[Python] 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

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