FastAPI / Pydantic circular references in separate files

circular-dependency, fastapi, pydantic, python

Solution

There are three cases when circular dependency may work in Python:

- Top of module: `import package.module`

- Bottom of module: `from package.module import attribute`

- Top of function: works both

In your situation, the second case "bottom of module" will help. Because you need to use `update_forward_refs` function to resolve pydantic postponed annotations like this:

# project.py
from typing import List
from pydantic import BaseModel


class Project(BaseModel):
    members: "List[User]"


from user import User
Project.update_forward_refs()

# user.py
from typing import List
from pydantic import BaseModel


class User(BaseModel):
    projects: "List[Project]"


from project import Project
User.update_forward_refs()

Nonetheless, I would strongly discourage you from intentionally introducing circular dependencies

Problem

I would love to use a schema that looks something like the following in FastAPI: ``` from __future__ import annotations from typing import List from pydantic import BaseModel class Project(BaseModel): members: List[User] class User(BaseModel): projects: List[Project] Project.update_forward_refs() ``` but in order to keep my project structure clean, I would ofc. like to define these in separate files. How could I do this without creating a circular reference? With the code above the schema generation in FastAPI works fine, I just dont know how to separate it out into separate files. In a later step I would then instead of using attributes use `@property`s to define the getters for these objects in subclasses of them. But for the OpenAPI doc generation, I need this combined - I think.

Original source