What does mutability mean in Python?

Defines mutable and immutable objects through observable state changes, aliasing, hashing, and API ownership.

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A mutable object can change its value while keeping the same identity; a list can append an item and remain that list. An immutable object cannot change its value after creation, so an operation that appears to update a string or integer creates or selects another object and rebinds a name. Lists, dictionaries, and sets are mutable. Numbers, strings, bytes, and tuples are immutable, although an immutable tuple can still contain a mutable object.

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The ability to distinguish object mutation from name rebinding and reason about shared state without oversimplifying containers.

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Core answer
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Overview
Mutability affects aliasing, safe sharing, dictionary keys, defaults, and function contracts, so it is more than a list of built-in types.
A mutable object can change its value while keeping the same identity; a list can append an item and remain that list. An immutable object cannot change its value after creation, so an operation that appears to update a string or integer creates or selects another object and rebinds a name. Lists, dictionaries, and sets are mutable. Numbers, strings, bytes, and tuples are immutable, although an immutable tuple can still contain a mutable object.
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The distinction becomes visible through aliases. If two names refer to one dictionary, updating it through either name affects the same object. Rebinding one name to another dictionary does not. In-place syntax is not a universal guarantee of mutation: += can mutate a list but produces another integer or tuple. That is why I reason from the type's contract rather than punctuation. Immutable values are often easier to share and may be hashable, while mutable inputs need explicit ownership expectations.

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Saying immutable means the name cannot be rebound.
Claiming a tuple guarantees every nested value is immutable.
Assuming += always mutates or always creates a new object.
Treating defensive copying as mandatory for every argument.

What does mutability mean in Python?

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