Fix TypeError: get_current_user() missing 1 required positional argument: 'db' in FastAPI

This error occurs when a FastAPI dependency function signature does not match what the dependency injection system expects. The DI container cannot resolve a parameter because it is not declared as a Depends() sub-dependency. Fix it by wrapping nested dependencies with Depends() so FastAPI can resolve the full chain automatically.

Reading the Stack Trace

Here's what each line means:

• File "/app/src/dependencies/auth.py", line 18, in get_current_user: The get_current_user dependency tries to use a 'db' parameter that FastAPI could not inject because it was not declared as a Depends().
• File "/app/venv/lib/python3.11/site-packages/fastapi/dependencies/utils.py", line 621, in solve_dependencies: FastAPI's dependency solver failed to resolve all parameters for the dependency function.
• File "/app/venv/lib/python3.11/site-packages/fastapi/routing.py", line 234, in app: The route handler initiated dependency resolution before executing the endpoint logic.

Common Causes

from sqlalchemy.orm import Session
from app.database import get_db

def get_current_user(token: str = Depends(oauth2_scheme), db: Session):
    token_data = decode_token(token)
    user = db.query(User).filter(User.id == token_data.sub).first()
    if not user:
        raise HTTPException(status_code=401)
    return user

def get_current_user(db: Session, token: str = Depends(oauth2_scheme)):
    # db has no default and no Depends(), so Python treats it as positional
    token_data = decode_token(token)
    return db.query(User).filter(User.id == token_data.sub).first()

@app.get("/users/me")
async def read_current_user():
    user = get_current_user()  # Called directly, not injected
    return user

The Fix

Wrap the db parameter with Depends(get_db) so FastAPI's dependency injection system knows how to resolve the database session. Every sub-dependency must be declared with Depends() for the DI chain to work.

from sqlalchemy.orm import Session
from fastapi import Depends
from app.database import get_db

def get_current_user(token: str = Depends(oauth2_scheme), db: Session):
    token_data = decode_token(token)
    user = db.query(User).filter(User.id == token_data.sub).first()
    if not user:
        raise HTTPException(status_code=401, detail="User not found")
    return user

from sqlalchemy.orm import Session
from fastapi import Depends
from app.database import get_db

def get_current_user(
    token: str = Depends(oauth2_scheme),
    db: Session = Depends(get_db),
):
    token_data = decode_token(token)
    user = db.query(User).filter(User.id == token_data.sub).first()
    if not user:
        raise HTTPException(status_code=401, detail="User not found")
    return user

Testing the Fix

import pytest
from fastapi.testclient import TestClient
from unittest.mock import patch, MagicMock
from app.main import app

client = TestClient(app)


def test_get_current_user_returns_user():
    with patch("app.dependencies.auth.decode_token") as mock_decode:
        mock_decode.return_value = MagicMock(sub=1)
        response = client.get(
            "/users/me",
            headers={"Authorization": "Bearer valid-token"},
        )
        assert response.status_code == 200


def test_get_current_user_invalid_token():
    response = client.get(
        "/users/me",
        headers={"Authorization": "Bearer invalid-token"},
    )
    assert response.status_code == 401


def test_get_current_user_no_auth_header():
    response = client.get("/users/me")
    assert response.status_code in (401, 403)

Run your tests:

pytest tests/test_auth.py -v

Pushing Through CI/CD

git checkout -b fix/fastapi-dependency-injection,git add src/dependencies/auth.py tests/test_auth.py,git commit -m "fix: wrap db param with Depends(get_db) in get_current_user",git push origin fix/fastapi-dependency-injection

Your CI config should look something like this:

name: CI
on:
  pull_request:
    branches: [main]
jobs:
  test:
    runs-on: ubuntu-latest
    services:
      postgres:
        image: postgres:15
        env:
          POSTGRES_DB: testdb
          POSTGRES_USER: postgres
          POSTGRES_PASSWORD: postgres
        ports:
          - 5432:5432
        options: >-
          --health-cmd pg_isready
          --health-interval 10s
          --health-timeout 5s
          --health-retries 5
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-python@v5
        with:
          python-version: '3.11'
          cache: 'pip'
      - run: pip install -r requirements.txt
      - run: pytest --tb=short -q
        env:
          DATABASE_URL: postgresql://postgres:postgres@localhost:5432/testdb

The Full Manual Process: 18 Steps

Here's every step you just went through to fix this one bug:

1. Notice the error alert or see it in your monitoring tool
2. Open the error dashboard and read the stack trace
3. Identify the file and line number from the stack trace
4. Open your IDE and navigate to the file
5. Read the surrounding code to understand context
6. Reproduce the error locally
7. Identify the root cause
8. Write the fix
9. Run the test suite locally
10. Fix any failing tests
11. Write new tests covering the edge case
12. Run the full test suite again
13. Create a new git branch
14. Commit and push your changes
15. Open a pull request
16. Wait for code review
17. Merge and deploy to production
18. Monitor production to confirm the error is resolved

Total time: 30-60 minutes. For one bug.

pip install bugstack

import bugstack

bugstack.init(api_key=os.environ["BUGSTACK_API_KEY"])

Deploying the Fix (Manual Path)

1. Run the full test suite locally to confirm the dependency chain resolves.
2. Open a pull request with the Depends(get_db) fix.
3. Wait for CI checks including database integration tests to pass.
4. Have a teammate review and approve the PR.
5. Merge to main and verify the endpoint works in staging.

Frequently Asked Questions