Mutable vs Immutable Objects

A mutable object can be changed after it's created, and an immutable object can't.

For example, lists are mutable in Python:

int_list = [4][9]

int_list = 1
# int_list is now[1][9]

And tuples are immutable:

int_tuple = (4, 9)

int_tuple = 1
# Raises: TypeError: 'tuple' object does not support item assignment

Strings can be mutable or immutable depending on the language.

Strings are immutable in Python:

test_string = 'mutable?'

test_string[7] = '!'
# Raises: TypeError: 'str' object does not support item assignment

But in some other languages, like Ruby, strings are mutable:

test_string = 'mutable?'

test_string[7] = '!'
# test_string is now 'mutable!'

Mutable objects are nice because you can make changes in-place, without allocating a new object. But be careful—whenever you make an in-place change to an object, all references to that object will now reflect the change.