Lists and Non-Null

Object types, scalars, and enums are the only kinds of types you can define in Graphene. But when you use the types in other parts of the schema, or in your query variable declarations, you can apply additional type modifiers that affect validation of those values.

NonNull

import graphene

class Character(graphene.ObjectType):
    name = graphene.NonNull(graphene.String)

Here, we’re using a String type and marking it as Non-Null by wrapping it using the NonNull class. This means that our server always expects to return a non-null value for this field, and if it ends up getting a null value that will actually trigger a GraphQL execution error, letting the client know that something has gone wrong.

The previous NonNull code snippet is also equivalent to:

import graphene

class Character(graphene.ObjectType):
    name = graphene.String(required=True)

List

import graphene

class Character(graphene.ObjectType):
    appears_in = graphene.List(graphene.String)

Lists work in a similar way: We can use a type modifier to mark a type as a List, which indicates that this field will return a list of that type. It works the same for arguments, where the validation step will expect a list for that value.

NonNull Lists

By default items in a list will be considered nullable. To define a list without any nullable items the type needs to be marked as NonNull. For example:

import graphene

class Character(graphene.ObjectType):
    appears_in = graphene.List(graphene.NonNull(graphene.String))

The above results in the type definition:

type Character {
    appearsIn: [String!]
}
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