What does frozen mean for dataclasses?
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.
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
. Afrozen
dataclass is by default hashable and suitable as adict
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.
Fl4ggi LP
Updated on June 04, 2022Comments
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Fl4ggi LP about 2 years
What's the difference between
@dataclass(frozen=True)
and@dataclass(frozen=False)
? When should I use which?-
khelwood over 3 yearsDo you want instances of your data class to be mutable or immutable?
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Fl4ggi LP over 3 years@khelwood That's the question... What's the difference between mutable and immutable instances? I know that strings and tuples are immutable. So do I have to set frozen True if I'm using using Strings or Tuples?
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khelwood over 3 yearsImmutable means you can't change the attributes or characteristics of an object after it's initialised. What is immutability?
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cikatomo about 3 yearsis there a dataclass so frozen that we cannot even change the default values when instantiating?
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MisterMiyagi about 3 yearsNo. What would be the point? If the attributes are already fixed, they will all be the same instance - so just create that instance once and be done with it.
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cikatomo about 3 yearsActually we can. Either by putting init=False in the decorator or by field(init=False). Together with frozen