Lazy Types

Strawberry supports lazy types, which are useful when you have circular dependencies between types.

For example, let’s say we have a User type that has a list of Post types, and each Post type has a User field. In this case, we can’t define the User type before the Post type, and vice versa.

To solve this, we can use lazy types:

# posts.py
from typing import TYPE_CHECKING, Annotated

import strawberry

if TYPE_CHECKING:
    from .users import User


@strawberry.type
class Post:
    title: str
    author: Annotated["User", strawberry.lazy(".users")]

# users.py
from typing import TYPE_CHECKING, Annotated, List

import strawberry

if TYPE_CHECKING:
    from .posts import Post


@strawberry.type
class User:
    name: str
    posts: List[Annotated["Post", strawberry.lazy(".posts")]]

strawberry.lazy in combination with Annotated allows us to define the path of the module of the type we want to use, this allows us to leverage Python’s type hints, while preventing circular imports and preserving type safety by using TYPE_CHECKING to tell type checkers where to look for the type.

Annotated is only available in Python 3.9+, if you are using an older version of Python you can use typing_extensions.Annotated instead.

# users.py
from typing import TYPE_CHECKING, List
from typing_extensions import Annotated

import strawberry

if TYPE_CHECKING:
    from .posts import Post


@strawberry.type
class User:
    name: str
    posts: List[Annotated["Post", strawberry.lazy(".posts")]]