[Py]python deep copy and shallow copy

Python deep copy shallow copy or deep copy shallow copy
Simply put.

1. Copy.copy a shallow copy copies only the parent object and does not copy the inner child objects of the object.
2. Copy.deepcopy deep Copy Copy objects and their sub-objects

A simple example illustrates the following:

>>>import copy>>>a = [1, 2, 3, 4, ['a', 'b', 'c']]>>> b = a>>> c = copy.copy(a)>>> d = copy.deepcopy(a)

It is easy to understand: A is a list, the table element A[4] is also a list (that is, an internal sub-object), B is another reference to a list, so a, B is exactly the same, can be proved by ID (a) ==id (b).

The 4th line is a shallow copy, the fifth line is a deep copy, and the ID (c) and ID (d) can be found to be different and not the same as the ID (a):

>>> id(a)19276104>>> id(b)19276104>>> id(c)19113304>>> id(d)19286976

As for how to see the difference between a deep/shallow copy, the following actions can be shown:

>>> a.append(5)    #操作1>>> a[4].append('hello')   #操作2

Then look at the results:

>>> a[1, 2, 0, 4, ['a', 'b', 'c', 'hello'], 5]>>> b[1, 2, 0, 4, ['a', 'b', 'c', 'hello'], 5]>>> c[1, 2, 3, 4, ['a', 'b', 'c', 'hello']]>>> d[1, 2, 3, 4, ['a', 'b', 'c']]

It can be found that a, B is affected by Operation 1, 2, C is only affected by Operation 2, D is unaffected. A, b the same result is very good understanding. Because C is a shallow copy of a, only the parent object is copied, so a sub-object ([' A ', ' B ', ' C ', ' hello ']) will change to affect the c;d is a deep copy, completely unaffected by a

Simply put.

1. copy.copy 浅拷贝 只拷贝父对象，不会拷贝对象的内部的子对象。2. copy.deepcopy 深拷贝 拷贝对象及其子对象

A simple example illustrates the following:

>>>import copy>>>a = [1, 2, 3, 4, ['a', 'b', 'c']]>>> b = a>>> c = copy.copy(a)>>> d = copy.deepcopy(a)

It is easy to understand: A is a list, the table element A[4] is also a list (that is, an internal sub-object), B is another reference to a list, so a, B is exactly the same, can be proved by ID (a) ==id (b).

The 4th line is a shallow copy, the fifth line is a deep copy, and the ID (c) and ID (d) can be found to be different and not the same as the ID (a):

>>> id(a)19276104>>> id(b)19276104>>> id(c)19113304>>> id(d)19286976

As for how to see the difference between a deep/shallow copy, the following actions can be shown:

>>> a.append(5)    #操作1>>> a[4].append('hello')   #操作2

Then look at the results:

>>> a[1, 2, 0, 4, ['a', 'b', 'c', 'hello'], 5]>>> b[1, 2, 0, 4, ['a', 'b', 'c', 'hello'], 5]>>> c[1, 2, 3, 4, ['a', 'b', 'c', 'hello']]>>> d[1, 2, 3, 4, ['a', 'b', 'c']]

It can be found that a, B is affected by Operation 1, 2, C is only affected by Operation 2, D is unaffected. A, b the same result is very good understanding. Because C is a shallow copy of a, only the parent object is copied, so a sub-object ([' A ', ' B ', ' C ', ' hello ']) will change to affect the c;d is a deep copy, completely unaffected by a

Simply put.

1. Copy.copy a shallow copy copies only the parent object and does not copy the inner child objects of the object.
2. Copy.deepcopy deep Copy Copy objects and their sub-objects

A simple example illustrates the following:

>>>import copy>>>a = [1, 2, 3, 4, ['a', 'b', 'c']]>>> b = a>>> c = copy.copy(a)>>> d = copy.deepcopy(a)

It is easy to understand: A is a list, the table element A[4] is also a list (that is, an internal sub-object), B is another reference to a list, so a, B is exactly the same, can be proved by ID (a) ==id (b).

The 4th line is a shallow copy, the fifth line is a deep copy, and the ID (c) and ID (d) can be found to be different and not the same as the ID (a):

ID (a)
19276104
ID (b)
19276104
ID (c)
19113304
ID (d)
19286976

至于如何看深/浅拷贝的区别，可以通过下面的操作来展现：

A.append (5) #操作1
A[4].append (' Hello ') #操作2

这时再查看结果：

A
[1, 2, 0, 4, [' A ', ' B ', ' C ', ' Hello '], 5]
B
[1, 2, 0, 4, [' A ', ' B ', ' C ', ' Hello '], 5]
C
[1, 2, 3, 4, [' A ', ' B ', ' C ', ' hello ']
D
[1, 2, 3, 4, [' A ', ' B ', ' C ']]

可以发现a、b受了操作1、2的影响，c只受操作2影响，d不受影响。a、b结果相同很好理解。由于c是a的浅拷贝，只拷贝了父对象，因此a的子对象（ ['a', 'b', 'c', 'hello']）改变时会影响到c；d是深拷贝，完全不受a的影响? ===========浅拷贝是指拷贝的只是原对象元素的引用，换句话说，浅拷贝产生的对象本身是新的，但是它的内容不是新的，只是对原对象的一个引用。这里有个例子

Alist=[[1, 2], 3, 4]
Blist = alist[:] #利用切片完成一次浅拷贝
ID (alist)
3084416588L
ID (blist)
3084418156L
Alist[0][0] = 5
Alist
[[5, 2], 3, 4]
Blist
[[5, 2], 3, 4]

可以看到，浅拷贝生产了一个新的对象bList,但是aList的内容确实对aList的引用，所以但改变aList中值的时候，bList的值也跟着变化了。但是有点需要特别提醒的，如果对象本身是不可变的，那么浅拷贝时也会产生两个值，见这个例子：

Alist = [1, 2]
Blist = alist[:]
Blist
[1, 2]
Alist
[1, 2]
alist[1]=111
Alist
[1, 111]
Blist
[1, 2]

为什么bList的第二个元素没有变成111呢？因为数字在python中是不可变类型！！这个顺便回顾下Python标准类型的分类：可变类型： 列表，字典不可变类型：数字，字符串，元组理解了浅拷贝，深拷贝是什么自然就很清楚了。python中有一个模块copy，deepcopy函数用于深拷贝，copy函数用于浅拷贝。最后，对象的赋值是深拷贝还是浅拷贝？对象赋值实际上是简单的对象引用

A = 1
ID (a)
135720760
b = A
ID (b)
135720760
```
A and B are exactly the same thing.

Reference: Python's depth copy

[Py]python deep copy and shallow copy