Separate factory from class

factory.py

import dataclasses

class DataFactory(type):
    def from_number(cls, num: int) -> "Data":
        return cls(str(num))

@dataclasses.dataclass
class Data(metaclass=DataFactory):
    value: str

d = Data.from_number(123) # <-- Method only available on the class, not on the instance
print(d)
assert "from_number" not in dir(d)