Editing Immutable object (section)

=== Python ===
In [[Python (programming language)|Python]], some built-in types (numbers, Booleans, strings, tuples, frozensets) are immutable, but custom classes are generally mutable. To simulate immutability in a class, one could override attribute setting and deletion to raise exceptions:

<syntaxhighlight lang="python">
class ImmutablePoint:
    """An immutable class with two attributes 'x' and 'y'."""

    __slots__ = ['x', 'y']

    def __setattr__(self, *args):
        raise TypeError("Can not modify immutable instance.")

    __delattr__ = __setattr__

    def __init__(self, x, y):
        # We can no longer use self.value = value to store the instance data
        # so we must explicitly call the superclass
        super().__setattr__('x', x)
        super().__setattr__('y', y)
</syntaxhighlight>

The standard library helpers [https://docs.python.org/3/library/collections.html#collections.namedtuple <code>collections.namedtuple</code>] and [https://docs.python.org/3/library/typing.html#typing.NamedTuple <code>typing.NamedTuple</code>], available from Python 3.6 onward, create simple immutable classes. The following example is roughly equivalent to the above, plus some tuple-like features:

<syntaxhighlight lang="python">
from typing import NamedTuple
import collections

Point = collections.namedtuple('Point', ['x', 'y'])

# the following creates a similar namedtuple to the above
class Point(NamedTuple):
    x: int
    y: int
</syntaxhighlight>

Introduced in Python 3.7, [https://docs.python.org/3/library/dataclasses.html <code>dataclasses</code>] allow developers to emulate immutability with [https://docs.python.org/3/library/dataclasses.html#frozen-instances frozen instances].  If a frozen dataclass is built, <code>dataclasses</code> will override <code>__setattr__()</code> and <code>__delattr__()</code> to raise <code>FrozenInstanceError</code> if invoked.

<syntaxhighlight lang="python">
from dataclasses import dataclass

@dataclass(frozen=True)
class Point:
    x: int
    y: int
</syntaxhighlight>