Trouble creating DefaultDict in Dataclass

Question

I am having trouble setting up a simple dataclass using a new defaultdict(dict).

If I tell the factory to use 'dict' as below , instantiation fails with typerror collection.defaultdict object is not callable

from collections import defaultdict
from dataclasses import dataclass, field

@dataclass
class ResultSet:
    changed: bool = False
    mqttdata:  defaultdict(dict) = field(default_factory=defaultdict(dict)) # does not work!

It does sorta work using field(default_factory=defaultdict) but then my code will fail later when it encounters missing keys - presumably because defaultdict was not set up for dict.

How do I properly set up a new defaultdict(dict) in a dataclass?

Answer 1

You have a few problems with the code and how you are using dataclasses currently:

• Type generics in annotations need to be specified through square brackets [] generally, so default[dict] instead of defaultdict(dict) for example.

• The default_factory argument to dataclasses.field() needs to be a no-arg callable which returns a new object with default values set. For example, assuming you have a nested dataclass Inner which specifies defaults for all fields, you could use default_factory=Inner to create a new Inner object each time the main dataclass is instantiated.

  Note that the default_factory argument is mainly useful for mutable types such as set, list, and dict, so that the same object isn't shared (and potentially mutated) between dataclass instances.

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Putting it all together, here is the working code which sets a default value for a field of type defaultdict[dict]:

from collections import defaultdict
from dataclasses import dataclass, field


@dataclass
class ResultSet:
    changed: bool = False
    mqttdata: defaultdict[dict] = field(default_factory=lambda: defaultdict(dict))  # works!


print(ResultSet())

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In Python versions earlier than 3.9, which is when PEP 585 was introduced, you'll need to add the following import at the top so that any type annotations are lazy-evaluated:

from __future__ import annotations