How do I get Cognito user pool information from lambda (python)?

We are considering sending mail only to users who have specified attributes among users registered with awscognito.
The user pool was accessible and user information could not be retrieved, but the value specified for the custom attribute could not be retrieved.

response=client.list_users(
  UserPoolId = USER_POOL_ID,
  AttributesToGet=['email',
  Limit = LIMIT
)

We are trying to decide whether or not to send an email based on the data of the custom attributes, so we are having trouble meeting the requirements.I'm thinking of using a user group instead of a custom attribute, but I don't know how to retrieve it in the same way. (I looked it up on the web, but I couldn't find it because I didn't get many references.)
If you know how to get custom attributes or user groups from lambda(python), please let me know.If you have information that you can't do it, that's fine.
Thank you for your cooperation.