title: How to require predefined string values in python pydantic basemodels?
body: <p>Is there any in-built way in pydantic to specify options? For example, let's say I want a string value that must either have the value "foo" or "bar".</p>
  
  <p>I know I can use regex validation to do this, but since I use pydantic with FastAPI, the users will only see the required input as a string, but when they enter something, it will give a validation error. All in-built validations of pydantic are displayed in the api interface, so would be great if there was something like </p>
  
  <pre><code>class Input(BaseModel):
       option: "foo" || "bar"
  </code></pre>