What is the difference between a mutable and an immutable object?

Question

Accepted Answer

**Mutable** objects can be changed after creation. **Immutable** objects cannot be changed.

**Mutable:**
- `list`.
- `dict`.
- `set`.
- Custom objects.

```python
my_list = [1, 2, 3]
my_list[0] = 999  # OK
```

**Immutable:**
- `int`, `float`, `complex`.
- `str`.
- `tuple`.
- `frozenset`.
- `bytes`.

```python
my_tuple = (1, 2, 3)
# my_tuple[0] = 999  # Error!

# But you can "change" via new object
my_tuple = (999, 2, 3)  # New object
```

**Why it matters:**
- Immutable objects can be used as dict keys.
- Immutable objects are thread-safe.

Python: What is the difference between a mutable and an immutable object?