Set default based on another field in pydantic BaseModel

I'd like to make the default value of the second filed in a BaseModel class dependend on the first filed + some external dictionary.

Please, how to achieve that with Pydantic?

Here is my naive and not working code:

from pydantic import BaseModel

MY_DICT: dict[str, int] = {
    "a": 1,
    "b": 2,
}

class MyConfig(BaseModel):
    letter: str
    plus_one_by_default_or_any_int: int = MY_DICT[letter] + 1

You could use a pydantic validator to achieve that.

from typing import Any, Optional, Dict

from pydantic import BaseModel, root_validator

class MyConfig(BaseModel):
    letter: str
    plus_one_by_default_or_any_int: int = 0

    @root_validator(pre=True)
    def default_val(cls, values: Dict[str, Any]):
        if values.get("plus_one_by_default_or_any_int") is None:
            values["plus_one_by_default_or_any_int"] = MY_DICT[values["letter"]] + 1
        return values