Resolvers

When defining a GraphQL schema, you usually start with the definition of the schema for your API, for example, let’s take a look at this schema:

import strawberry
 
@strawberry.type
class User:
    name: str
 
 
@strawberry.type
class Query:
    last_user: User

type User {
    name: String!
}
 
type Query {
    lastUser: User!
}

We have defined a User type and a Query type. Next, to define how the data is returned from our server, we will attach resolvers to our fields.

Let’s define a resolver

Let’s create a resolver and attach it to the lastUser field. A resolver is a Python function that returns data. In Strawberry there are two ways of defining resolvers; the first is to pass a function to the field definition, like this:

def get_last_user() -> User:
    return User(name="Marco")
 
 
@strawberry.type
class Query:
    last_user: User = strawberry.field(resolver=get_last_user)

Now when Strawberry executes the following query, it will call the get_last_user function to fetch the data for the lastUser field:

{
  lastUser {
    name
  }
}

{
  "data": {
    "lastUser": {
      "name": "Marco"
    }
  }
}

Defining resolvers as methods

The other way to define a resolver is to use strawberry.field as a decorator, like here:

@strawberry.type
class Query:
    @strawberry.field
    def last_user(self) -> User:
        return User(name="Marco")

This is useful when you want to co-locate resolvers and types or when you have very small resolvers.

Defining arguments

Fields can also have arguments; in Strawberry the arguments for a field are defined on the resolver, as you would normally do in a Python function. Let’s define a field on a Query that returns a user by ID:

import strawberry
 
 
@strawberry.type
class User:
    name: str
 
 
@strawberry.type
class Query:
    @strawberry.field
    def user(self, id: strawberry.ID) -> User:
        # here you'd use the `id` to get the user from the database
        return User(name="Marco")

type User {
    name: String!
}
 
type Query {
    user(id: ID!): User!
}

Optional arguments

Optional or nullable arguments can be expressed using Optional . If you need to differentiate between null (maps to None in Python) and no arguments being passed, you can use UNSET :

from typing import Optional
import strawberry
 
@strawberry.type
class Query:
    @strawberry.field
    def hello(self, name: Optional[str] = None) -> str:
        if name is None:
            return "Hello world!"
        return f"Hello {name}!"
 
    @strawberry.field
    def greet(self, name: Optional[str] = strawberry.UNSET) -> str:
        if name is strawberry.UNSET:
            return "Name was not set!"
        if name is None:
            return "Name was null!"
        return f"Hello {name}!"

type Query {
    hello(name: String = null): String!
    greet(name: String): String!
}

Like this you will get the following responses:

{
  unset: greet
  null: greet(name: null)
  name: greet(name: "Dominique")
}

{
  "data": {
    "unset": "Name was not set!",
    "null": "Name was null!",
    "name": "Hello Dominique!"
  }
}

Accessing execution information

Sometimes it is useful to access the information for the current execution context. Strawberry allows to declare a parameter of type Info that will be automatically passed to the resolver. This parameter contains the information for the current execution context.

import strawberry
from strawberry.types import Info
 
 
def full_name(root: "User", info: strawberry.Info) -> str:
    return f"{root.first_name} {root.last_name} {info.field_name}"
 
 
@strawberry.type
class User:
    first_name: str
    last_name: str
    full_name: str = strawberry.field(resolver=full_name)

API

Info objects contain information for the current execution context:

class Info(Generic[ContextType, RootValueType])