Note: This post is part 2 in a series of blog posts I'm writing as I work on the official documentation for Strawberry.

In the last post, we looked at how to query data using Strawberry, a modern GraphQL server library for Python. In this post, we'll learn how to use Strawberry to change data on our server using GraphQL Mutations. Let's dive in!

Hello Mutation

Mutations are used to modify data in GraphQL APIs. You can use them to create, modify, and update data on your server. Let's start with a simple example: 

import strawberry

count = 0

@strawberry.type
class Mutation:
    @strawberry.mutation
    def increment(self, info) -> int:
        global count
        count += 1
        return count

schema = strawberry.Schema(query=Query, mutation=Mutation)

In this example, we've got a global variable called count. In order to increment that variable from the client side, we'll need to define a GraphQL mutation. We start by defining a base Mutation class, much like we defined a base Query type in part 1. Then we're going to pass it into our strawberry.Schema. Finally, we'll define a resolver function called increment which increments count, and decorate it using the strawberry.mutation decorator so that Strawberry knows it's a mutation.

To test out your mutation, start the Strawberry server and visit the GraphQL playground in your browser. You can do that by running the following command:

$ strawberry server app
Running strawberry on http://0.0.0.0:8000/graphql 🍓

If you execute the the increment mutation multiple times, you should see the value of count increase each time you run it:

mutation {
    increment
}

Great job! Let's move on to something a little more useful.

Creating Todos

somethign something input types...

dict instead of array to make it easier

Let's go back to our todo list example from part 1. If we want to create a new todo item, we'll need to write a mutation. Mutations in strawberry are defined similarly to queries. We'll create a method on our Mutation class called create_todo, and decorate it with the strawberry.mutation decorator so that Strawberry knows that this function is a mutation. Then, inside our create_todo methods, we'll create the todo item and store it in our todos array.

import strawberry

@strawberry.input
class TodoInputType:
    name: str
    done: typing.Optional[bool]

todos = {
    '1': {
        "id": 1,
        "name": "Finish blog post"
        "done": False
    }
}

@strawberry.type
class Mutation:
    @strawberry.mutation
    def create_todo(self, info, todo: TodoInputType) -> TodoType:
        todo = TodoType(
            id=
            name=todo.name,
            done=bool(todo.done)
        )
        todos.append(todo)
        return todo

Updating Todos

import strawberry

@strawberry.type
class TodoInputType:
    name: str
    done: bool

todos = [
  TodoType(name="Todo #1", done=False),
  TodoType(name="Todo #2", done=False),
  TodoType(name="Todo #3", done=True)
]

@strawberry.type
class Mutation:
    @strawberry.mutation
    def update_todo(self, info, id: int, todo: TodoInputType) -> TodoType:
        old_todo = todos[id]
        old_todo.name = todo.name
        old_todo.done = todo.done
        return old_todo

Deleting Todos

import strawberry

@strawberry.type
class TodoInputType:
    name: str
    done: bool

todos = [
  TodoType(name="Todo #1", done=False),
  TodoType(name="Todo #2", done=False),
  TodoType(name="Todo #3", done=True)
]

@strawberry.type
class Mutation:
    @strawberry.mutation
    def delete_todo(self, info, id: int) -> TodoType:
        old_todo = todos[id]
        del todos[id]
        return old_todo