I am creating an app that will involve a lot of users which they can Assign any username they want to, which will be used as the entity ID/Name.
But when I tried the username '__something__' , GAE gave me an error:
raised BadRequestError(The key path element name "__something__" is reserved.)
The problem with that specific key are the two '_' (underscore) at the beginning and ending of the username:
So you don't list to blacklist a list of terms. You just need to validate that your user doesn't use
__ at the beginning and end of the username.