schema_with_field.py

# output-json
from typing import Any, Callable, Dict, Generator, Optional

from pydantic import BaseModel, Field
from pydantic.fields import ModelField


class RestrictedAlphabetStr(str):
    @classmethod
    def __get_validators__(cls) -> Generator[Callable, None, None]:
        yield cls.validate

    @classmethod
    def validate(cls, value: str, field: ModelField):
        alphabet = field.field_info.extra['alphabet']
        if any(c not in alphabet for c in value):
            raise ValueError(f'{value!r} is not restricted to {alphabet!r}')
        return cls(value)

    @classmethod
    def __modify_schema__(
        cls, field_schema: Dict[str, Any], field: Optional[ModelField]
    ):
        if field:
            alphabet = field.field_info.extra['alphabet']
            field_schema['examples'] = [c * 3 for c in alphabet]


class MyModel(BaseModel):
    value: RestrictedAlphabetStr = Field(alphabet='ABC')


print(MyModel.schema_json(indent=2))