How to Create Immutable Data Classes with frozen=True in Python

Create immutable data classes in Python using @dataclass(frozen=True) to prevent attribute modifications after instantiation.

Easy Python 3.7+ Aug 9, 2026 OOP & classes 13 views 0 copies

dataclass frozen immutable oop

Python code

19 lines

Python 3.7+

from dataclasses import dataclass

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

    def distance_from_origin(self) -> float:
        return (self.x**2 + self.y**2) ** 0.5

if __name__ == "__main__":
    p = Point(3.0, 4.0)
    print(p)
    print(f"Distance from origin: {p.distance_from_origin():.2f}")

    try:
        p.x = 10.0
    except AttributeError as e:
        print(f"Cannot modify frozen dataclass: {e}")

Output

stdout

Point(x=3.0, y=4.0)
Distance from origin: 5.00
Cannot modify frozen dataclass: cannot assign to field 'x'

How it works

The @dataclass(frozen=True) decorator automatically generates an __init__, __repr__, and other dunder methods while making the instance immutable. When you try to set an attribute, Python raises an AttributeError because the __setattr__ method is overridden to disallow changes. This immutability makes instances hashable (if fields are hashable), which is useful for using them as dictionary keys or in sets. The class-level distance_from_origin method remains usable since it only reads attributes.

Common mistakes

Assuming frozen dataclasses cannot have any methods—they can, as long as they don't modify state.
Forgetting that frozen=True makes instance hashable only if all fields are hashable.
Trying to mutate a field inside a method will raise AttributeError at runtime.

Variations

Use `@dataclass(frozen=True, order=True)` to get ordering methods for sorting.
Use `frozen=True` with `slot=True` (Python 3.10+) for memory efficiency and speed.

Real-world use cases

Modeling immutable value objects like coordinates or configuration records in a data pipeline.
Using frozen dataclass instances as dictionary keys for caching or counting occurrences.
Defining API request/response schemas that should not be modified after creation.

How to Create Immutable Data Classes with frozen=True in Python

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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Python code

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How it works

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Real-world use cases

More from OOP & classes

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