What does frozen mean for dataclasses?

Question

What's the difference between @dataclass(frozen=True) and @dataclass(frozen=False)? When should I use which?

Answer 1

In Python, "frozen" means an object cannot be modified. For example, consider set and frozenset:

>>> s = set((1, 2, 3))
>>> s
{1, 2, 3}
>>> s.add(4)
>>> s
{1, 2, 3, 4}
>>> fs = frozenset((1, 2, 3))
>>> fs
frozenset({1, 2, 3})
>>> fs.add(4)
...
AttributeError: 'frozenset' object has no attribute 'add'

Likewise, creating a dataclass with frozen=True means its instances are frozen and cannot be changed.

Be aware that frozen only applies to the dataclass instance itself – a frozen dataclass can contain mutable items such as lists, and a regular dataclass can contain frozen/immutable items such as tuples.

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The point of frozen objects is to avoid accidental modification, and to guarantee a consistent value.

• The former is advantageous to avoid bugs. When an object is not intended to be modified, making it frozen reveals accidental modification via an immediate error.
• The latter allows use as immutable object, for example the keys of a dict. A frozen dataclass is by default hashable and suitable as a dict key.

from dataclasses import dataclass

@dataclass(frozen=True)
class Frozen:
    x: int
    y: int

named_points = {Frozen(0, 0): "Origin"}

Note that hashability does not just depend on the dataclass but is recursive – a frozen dataclass containing a list is not hashable, because the list is not hashable.