FrozenInstanceError: Cannot Assign to Field

Why This Happens

Frozen dataclasses (frozen=True) are immutable. All fields are set in __init__ and cannot be changed. Use replace() to create new instances with modified values.

The Problem

from dataclasses import dataclass

@dataclass(frozen=True)
class Point:
    x: float
    y: float

p = Point(1.0, 2.0)
p.x = 3.0

The Fix

from dataclasses import dataclass, replace

@dataclass(frozen=True)
class Point:
    x: float
    y: float

p = Point(1.0, 2.0)
p2 = replace(p, x=3.0)

Step-by-Step Fix

1. 1

   Use replace()

   Create new instances: replace(old, field=value).

2. 2

   Remove frozen if not needed

   Use @dataclass without frozen=True for mutable instances.

3. 3

   Keep frozen for immutability

   Use frozen=True for hashable, thread-safe objects.