Python - several ways to copy a list (list)

• When I was writing code recently, the script couldn't run no matter what. After checking for a long time, it turned out that I made a low-level mistake.
• The value of the original list needs to be copied in the script, and I directly use "=" to assign the value. The code example is as follows:

  list_old = [1,2,3]
  list_new = list_old
  

• The result of this is that when I try to modify the new list, the values ​​​​of the original list will also be modified.
• The correct way should be to use a method that just copies the list values.
• This article records the various methods of copying the list. It can be regarded as a summary and a reminder to myself. I hope that I will not make such low-level mistakes again.

1. Direct assignment

• This method is the case above

  list_old = [1,2,3]
  list_new = list_old
  

• In the case of direct assignment, both the original list and the new list point to the same list object.
  
• For example, add an element to the new list, print the old list data and you will find that an element is also added
  

Second, the list copy method ( list.copy() )

• This method will only copy the values ​​inside the list

  list_old = [1,2,3]
  list_new = list_old.copy()
  

• The old and new lists point to two list objects with the same value
  

• Adding an element to the new list leaves the old list unaffected.
  

3. Restructure the list

• The next few methods are relatively similar, all of which take out the data in the old list and reconstruct it into a new list
• So just make a brief introduction and choose the appropriate method.

1. Utilize list index

list_old = [1,2,3]
list_new = list_old[:]

2. Use list() to construct a list

list_old = [1,2,3]
list_new = list(list_old)

3. Use extend() to construct a list

extend() function : Append multiple values ​​in another sequence at the end of the list at once

list_old = [1,2,3]
list_new = []
list_new = list_new.extend(list_old)

4. Use for + append()

list_old = [1,2,3]
list_new = []
for i in list_old:
	list_new.append(i)

Four, deep copy copy.deepcopy()

• Note: The above method is just a shallow copy, in simple terms, only one layer of the list will be copied.
• for example:

  list_old = [1,2,3,[4,5,6]]
  list_new = list_old.copy()
  

  • In this case, only the first list elements are copied, but if the operation is performed on the second-level list elements "[4,5,6]", it will affect both the old and new lists.
  • At this time, in the old and new lists, the second-level elements still point to the same list "[4,5,6]"
    
• Then you need to use deep copy

  import copy
  list_old = [1,2,3,[4,5,6]]
  list_new = copy.deepcopy(list_old)
  

  • In this way, the element "[4,5,6]" of the new list will not be affected by the old list
    

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