Make an immutable coordinate record with namedtuple

Make an immutable coordinate record with namedtuple

By Batu. AI assistance was used to prepare this article.

collections.namedtuple() creates a tuple subclass whose positions also have readable attribute names. Here, Coordinate has x and y fields. The resulting instances can be indexed, unpacked, compared as tuples, and accessed as point.x or point.y. Because the underlying structure is a tuple, assigning to a field raises AttributeError instead of altering the original coordinate.

from collections import namedtuple

Coordinate = namedtuple("Coordinate", ["x", "y"])
origin = Coordinate(2, 3)
moved = origin._replace(y=9)

try:
    origin.x = 5
except AttributeError:
    immutable = True
else:
    immutable = False

assert immutable is True
assert origin == Coordinate(2, 3)
assert moved == Coordinate(2, 9)
assert moved.x == 2 and moved[1] == 9

print(f"origin={origin}")
print(f"moved={moved}")
print(f"x={moved.x}")
print(f"immutable={immutable}")

Expected stdout

origin=Coordinate(x=2, y=3)
moved=Coordinate(x=2, y=9)
x=2
immutable=True

_replace(y=9) is the update mechanism shown here: it returns a new Coordinate, leaving origin at (2, 3). The printed representation includes field names, which is useful when inspecting a result. The assertions check this specific construction and update result, while the caught exception records the field-assignment behavior used by the recipe.

Immutability is shallow. A named tuple can hold a mutable object, such as a list, whose contents may still be changed. namedtuple() also does not enforce that x and y are numeric, share units, or lie in a permitted range; validate those constraints before construction when they matter. Field names must be valid identifiers and cannot begin with an underscore. For defaults or static type annotations, consider a dataclass or typing.NamedTuple when those features better fit the program.

Source: Python namedtuple() documentation.

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