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Immutability

Notice that in the prior examples, we were not changing the original string with any of the operations we ran on it. Every string operation is defined to produce a new string as its result, because strings are immutable in Python—they cannot be changed in-place after they are created. For example, you can’t change a string by assigning to one of its positions, but you can always build a new one and assign it to the same name. Because Python cleans up old objects as you go (as you’ll see later), this isn’t as inefficient as it may sound:

>>> S
'Spam'
>>> S[0] = 'z'             # Immutable objects cannot be changed
...error text omitted...
TypeError: 'str' object does not support item assignment

>>> S = 'z' + S[1:]        # But we can run expressions to make new objects
>>> S
'zpam'

Every object in Python is classified as either immutable (unchangeable) or not. In terms of the core types, numbers, strings, and tuples are immutable; lists and dictionaries are not (they can be changed in-place freely). Among other things, immutability can be used to guarantee that an object remains constant throughout your program.