Problem overriding dependency with a functools.partial(Dep, ...) where Dep takes **kwargs

[estan]

First Check

• I added a very descriptive title to this issue.
• I used the GitHub search to find a similar issue and didn't find it.
• I searched the FastAPI documentation, with the integrated search.
• I already searched in Google "How to X in FastAPI" and didn't find any information.
• I already read and followed all the tutorial in the docs and didn't find an answer.
• I already checked if it is not related to FastAPI but to Pydantic.
• I already checked if it is not related to FastAPI but to Swagger UI.
• I already checked if it is not related to FastAPI but to ReDoc.

Commit to Help

• I commit to help with one of those options 👆

Example Code

import functools

from fastapi import FastAPI, Depends
from fastapi.testclient import TestClient

app = FastAPI()

class Dep:
    def __init__(self, _, **kwargs):
        pass


def get_dep():
    return Dep(1)


@app.get("/")
async def root(dep: Dep = Depends(get_dep)):
    print("root called")
    return "a"


def test_failing():
    app.dependency_overrides[get_dep] = functools.partial(Dep, 1)
    assert TestClient(app).get("/").json() == "a"
    app.dependency_overrides = {}


def test_passing():
    app.dependency_overrides[get_dep] = lambda: Dep(1)
    assert TestClient(app).get("/").json() == "a"
    app.dependency_overrides = {}

Description

I'm attempting to override a dependency with a functools.partial(Dep, ...).

• In the above MRE, the test_failing test case is failing with

  E       AssertionError: assert {'detail': [{'loc': ['query', 'kwargs'], 'msg': 'field required', 'type': 'value_error.missing'}]} == 'a'
  

  and as seen in the output if running with pytest -s, the root endpoint is never called when the test case runs.

• The test_passing test case, which uses a lambda instead of a partial, passes.

• I expect both test cases to pass.

It seems to be due to Dep taking **kwargs.

Operating System

Linux

Operating System Details

Ubuntu 20.04.3 LTS

FastAPI Version

0.68.0

Python Version

Python 3.8.10

Additional Context

No response

[Kludex]

Can you change the order of those tests?

[estan]

Yes, the result is the same.

But now I got a little further and can cut Databases out of the equation with the following revised MRE:

import functools

from fastapi import FastAPI, Depends
from fastapi.testclient import TestClient

app = FastAPI()

class Dep:
    def __init__(self, _, **kwargs):
        pass


def get_dep():
    return Dep(1)


@app.get("/")
async def root(dep: Dep = Depends(get_dep)):
    print("root called")
    return "a"


def test_failing():
    app.dependency_overrides[get_dep] = functools.partial(Dep, 1)
    assert TestClient(app).get("/").json() == "a"
    app.dependency_overrides = {}


def test_passing():
    app.dependency_overrides[get_dep] = lambda: Dep(1)
    assert TestClient(app).get("/").json() == "a"
    app.dependency_overrides = {}

This fails in the same way. The problem seems to appear because Dep takes a **kwargs.

I'll update issue title and description. EDIT: Done.

[YuriiMotov]

When you override dependency by function with **kwargs, FastAPI will try to resolve dependencies, find kwargs parameter and treat it as a required Query parameter.
You don't pass it, so you have error 422.

Just use lambda: Dep(1) or create new function that will declare parameters that are really needed and pass them to the __init__ of Dep