What is the difference between mutable and immutable objects?

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Every Python value is an object , and every object is either mutable (its state can change after creation) or immutable (its state is fixed for life). The distinction affects how assignment, function arguments, hashing, and aliasing behave. Which is which: Immutable — int , float , complex , bool , str , bytes , tuple , frozenset , range , None . Mutable — list , dict , set , bytearray , most user-defined classes, many standard-library objects ( deque , array.array ). What mutability actually means: for an immutable object, any “modification” creates a new object — the original is unchanged. The mutable default argument gotcha is one of the most asked Python questions. Show the bug AND the fix (None default). Know which types are mutable vs immutable. The aliasing issue (two names, one object) is equally important.