FastAPI depends extension

Introduction

Sometimes your FastAPI dependencies have to get value from functions cannot be available on initialization. The problem is particularly acute to use class dependencies with inheritance. This project try to solve problem of modify Depends after application initialization.

Installation

pip install fastapi-depends-ext

Tutorial

DependsAttr

from typing import List

from fastapi import Depends
from fastapi import FastAPI
from fastapi import Query
from pydantic import conint

from fastapi_depends_ext import DependsAttr
from fastapi_depends_ext import DependsAttrBinder


class ItemsPaginated(DependsAttrBinder):
    _items = list(range(100))

    async def get_page(self, page: conint(ge=1) = Query(1)):
        return page

    async def items(self, page: int = DependsAttr("get_page")):
        _slice = slice(page * 10, (page + 1) * 10)
        return self._items[_slice]


class ItemsSquarePaginated(ItemsPaginated):
    async def items(self, items: List[int] = DependsAttr("items", from_super=True)):
        return [i**2 for i in items]


app = FastAPI()


@app.get("/")
def items_list(items: List[int] = Depends(ItemsPaginated().items)) -> List[int]:
    return items


@app.get("/square")
def items_list_square(items: List[int] = Depends(ItemsSquarePaginated().items)) -> List[int]:
    return items

Use DependsAttr to Depends from current instance attributes. All examples use asyncio, but you can write all methods synchronous.

DependsAttr support next properties:

• class variables (must contains callable object)
• class methods
• static methods
• instance methods
• property (must return callable that will be used as dependency)

Your class must inherit from DependsAttrBinder and attributes must be DependsAttr. DependsAttrBinder automatically patch all methods with DependsAttr by instance attributes.

DependsAttr arguments:

• method_name - str, name of instance attribute to use as dependency
• from_super - bool, on true, will use attribute method_name from super class like super().method_name()
• use_cache - bool, allow to cache depends result for the same dependencies in request

DependsExt

Useless(?) class created to proof of concept of patching methods and correct work FastAPI applications.

DependsExt allow you define default values of method arguments after FastAPI endpoint has been created.

Example:

from fastapi import FastAPI
from fastapi import Query

from fastapi_depends_ext import DependsExt


def pagination(page: int = Query()):
    return page


depends = DependsExt(pagination)


app = FastAPI()


@app.on_event("startup")
def setup_depends():
    depends.bind(page=Query(1))


@app.get("/")
def get_method(value: int = depends) -> int:
    return value

Is equivalent for

from fastapi import Depends
from fastapi import FastAPI
from fastapi import Query


def pagination(page: int = Query(1)):
    return page


app = FastAPI()


@app.get("/")
def get_method(value: int = Depends(pagination)) -> int:
    return value