ‘c’, ‘d’, ‘e’, ‘f’, ‘1’, ‘2’, ‘3’, ‘4’, ‘5’, ‘6’]
l1 = [‘x’, ‘b’, ‘c’, ‘d’, ‘e’, ‘f’, ‘1’, ‘2’, ‘3’, ‘4’, ‘5’, ‘6’]
l2 = [‘y’, ‘b’, ‘c’, ‘d’, ‘e’, ‘f’, ‘1’, ‘2’, ‘3’, ‘4’, ‘5’, ‘6’]

嵌套的可变对象如果被修改，会影响到原列表。参考代码如下：

l1 = [123,"abc", ["1a2b", 666], 'x']
l2 = l1.copy()
print(f"l1 = {l1}") # l1 = [123, 'abc', ['1a2b', 666], 'x']
print(f"l2 = {l2}") # l2 = [123, 'abc', ['1a2b', 666], 'x']
print(f"id(l1) = {id(l1)}") # id(l1) = 139777658463680
print(f"id(l2) = {id(l2)}") # id(l2) = 139777658164800
print(f"id(l1[2]) = {id(l1[2])}") # id(l1[2]) = 139777658463488
print(f"id(l2[2]) = {id(l2[2])}") # id(l2[2]) = 139777658463488
print(f"id(l1[0]) = {id(l1[0])}") # id(l1[0]) = 9788800
print(f"id(l2[0]) = {id(l2[0])}") # id(l2[0]) = 9788800
l1[2][0] = "aaa"
print(f"l1 = {l1}") # l1 = [123, 'abc', ['aaa', 666], 'x']
print(f"l2 = {l2}") # l2 = [123, 'abc', ['aaa', 666], 'x']
l2[2][1] = 888
print(f"l1 = {l1}") # l1 = [123, 'abc', ['aaa', 888], 'x']
print(f"l2 = {l2}") # l2 = [123, 'abc', ['aaa', 888], 'x']

深拷贝 —— Deep Copy

深拷贝是指复制对象及其内部所有嵌套对象。深拷贝会递归地复制每个对象，不仅复制对象本身，还复制所有嵌套的子对象，这样原始对象和拷贝对象完全独立，修改其中一个不会影响另一个。参考代码如下：

import copy # 需要用到copy模块
# copy模块里面也有浅拷贝函数 copy.copy() 使用所有数据类型

t1 = [123,"abc", ["1a2b", 666], 'x']
t2 = copy.deepcopy(t1)
print(f"t1 = {t1}") # t1 = [123, 'abc', ['1a2b', 666], 'x']
print(f"t2 = {t2}") # t2 = [123, 'abc', ['1a2b', 666], 'x']
print(f"id(t1) = {id(t1)}") # id(t1) = 140355517295168
print(f"id(t2) = {id(t2)}") # id(t2) = 140355517406720
print(f"id(t1[2][0]) = {id(t1[2][1])}") # id(t1[2][0]) = 140355517792112
print(f"id(t2[2][0]) = {id(t2[2][1])}") # id(t2[2][0]) = 140355517792112 不可变数据，所以两者id一样
t1[2][0] = 111
print(f"t1 = {t1}") # t1 = [123, 'abc', [111, 666], 'x']
print(f"t2 = {t2}") # t2 = [123, 'abc', ['1a2b', 666], 'x']
t2[2][1] = "xxx"
print(f"t1 = {t1}") # t1 = [123, 'abc', [111, 666], 'x']
print(f"t2 = {t2}") # t2 = [123, 'abc', ['1a2b', 'xxx'], 'x']

总结

浅拷贝：复制对象，但嵌套对象仍然指向相同的内存地址。
深拷贝：递归复制对象以及所有嵌套的子对象，确保原始对象和拷贝对象之间完全独立。

常见的应用场景

浅拷贝：适用于嵌套对象不需要被修改时。
深拷贝：适用于嵌套对象也需要被独立复制，确保修改拷贝时不影响原始对象。

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