Mutable vs immutable objects

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()
>>> greeting

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

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.

For an in-depth guide on mutability see Ned Batchelder's video on names and values.