FastAPI dependency override not applied in tests in CI

FastAPI replaces a dependency only when the override dictionary key is the exact same callable passed to Depends. If the key is a different function object, the override is silently ignored and the real dependency runs, which in CI often means a missing database or service.

What this error means

A test that sets app.dependency_overrides[get_db] = ... still runs the real dependency: you see a connection error or the production code path, and the override appears to have no effect.

python

# override set, yet the real get_db still runs:
sqlalchemy.exc.OperationalError: (psycopg.OperationalError) connection to
server at "localhost" (127.0.0.1), port 5432 failed: Connection refused

Common causes

The override key is not the exact Depends callable

The key is a re-imported or wrapped copy of the dependency, not the identical object used in Depends(...), so FastAPI never matches it.

The override was set on a different app instance

Tests build their own app or import a different module, so overrides are attached to an app the client does not use.

How to fix it

Override the exact callable on the same app

Import the identical dependency function used in the routes.
Set the override on the same app the TestClient wraps.
Clear overrides in teardown to avoid leaking between tests.

tests/conftest.py

from app.main import app
from app.deps import get_db

def override_get_db():
    yield test_session

app.dependency_overrides[get_db] = override_get_db
# in teardown:
app.dependency_overrides.clear()

Confirm the client uses the overridden app

Construct the TestClient from the same app object that holds the overrides.