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Copying Lists

Assume that var1 is a variables of type int, float, str, or bool. We have seen before that if we create a new variable var2 and set its initial value to be equal to that of var1, then var2 will initially contain the same value as var1, but will be its own distinct variable whose value can be changed independent of var1. This is illustrated in the following two cells.

var1 = 37
var2 = var1 
print(var1)
print(var2)

37
37

var2 = "blah"
print(var1)
print(var2)

37
blah

The situation is different for lists, however. If we have a list called list1 and we set list2 to be equal to list1, then list1 and list2 will be two different names of the same list. Any changes made to list1 will also effect list2 and vice versa.

list1 = [3, 8, 2]
list2 = list1
print(list1)
print(list2)

[3, 8, 2]
[3, 8, 2]

list2.append(37)
print(list1)
print(list2)

[3, 8, 2, 37]
[3, 8, 2, 37]

What if we want to actually create a new, entirely separate, copy of an already existing list? It turns out that Python provides two ways of doing a such.

  1. We may use the copy() method of the list that we wish to duplicate.

  2. We can return a copy of a list by using slicing.

Let’s see first see how to duplicate a list using the copy() method.

dup1 = list1.copy()
print(list1)
print(dup1)

[3, 8, 2, 37]
[3, 8, 2, 37]

dup1.append(42)
print(list1)
print(dup1)

[3, 8, 2, 37]
[3, 8, 2, 37, 42]

We will now see how to duplicate a list by using slicing.

dup2 = list1[:]
print(list1)
print(dup2)

[3, 8, 2, 37]
[3, 8, 2, 37]

dup2.remove(8)
print(list1)
print(dup2)

[3, 8, 2, 37]
[3, 2, 37]

Example: Copying and Sorting Lists

A list called Avengers is provided in the cell below. Add code to accomplish the following tasks:

  1. Create two new copies of the list, one called Avengers_Asc and one called Avengers_Desc.

  2. Sort Avengers_Asc in ascending alphabetical order.

  3. Sort Avengers_Desc in descending alphabetical order.

  4. Print all three lists.

Avengers = ['Capt. America', 'Black Widow', 'Iron Man', 'Hulk', 'Thor', 'Hawkeye']

Avengers_Asc = Avengers.copy()
Avengers_Desc = Avengers.copy()

Avengers_Asc.sort()
Avengers_Desc.sort(reverse=True)

print(Avengers)
print(Avengers_Asc)
print(Avengers_Desc)

['Capt. America', 'Black Widow', 'Iron Man', 'Hulk', 'Thor', 'Hawkeye']
['Black Widow', 'Capt. America', 'Hawkeye', 'Hulk', 'Iron Man', 'Thor']
['Thor', 'Iron Man', 'Hulk', 'Hawkeye', 'Capt. America', 'Black Widow']

Avengers_Asc = sorted(Avengers)
Avengers_Desc = sorted(Avengers)
Avengers_Desc.reverse()

print(Avengers)
print(Avengers_Asc)
print(Avengers_Desc)

['Capt. America', 'Black Widow', 'Iron Man', 'Hulk', 'Thor', 'Hawkeye']
['Black Widow', 'Capt. America', 'Hawkeye', 'Hulk', 'Iron Man', 'Thor']
['Thor', 'Iron Man', 'Hulk', 'Hawkeye', 'Capt. America', 'Black Widow']
Finding Index of List Elements Working with Data Stored in Parallel Lists