A detailed copy of the depth in Python

To get a clear picture of a deep copy in Python, you need to figure out the following concepts:

Variables-References-Objects (Mutable objects, immutable objects)-slices-copy (shallow copy, deep copy)

" Variable-Object-Reference "

In Python everything is an object, such as:3, 3.14, ' Hello ', [1,2,3,4],{' a ': 1} ...

Even the type itself is an object, type Object

The variables in Python are different from C/c++/java, which refers to the object's reference, Python is a dynamic type, and when the program runs, it is based on the type of the object

To determine what type of variable it is.

assign a value individually : Say, for example:

>>> A = 3

After running a=3, variable a becomes a reference to object 3. Internally, the variable is actually a pointer to the object memory space

Because a python variable is simply a reference to an object, or to the ruling of an object, variable references can often be changed in a program

>>> x = #变量绑定到整型对象
>>> x = ' Hello ' #现在又成了字符串
>>> x = [#现在又成了列表]

The professional statement is as follows:

A variable is an element of a system table that has a connection to an object

Object is a piece of memory that is allocated, storing the value it represents

A reference is an automatically formed pointer from a variable to an object

Special Note: types belong to objects, not variables

For example, like the A=3, the integer object 3 contains the dual information

1. value is 3

2. A header message: Tell Pthyon that this is an integer object [equivalent to a pointer to int]

shared references : for example,

>>> A = 3

>>> B = A

After you run assignment statement B = A, the variable A and variable B point to the memory space of the same object.

As you can see, A and B, whose IDs are exactly the same, point to the same integer object 3, or the same piece of memory.

If you delete a, it will not affect B

The introduction of copy concepts is based on the potential side effects of shared references to mutable objects.

" mutable objects- immutable Objects "

Immutable objects in Python refer to objects that cannot be modified once created, including strings, tuples, numbers

mutable objects in Python refer to: Objects that can be modified, including: lists, dictionaries

The above said a, b are integers, integers are immutable objects, if they are mutable objects, it is another matter.

2. >>> L1 = [2,3,4] #L1变量指向的是一个可变对象: List
4. >>> L2 = L1 #将L1值赋给L2后, both share references to the same list object [1,2,3,4]
6. >>> l1[0] = #因为列表可变, change the value of the first element in the L1
8. >>> L1; L2 #改变后, l1,l2 change at the same time, because the value of the object itself has changed.
10. [3, 4]
12. [3, 4]

If you do not want to change the value of the list L2, there are two ways : slice and copy module

2. >>> L1 = [2,3,4]
4. >>> L2 = L1
6. >>> ID (L1); ID (L2) #共享引用一个可变对象
8. 45811784L
10. 45811784L
12. >>> L2 = l1[:] #切片操作
14. >>> ID (L1); ID (L2) #切片后, the object is different.
16. 45811784L
18. 45806920L
20. >>> l1[0] =
22. >>> L1; L2 #L1发生改变, L2 no change
24. [3, 4]
26. [2, 3, 4]

" copy "

1. slicing techniques are applied to all sequences, including: List, String, Ganso

>>> but slices cannot be applied to dictionaries. You can only use the d.copy () method or the d.deepcopy () method for a dictionary.

2. deep Copy, can be used for sequence, also can be used in dictionary

>>> Import Copy

>>> X = copy.copy (Y) #浅拷贝: Copies only top-level objects, or: parent objects

>>> X = copy.deepcopy (Y) #深拷贝: Copies all objects, top-level objects, and their nested objects. Or, the parent object and its child objects

• if the dictionary has only top-level objects :

• If the object is nested in the dictionary:

" Conclusion"

• A deep copy is a copy of the source object, taking up different memory space
• If the source object has only one level directory, the source makes any changes, does not affect the dark Copy object
• If the source object has more than one level of directory, the source makes any changes to affect the shallow copy, but does not affect the deep copy
• The slices of a sequence object are actually shallow copies, that is, only top-level objects are copied

A detailed copy of the depth in Python