Python 修改和编辑列表

Question

I am just starting to learn python, I was trying out some code and I wanted to ask why the two following codes function differently?

First code sample:

def combine_lists(list1, list2):
  new_list = list2
  index = len(list2)
  for items in list1:
      new_list.insert(index, items)
  return(new_list)
    
Jamies_list = ["Alice", "Cindy", "Bobby", "Jan", "Peter"]
Drews_list = ["Mike", "Carol", "Greg", "Marcia"]

print(combine_lists(Jamies_list, Drews_list))

The output of this first code is correct (where I want list 1 reversed):

['Mike', 'Carol', 'Greg', 'Marcia', 'Peter', 'Jan', 'Bobby', 'Cindy', 'Alice']

Second code sample:

def combine_lists(list1, list2):
  new_list = list2
  for items in list1:
      new_list.insert(len(list2), items)
  return(new_list)
    
Jamies_list = ["Alice", "Cindy", "Bobby", "Jan", "Peter"]
Drews_list = ["Mike", "Carol", "Greg", "Marcia"]

print(combine_lists(Jamies_list, Drews_list))

But this second code outputs:

['Mike', 'Carol', 'Greg', 'Marcia', 'Alice', 'Cindy', 'Bobby', 'Jan', 'Peter']

Why is there a difference in these two outputs by just assigning the len(list2) to a variable and using that for the index insertion?

Thanks!

Answer 1

This line:

    new_list = list2

Just creates a new name for the list2 list, but both variables refer to the same list.

So, in the first example, you get the length of that list with index = len(list2) and then use the same value for index on each iteration through the for loop.

In the second example, you get len(list2) for each iteration, but since you're modifying new_list , you're also modifying list2 and its length will increase as you go.

If you need a copy of the list, create a copy with list(new_list) = list2 or new_list = list2.copy()

Answer 2

First code snippet

---

def combine_lists(list1, list2):
  new_list = list2
  index = len(list2)
  for items in list1:
      new_list.insert(index, items)
      print(new_list)
  return(new_list)
    
Jamies_list = ["Alice", "Cindy", "Bobby", "Jan", "Peter"]
Drews_list = ["Mike", "Carol", "Greg", "Marcia"]

print(combine_lists(Jamies_list, Drews_list))

Output:

['Mike', 'Carol', 'Greg', 'Marcia', 'Alice']
['Mike', 'Carol', 'Greg', 'Marcia', 'Cindy', 'Alice']
['Mike', 'Carol', 'Greg', 'Marcia', 'Bobby', 'Cindy', 'Alice']
['Mike', 'Carol', 'Greg', 'Marcia', 'Jan', 'Bobby', 'Cindy', 'Alice']
['Mike', 'Carol', 'Greg', 'Marcia', 'Peter', 'Jan', 'Bobby', 'Cindy', 'Alice']
['Mike', 'Carol', 'Greg', 'Marcia', 'Peter', 'Jan', 'Bobby', 'Cindy', 'Alice']

For your first code, you're inserting new_list.insert(index, items) every new item on the same position for a certain period ( len(list2) ) . Every time when you insert it get the same index and last insert element get move to the index+1 position and that's why it seems Jamies_list the list are inserting reversely.

Second code snippet

Since you're allocating len(list2) every time of for loop , it get dynamically update the len and taking the new the position of the new item