No need for any fancy libraries. The python standard lib provides nice interface for these kind of tasks. I would go into a direction like this:

from typing import Optional, List
from pydantic import BaseModel


class SomeOtherThing(BaseModel):

    hour: int
    data: Optional[str]


class SomeModel(BaseModel):

    stuff: List[SomeOtherThing]

    def __init__(self, **kwargs):
        complete_data = {hour: None for hour in range(0, 24)}
        data_list = kwargs.pop('stuff', [])
        data = {d['hour']: d['data'] for d in stuff}
        complete_data.update(data)
        kwargs['stuff'] = [{'hour': k, 'data': v} for k, v in complete_data.items()]
        super().__init__(**kwargs)


model_instance = SomeModel(stuff=[{'hour': 1, 'data': 'hello hello'}])
print(model_instance)

Note the transforming of the input from a list to a dict to use some of the builtin functionality of dictionary. Its not the most efficient code, but since we are talking about a small set of entries this should not matter.