What are mutable and immutable types in Python?

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Every Python object is either **mutable** (its state can change in place) or **immutable** (any 'change' actually produces a new object). The distinction is about identity over time, not about whether a *name* can be reassigned.

- **Immutable:** `int`, `float`, `bool`, `complex`, `str`, `bytes`, `tuple`, `frozenset`, `range`. Operations return new objects; the original is untouched.
- **Mutable:** `list`, `dict`, `set`, `bytearray`, and most user-defined class instances.

Three practical consequences interviewers probe:

1. **Hashability** — immutable built-ins are hashable, so only they can be dict keys / set members.
2. **Function arguments** — Python passes references. Mutating a mutable argument is visible to the caller; rebinding a name inside the function is not.
3. **Mutable default arguments** — a classic bug (its own question), caused by a mutable object being created once and shared.

`x += 1` on an int rebinds `x` to a brand-new int (the old object is unchanged), whereas `l

What are mutable and immutable types in Python?

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