how I'd like a no-op type to behave

sensitive.py

from pydantic import BaseModel


Sensitive = ...


class Config(BaseModel):
    user: str
    password: Sensitive[str]

    def to_anonymized_dict(self):
        values = self.dict()
        for name in values:
            if isinstance(values[name], Sensitive):
                value = str(values[name])[:]
                peek = len(value) // 5
                values[name] = value[:peek] + ("*"*(len(value)-(2*peek))) + value[-peek:]
        return values


cfg = Config(user="foo", password="12345678")  # should create an instance as if `Sensitive[str]` were the same as `str`
print(cfg.to_anonymized_dict())  # {'user': 'foo', 'password': '1******8'}