How do I constrain the possible values of an environment variable using pydantic BaseSettings?

Question

I'm using pydantic for the first time and trying to use it to read configuration values from environment variables. What I'd like to do is check that the supplied value is one of a finite list of values. I want to check that the value supplied for format is one of csv or parquet.

From reading the documentation it seems what I need is a ConstrainedType so I think I need something like this:

format: str = constr(regex=r'(csv|parquet)')

however I'm not clear on how to use this in BaseSettings. The environment variable that stores the value is called FORMAT. I have tried:

from pydantic import BaseSettings, Field, constr

class Settings(BaseSettings):
    format: str = constr(regex=r'(csv|parquet)')
    bucket: str = Field(..., env="BUCKET") # bucket is just another setting that I need

but this doesn't work because the value is stored in a environment variable called FORMAT.

How can I have a value read from an environment variable of my choosing and still validate that that the value is an expected value?