fastapi: how to cache the value of fastapi dependency in a class-based (callable) dependency?

I have a small fastapi example here

# run.py
from functools import cache

import uvicorn
from fastapi import FastAPI, Depends

app = FastAPI()


class Dep:

    def __init__(self, inp):
        self.inp = inp

    @cache
    def __call__(self):
        print(self.inp * 100)
        return self.inp


@app.get("/a")
def a(v: str = Depends(Dep("a"))):
    return v


@app.get("/a/a")
def aa(v: str = Depends(Dep("a"))):
    return v


@app.get("/b")
def b(v: str = Depends(Dep("b"))):
    return v


@app.get("/b/b")
def bb(v: str = Depends(Dep("b"))):
    return v


def main():
    uvicorn.run(
        "run:app",
        host="0.0.0.0",
        reload=True,
        port=8000,
        workers=1
    )


if __name__ == "__main__":
    main()

I run python run.py and the application spins up.

What I expect is that:

the first time I hit /a or /a/a endpoints it shows logs to me and print 100 "a" for me. the next times no logging happens because of the @cache decorator

and

the first time I hit /b or /b/b endpoints it shows logs to me and print 100 "b"s for me. the next times no logging happens because of the @cache decorator

What happens

The first time I hit /b, it shows logs. the next times no log
The first time I hit /b/b, it shows logs. the next times no log
The first time I hit /a, it shows logs. the next times no log
The first time I hit /a/a, it shows logs. the next times no log

the reason is that each time I am passing a value to the Dep class it is creating a new object and the call method is that of a new object each time. that is why the caching is not working properly.

and the caching is being done by fastapi dependency . That is why when I call an endpoint for the next times, I do not see new logs

But the question is how do I

• pass arguments to the callable dependency (a class)
• and also cache the value the function returns

Extra information

I tried to achieve the same using the approach below:

# run.py
from functools import cache

import uvicorn
from fastapi import FastAPI, Depends, Request

app = FastAPI()


@cache
def dep(inp):
    @cache
    def sub_dep():
        print(inp*100)
        return inp

    return sub_dep



@app.get("/a")
def a(v: str = Depends(dep("a"))):
    return v


@app.get("/a/a")
def aa(v: str = Depends(dep("a"))):
    return v


@app.get("/b")
def b(v: str = Depends(dep("b"))):
    return v


@app.get("/b/b")
def bb(v: str = Depends(dep("b"))):
    return v


def main():
    uvicorn.run(
        "run:app",
        host="0.0.0.0",
        reload=True,
        port=8000,
        workers=1
    )


if __name__ == "__main__":
    main()

and it working as expected above

> The first time I hit /a, it shows logs. The next times, no log. The first time I hit /a/a, it shows logs. The next times, no log.

You created four distinct objects at import time
when you defined those functions, a pair of "a" functions and
a pair of "b" functions.
There's no relationship among those four objects,
they are all separate from one another.
So a GET that uses one has no effect on the other three.

Consider creating a
cached
a_result() helper function which both the /a and /a/a endpoints call.
And similarly a b_result() helper shared by both /b and /b/b.
Now there is a linkage across endpoints.

(If web clients can send an arbitrary arg to such a helper, then prefer
lru_cache
instead, so there's no memory leak.)