什么是浅拷贝，深拷贝和普通赋值操作之间的区别？

Question

import copy

a = "deepak"
b = 1, 2, 3, 4
c = [1, 2, 3, 4]
d = {1: 10, 2: 20, 3: 30}

a1 = copy.copy(a)
b1 = copy.copy(b)
c1 = copy.copy(c)
d1 = copy.copy(d)


print("immutable - id(a)==id(a1)", id(a) == id(a1))
print("immutable - id(b)==id(b1)", id(b) == id(b1))
print("mutable - id(c)==id(c1)", id(c) == id(c1))
print("mutable - id(d)==id(d1)", id(d) == id(d1))

我得到以下结果：

immutable - id(a)==id(a1) True
immutable - id(b)==id(b1) True
mutable - id(c)==id(c1) False
mutable - id(d)==id(d1) False

如果我执行deepcopy：

a1 = copy.deepcopy(a)
b1 = copy.deepcopy(b)
c1 = copy.deepcopy(c)
d1 = copy.deepcopy(d)

结果相同：

immutable - id(a)==id(a1) True
immutable - id(b)==id(b1) True
mutable - id(c)==id(c1) False
mutable - id(d)==id(d1) False

如果我从事分配操作：

a1 = a
b1 = b
c1 = c
d1 = d

结果是：

immutable - id(a)==id(a1) True
immutable - id(b)==id(b1) True
mutable - id(c)==id(c1) True
mutable - id(d)==id(d1) True

有人能解释一下这些复制品到底有什么区别吗？它是否与可变对象相关？如果是，你能解释给我听吗？

Answer 1

对于不可变对象，不需要复制，因为数据永远不会更改，所以Python使用相同的数据；id总是相同的。对于可变对象，由于它们可能发生更改，[浅层]复制将创建一个新对象。

深度复制与嵌套结构相关。如果有列表列表，那么deepcopycopies嵌套列表也会，因此它是递归副本。只需复制，您就有了一个新的外部列表，但内部列表是引用。

分配不复制。它只是设置对旧数据的引用。因此，您需要复制以创建具有相同内容的新列表。

Answer 2

对于不可变对象，创建副本没有多大意义，因为它们不会改变。对于可变对象^{}，^{}和^{}的行为不同。让我们用例子来讨论它们。

赋值操作只是将源的引用赋值给目标，例如：

>>> i = [1,2,3]
>>> j=i
>>> hex(id(i)), hex(id(j))
>>> ('0x10296f908', '0x10296f908') #Both addresses are identical

现在i和j在技术上指的是同一个列表。i和j都有相同的内存地址。任何更新 它们中的一个会反射到另一个。e、 克：

>>> i.append(4)
>>> j
>>> [1,2,3,4] #Destination is updated

>>> j.append(5)
>>> i
>>> [1,2,3,4,5] #Source is updated

另一方面，copy和deepcopy创建变量的新副本。所以现在对原始变量的更改将不会反映出来 复制变量，反之亦然。但是copy(shallow copy)，不要创建嵌套对象的副本，而只是 复制嵌套对象的引用。Deepcopy递归地复制所有嵌套对象。

演示copy和deepcopy行为的一些示例：

使用copy：的平面列表示例

>>> import copy
>>> i = [1,2,3]
>>> j = copy.copy(i)
>>> hex(id(i)), hex(id(j))
>>> ('0x102b9b7c8', '0x102971cc8') #Both addresses are different

>>> i.append(4)
>>> j
>>> [1,2,3] #Updation of original list didn't affected copied variable

使用copy：的嵌套列表示例

>>> import copy
>>> i = [1,2,3,[4,5]]
>>> j = copy.copy(i)

>>> hex(id(i)), hex(id(j))
>>> ('0x102b9b7c8', '0x102971cc8') #Both addresses are still different

>>> hex(id(i[3])), hex(id(j[3]))
>>> ('0x10296f908', '0x10296f908') #Nested lists have same address

>>> i[3].append(6)
>>> j
>>> [1,2,3,[4,5,6]] #Updation of original nested list updated the copy as well

使用deepcopy：的平面列表示例

>>> import copy
>>> i = [1,2,3]
>>> j = copy.deepcopy(i)
>>> hex(id(i)), hex(id(j))
>>> ('0x102b9b7c8', '0x102971cc8') #Both addresses are different

>>> i.append(4)
>>> j
>>> [1,2,3] #Updation of original list didn't affected copied variable

使用deepcopy：的嵌套列表示例

>>> import copy
>>> i = [1,2,3,[4,5]]
>>> j = copy.deepcopy(i)

>>> hex(id(i)), hex(id(j))
>>> ('0x102b9b7c8', '0x102971cc8') #Both addresses are still different

>>> hex(id(i[3])), hex(id(j[3]))
>>> ('0x10296f908', '0x102b9b7c8') #Nested lists have different addresses

>>> i[3].append(6)
>>> j
>>> [1,2,3,[4,5]] #Updation of original nested list didn't affected the copied variable    

Answer 3

正常的赋值操作只是将新变量指向现有对象。docs解释了浅拷贝和深拷贝之间的区别：

The difference between shallow and deep copying is only relevant for compound objects (objects that contain other objects, like lists or class instances):

• A shallow copy constructs a new compound object and then (to the extent possible) inserts references into it to the objects found in the original.

• A deep copy constructs a new compound object and then, recursively, inserts copies into it of the objects found in the original.

下面是一个小演示：

import copy

a = [1, 2, 3]
b = [4, 5, 6]
c = [a, b]

使用普通赋值操作复制：

d = c

print id(c) == id(d)          # True - d is the same object as c
print id(c[0]) == id(d[0])    # True - d[0] is the same object as c[0]

使用浅拷贝：

d = copy.copy(c)

print id(c) == id(d)          # False - d is now a new object
print id(c[0]) == id(d[0])    # True - d[0] is the same object as c[0]

使用深层副本：

d = copy.deepcopy(c)

print id(c) == id(d)          # False - d is now a new object
print id(c[0]) == id(d[0])    # False - d[0] is now a new object