frozen

Learn immutable data classes with @dataclass(frozen=True).

frozen=True

@dataclass(frozen=True) makes the dataclass immutable. After creating an instance you cannot assign new values to its fields; trying raises FrozenInstanceError. This is ideal for constant values.

Example: @dataclass(frozen=True)\nclass Color:\n r: int = 0\n g: int = 0\n b: int = 0. After c = Color(255, 0, 0), writing c.r = 100 raises an error. hash(c) works; frozen classes are hashable.

frozen and hash

frozen dataclasses are hashable. So you can use their instances as dictionary keys or set members. hash(c) returns a number; two instances with the same values give the same number.

This is useful for constant value types like coordinates, colors or currencies. You represent values you want to store as dictionary keys with frozen dataclasses; since they do not change, the key is not corrupted.

Exercise Make the Color class frozen

Start with from dataclasses import dataclass. Define a Color class with @dataclass(frozen=True); include fields r: int = 0, g: int = 0 and b: int = 0. Then create an instance with c = Color(255, 0, 0) and print with print(c.r).

Show example solution

# Write the Color class as a frozen dataclass
from dataclasses import dataclass

@dataclass(frozen=True)
class Color:
    r: int = 0
    g: int = 0
    b: int = 0

c = Color(255, 0, 0)
print(c.r)

Test yourself

What does frozen=True prevent?
- Field assignment
- Instance creation
- Class definition
- Print calls
Show answer

Field assignment — frozen=True prevents field assignment after the instance is created.

frozen decorator line

@dataclass(____)
class Color:
    r: int = 0
    g: int = 0
    b: int = 0

Show answer

@dataclass(frozen=True)
class Color:
    r: int = 0
    g: int = 0
    b: int = 0

Match the frozen parts

frozen=True

Argument
Color(255, 0, 0)

Instance
FrozenInstanceError

Assignment error
hash(c)

Hash call

Next lesson: __post_init__

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