Dependency injection failure for unhashable types

Example

Suppose you run the following script

export MY_PREFIX_PASSWORD=hello

and then

from fastapi import FastAPI

app = FastAPI()

class MySettings(BaseSettings):
    password: SecretStr
    base_url: AnyHttpUrl = "https://www.google.com"
    localdownload_dir: str = "~/mydir-import"

    class Config:
        env_prefix = "my_prefix"

def get_settings() -> MySettings:
    return MySettings()

   
@app.get("/")
def read_root(settings:MySettings = Depends(get_settings)):
    return {"Hello": "World"}

Description

When you invoke via curl the rest endpoint, you get the following failure:

File "/venv/lib/python3.7/site-packages/uvicorn/protocols/http/httptools_impl.py", line 390, in run_asgi result = await app(self.scope, self.receive, self.send) File "/venv/lib/python3.7/site-packages/uvicorn/middleware/proxy_headers.py", line 45, in __call__ return await self.app(scope, receive, send) File "/venv/lib/python3.7/site-packages/fastapi/applications.py", line 179, in __call__ await super().__call__(scope, receive, send) File "/venv/lib/python3.7/site-packages/starlette/applications.py", line 111, in __call__ await self.middleware_stack(scope, receive, send) File "/venv/lib/python3.7/site-packages/starlette/middleware/errors.py", line 181, in __call__ raise exc from None File "/venv/lib/python3.7/site-packages/starlette/middleware/errors.py", line 159, in __call__ await self.app(scope, receive, _send) File "/venv/lib/python3.7/site-packages/starlette/exceptions.py", line 82, in __call__ raise exc from None File "/venv/lib/python3.7/site-packages/starlette/exceptions.py", line 71, in __call__ await self.app(scope, receive, sender) File "/venv/lib/python3.7/site-packages/starlette/routing.py", line 566, in __call__ await route.handle(scope, receive, send) File "/venv/lib/python3.7/site-packages/starlette/routing.py", line 227, in handle await self.app(scope, receive, send) File "/venv/lib/python3.7/site-packages/starlette/routing.py", line 41, in app response = await func(request) File "/venv/lib/python3.7/site-packages/fastapi/routing.py", line 176, in app dependency_overrides_provider=dependency_overrides_provider, File "/venv/lib/python3.7/site-packages/fastapi/dependencies/utils.py", line 552, in solve_dependencies solved = await run_in_threadpool(call, **sub_values) File "/venv/lib/python3.7/site-packages/starlette/concurrency.py", line 34, in run_in_threadpool return await loop.run_in_executor(None, func, *args) File "/usr/local/lib/python3.7/concurrent/futures/thread.py", line 57, in run result = self.fn(*self.args, **self.kwargs) TypeError: unhashable type: 'MySettings'

Environment

• OS: [e.g. Linux / Windows / macOS]: MACOS
• FastAPI Version = “0.61.0”
• Python version: 3.7.3

[cbenz]

This documentation section shows that we can use a dependency with Depends which uses @lru_cache. I followed this docs and had the same problem described by this issue.

Should the docs be updated to remove the usage of @lru_cache or can it be useful in some cases?

[escaped]

Just ran into the same issue. Looks like pydantic does not implement a __hash__() method for its models as of now: https://github.com/samuelcolvin/pydantic/pull/1881. ~So, I guess we should update the documentation, or do I miss something?~ (edit: see next comment)

I guess, in general, it makes sense to document the use of lru_cache, as otherwise, the dependency generator is called for every request instead of just once. Additionally, it might be useful to explicitly state that lru_cache only works for hashable objects.

Anyway, we can work around that issue for now by either let fastapi call the method multiple times, implement a __hash__() method for the Settings class or implement a custom caching mechanism, eg.

def get_settings() -> config.Settings:
    cache = getattr(get_settings, "__cached", None)
    if not cache:
        settings = config.Settings()
        get_settings.__cache = settings
    return get_settings.__cache