Day2-python Basic 2---the difference between a shallow copy and a deep copy

Shallow copy

First, let's look at the following code:

11 >>> names = ["maqing","Peilin","xiaoming","Lilei","Hanmeimei",["maqing","Wangchuan","Lilong"]]22 >>> names2 =names.copy ()33 >>>Print(names)43 |'maqing','Peilin','xiaoming','Lilei','Hanmeimei', ['maqing','Wangchuan','Lilong']]55 >>> names[2] ="Xiao Ming"67 >>>Print(names)78 ['maqing','Peilin','Xiao Ming','Lilei','Hanmeimei', ['maqing','Wangchuan','Lilong']]89 >>>Print(Names2)910 ['maqing','Peilin','xiaoming','Lilei','Hanmeimei', ['maqing','Wangchuan','Lilong']]Ten>>> names[-1][2] ="Li Long" One>>>Print(names) A13 ['maqing','Peilin','Xiao Ming','Lilei','Hanmeimei', ['maqing','Wangchuan','Li Long']] ->>>Print(Names2) -15 ['maqing','Peilin','xiaoming','Lilei','Hanmeimei', ['maqing','Wangchuan','Li Long']]

First step: Modify the value of ' xiaoming ' in Names to Chinese "Xiao Ming", the output list found only names modified, but not modified in Names2, in line with the nature of copy.

The second step: modify the names in the level two list "Lilong" The value of "Li Long", output names and names2, we found that ' lilong ' value has been changed to Chinese, it seems that this does not conform to the nature of copy.

This copy we call a shallow copy.

There are three methods of shallow copy:

• Using the slice [:] Action
• Using factory functions (such as List/dir/set)
• Using the copy () function in the Copy module

1 person = ['name', ['saving', +]]2 P1 = copy.copy (person)3 p2 = person[:]4 p3 = list (person)

Deep copy

1>>>ImportCopy2>>> names = [" would", 28, ["Python","C #","JavaScript"]]3>>> Names2 =copy.deepcopy (names)4>>> Names[1] =' -'5>>>Print(names)6[' would',' -', ['Python','C #','JavaScript']]7>>>Print(Names2)8[' would', 28, ['Python','C #','JavaScript']]9>>> Names[2][1] ="Ruby"Ten>>>Print(names) One[' would',' -', ['Python','Ruby','JavaScript']] A>>>Print(Names2) -[' would', 28, ['Python','C #','JavaScript']]

Summarize

This article describes the assignment and copying of objects, and the differences between them:

• The assignment of an object in Python is an object reference (memory address) Pass
• With Copy.copy (), you can make a shallow copy of the object, which duplicates the object, but still uses the original reference for the element in the object.
• If you need to copy a container object and all of its elements (containing the element's child elements), you can use Copy.deepcopy () to make a deep copy
• For non-container types (such as numbers, strings, and other ' atomic ' types of objects) are not copied one says
• If the meta-ancestor variable contains only atomic type objects, it cannot be deep copied, see the following example

　　　

Day2-python Basic 2---the difference between a shallow copy and a deep copy