python 46 lines · 7 steps

Testing FastAPI routes with dependency overrides

A pytest fixture swaps FastAPI's database and auth dependencies for test doubles so routes run against an isolated SQLite database.

Explained by highlit

FastAPI Intermediate testing fixtures dependency-injection database-isolation authentication

1import pytest
2from fastapi.testclient import TestClient
3from sqlalchemy import create_engine
4from sqlalchemy.orm import sessionmaker
5 
6from app.main import app
7from app.database import Base, get_db
8from app.auth import get_current_user
9from app.models import User
10 
11engine = create_engine(
12    "sqlite:///./test.db",
13    connect_args={"check_same_thread": False},
14)
15TestingSessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
16 
17 
18@pytest.fixture
19def db_session():
20    Base.metadata.create_all(bind=engine)
21    session = TestingSessionLocal()
22    try:
23        yield session
24    finally:
25        session.close()
26        Base.metadata.drop_all(bind=engine)
27 
28 
29@pytest.fixture
30def client(db_session):
31    def override_get_db():
32        try:
33            yield db_session
34        finally:
35            pass
36 
37    def override_get_current_user():
38        return User(id=1, email="tester@example.com", is_active=True)
39 
40    app.dependency_overrides[get_db] = override_get_db
41    app.dependency_overrides[get_current_user] = override_get_current_user
42 
43    with TestClient(app) as test_client:
44        yield test_client
45 
46    app.dependency_overrides.clear()

01 / 01

STEP 01

Walkthrough

Space play ←→ step click any line

Three takeaways

1FastAPI's dependency_overrides lets tests inject fakes without touching route code.
2Yielding fixtures give you setup-then-teardown around each test in one function.
3Overriding authentication lets you exercise protected routes as a known user.

Related explainers

typescript

import { registerLocaleData } from '@angular/common';
import localeFr from '@angular/common/locales/fr';
import localeFrExtra from '@angular/common/locales/extra/fr';
import localeDe from '@angular/common/locales/de';

Locale-aware bootstrapping in Angular

i18n localization dependency-injection

Intermediate 8 steps

python

from fastapi import FastAPI, WebSocket, WebSocketDisconnect
 
app = FastAPI()

Building a WebSocket chat with FastAPI

websockets broadcast connection-management

Intermediate 9 steps

typescript

import { Module } from '@nestjs/common';
import { ConfigModule } from '@nestjs/config';
import * as Joi from 'joi';

Validating env config at boot in NestJS

configuration schema-validation environment-variables

Intermediate 8 steps

java

@Component
@Converter
public class EncryptedStringConverter implements AttributeConverter<String, String> {

Transparent column encryption in Spring & JPA

encryption aes-gcm jpa-converter

Advanced 10 steps

python

import time
import uuid
 
from django.utils.deprecation import MiddlewareMixin

Attaching per-request context in Django

middleware request lifecycle multi-tenancy

Intermediate 7 steps

typescript

import { Inject, Injectable, Logger } from '@nestjs/common';
import { CACHE_MANAGER } from '@nestjs/cache-manager';
import { Cache } from 'cache-manager';
import { InjectRepository } from '@nestjs/typeorm';