title: Validating python class attributes
body: <p>I want to add validations for my class attributes. On exploring, I came across multiple libraries <a href="https://cloud.google.com/appengine/docs/standard/python/datastore/typesandpropertyclasses" rel="nofollow noreferrer">google appengine</a>, <a href="https://docs.djangoproject.com/en/2.0/ref/models/fields/#field-types" rel="nofollow noreferrer">dj</a> have their own property classes to validate attributes. Since its a common problem, Is there any python library which just provides basic validators ?</p>