Mutable vs immutable objects

Last Updated: June 6, 2023 1:49 AM UTC

Last edited by:

Greg Taeger <[email protected]>

GitHub <[email protected]>

Update mutability tag to link Nedbat's talk (#2546)

Co-authored-by: wookie184 <[email protected]>

Co-authored-by: Bradley Reynolds <[email protected]>

Imagine that you want to make all letters in a string upper case. Conveniently, strings have an .upper() method.

You might think that this would work:

>>> greeting = "hello"
>>> greeting.upper()
'HELLO'
>>> greeting
'hello'

greeting didn't change. Why is that so?

That's because strings in Python are immutable. You can't change them, you can only pass around existing strings or create new ones.

>>> greeting = "hello"
>>> greeting = greeting.upper()
>>> greeting
'HELLO'

greeting.upper() creates and returns a new string which is like the old one, but with all the letters turned to upper case.

int, float, complex, tuple, frozenset are other examples of immutable data types in Python.

Mutable data types like list, on the other hand, can be changed in-place:

>>> my_list = [1, 2, 3]
>>> my_list.append(4)
>>> my_list
[1, 2, 3, 4]

Other examples of mutable data types in Python are dict and set. Instances of user-defined classes are also mutable.