interface UserError

Base Interface all user-facing GraphQL errors should implement. This is used to provide a common way to handle errors in the frontend.

A possible scenario would be having a create_user mutation that returns different types of error if the email is already in use, the password is too short, etc.

It could be declared like this:

class UsernameTakenError(ObjectType):
    class Meta:
        interfaces = (UserError, )

    suggestedUsernames = List(String)


class PasswordTooShortError(ObjectType):
    class Meta:
        interfaces = (UserError, )

class UserCreationError(Union):
    class Meta:
        types = (UsernameTakenError, PasswordTooShortError)

class CreateUser(Mutation):
    user = Field(User)
    errors = List(UserCreationError)

And it would be queried like this:

mutation {
    createUser(email: foo@bar.com, password: "123") {
        user {
            id
        }
        errors {
            __typename
            ... on UserError {
                message
            }
            ... on UsernameTakenError {
                message
                suggestedUsername
            }
            ... on PasswordTooShortError {
                message
            }
        }
    }
}

If new errors are added, and they implement the UserError interface, the frontend will be able to handle at least the message field without any changes.

based on: https://productionreadygraphql.com/2020-08-01-guide-to-graphql-errors