Mutable vs. Immutable Objects Explained

A mutable object can be changed after it's created, and an immutable object can't. When you "change" an immutable object, you're really creating a new object with the new value, and the original stays exactly the same.

For example, lists are mutable in Python:

int_list = [4, 9] int_list[0] = 1 # int_list is now [1, 9]

And tuples are immutable:

int_tuple = (4, 9) int_tuple[0] = 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.

Mutable vs. Immutable

	Mutable	Immutable
Can it change after it's created?	Yes, in place	No—"changing" it makes a new object
Shared references	A change through one reference shows up through all of them	Safe to share, since nobody can change it
Cost of an update	Cheap: just change it	Has to copy, so building one up piece by piece (like repeatedly adding to a string) can take O(n²) time overall
Using it across threads	Needs locks or other synchronization if any thread might change it	Safe to share between threads without locks
As a dictionary key	Risky: if the key changes after you insert it, you might not be able to find it again	Works great
Examples in Python 3.6	Lists, dictionaries, sets, and bytearrays	Numbers, strings, tuples, frozensets, and bytes

Neither one is better across the board. Use mutable objects when you need to make lots of updates efficiently, and immutable ones when you want values that can be shared freely without anyone changing them out from under you.

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Frequently Asked Questions

What's the difference between mutable and immutable objects?

Mutable objects can be modified in place after creation; immutable objects cannot—any 'change' creates a new object instead. Immutable objects are safe to share and to use as hash keys, while mutable objects are cheaper to update.

Are strings mutable?

It depends on the language. Strings are immutable in Java, Python, JavaScript, and C#, so every 'change' builds a new string. They're mutable in C++ (std::string) and in Ruby (unless they're frozen).

Which Python types are immutable?

Numbers (int, float), strings, tuples, frozensets, and booleans are immutable. Lists, dictionaries, and sets are mutable.

Why does mutability matter in interviews?

It explains gotchas like shared references, mutable default arguments, and why only immutable (hashable) objects can be used as dictionary keys or set elements.

Mutable vs Immutable Objects