List are Mutable

Lists are mutable data type. This means that its composition can be altered without recreation, reassignment, redeclaration. This can be dangerous for most new programmers.

Example 1: Altering via index:

# Manipulation by index

primes = [2,3,5,7,11]
print('primes currently:', primes)
primes[1] = 10000
primes[2] = 'Hello!'

print('primes now:', primes)

primes currently: [2, 3, 5, 7, 11]
primes now: [2, 10000, 'Hello!', 7, 11]

In example 1, you can see that we can change the composition of the list without redeclaring a new list for the variable: primes.

Example 2: Copying a List to another variable:

# Copying a list to a new variable

primes1 = [2,3,5,7,11]
primes2 = primes1
print('primes1 currently:', primes1)
print('primes2 currently:', primes2)

primes2[-1] = 13

print('primes1 now:', primes1)
print('primes2 now:', primes2)

primes1 currently: [2, 3, 5, 7, 11]
primes2 currently: [2, 3, 5, 7, 11]
primes1 now: [2, 3, 5, 7, 13]
primes2 now: [2, 3, 5, 7, 13]

In example 2, we have only manipulated the list called primes2. However, the original list called primes1 was also affected by the changes as well.

This is because when a new variable is created to hold another list variable. Python saves memory by pointing towards where the original list is located rather than creating a whole new copy.

Therefore, any changes will affect both variables.

Solution: Use a method called .copy()

This method will help us create a new list in memory with the same values from the original.

# Using .copy() method

primes1 = [2,3,5,7,11]
primes2 = primes1.copy()
print('primes1 currently:', primes1)
print('primes2 currently:', primes2)

primes2[-1] = 13

print('primes1 now:', primes1)
print('primes2 now:', primes2)

primes1 currently: [2, 3, 5, 7, 11]
primes2 currently: [2, 3, 5, 7, 11]
primes1 now: [2, 3, 5, 7, 11]
primes2 now: [2, 3, 5, 7, 13]

Example 3: Sorting & List Methods

List has a built-in method to help sort. Unlike the sorted() function, list has a method which allows us to manipulate the list you are sorting without creating a whole new variable.

# List. sort() method

a_list = list('Hello, World!')
print('a_list now:', a_list)

a_list.sort()

print('a_list sorted:', a_list)

a_list now: ['H', 'e', 'l', 'l', 'o', ',', ' ', 'W', 'o', 'r', 'l', 'd', '!']
a_list sorted: [' ', '!', ',', 'H', 'W', 'd', 'e', 'l', 'l', 'l', 'o', 'o', 'r']

Examine that there is no assignment operation occurring to help us sort. We are using a method that lists have to just manipulate the given list rather than needing to create a new container like strings.

Below are the list of methods that affect the composition of a list. They will not require recreation/reassignment.

• append()

• extend()

• insert()

• remove()

• pop()

• sort()

• reverse()