Testing Asynchronous Code with Shared Resources in Django

When testing asynchronous code in Django, you're facing a challenge with resource sharing across test methods. Django creates a new event loop for each async test method, which is causing issues with your Redis client.

Understanding the Issue

Django automatically detects async def test methods and wraps them to run in their own event loop. This isolation is intentional for test independence but creates problems when you need to share resources like a Redis client.

Possible Solutions

1. Use SerializeMixin for Sequential Test Execution

If your tests need to share resources, you can use SerializeMixin to ensure they run sequentially:

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import asyncio
from redis.asyncio import Redis
from django.test import TestCase
from django.test.testcases import SerializeMixin

class RedisTestMixin(SerializeMixin):
    lockfile = __file__  # Use the current file as the lock

    @classmethod
    def setUpClass(cls):
        super().setUpClass()
        # Create a shared Redis client in setup
        cls.loop = asyncio.new_event_loop()
        asyncio.set_event_loop(cls.loop)
        cls.redis_client = Redis()

    @classmethod
    def tearDownClass(cls):
        # Clean up resources
        cls.loop.run_until_complete(cls.redis_client.close())
        cls.loop.close()
        super().tearDownClass()

class MyRedisTests(RedisTestMixin, TestCase):
    async def test_redis_operation(self):
        # Use the shared redis client
        await self.redis_client.set("test_key", "test_value")
        value = await self.redis_client.get("test_key")
        self.assertEqual(value, b"test_value")

2. Initialize Redis Client in Each Test Method

If you need to run tests in parallel, you might need to accept the overhead of creating a new Redis client in each test:

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from django.test import TestCase
from redis.asyncio import Redis

class MyRedisTests(TestCase):
    async def test_redis_operation(self):
        # Create a new Redis client for this test
        redis_client = Redis()
        try:
            await redis_client.set("test_key", "test_value")
            value = await redis_client.get("test_key")
            self.assertEqual(value, b"test_value")
        finally:
            await redis_client.close()

3. Use async_to_sync for Special Cases

If you're using test decorators that aren't async-compatible, you can use async_to_sync:

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from asgiref.sync import async_to_sync
from django.test import TestCase
from redis.asyncio import Redis
import mock

class MyRedisTests(TestCase):
    @mock.patch('some.module')
    @async_to_sync
    async def test_redis_with_mock(self):
        redis_client = Redis()
        try:
            # Test code here
            await redis_client.set("key", "value")
        finally:
            await redis_client.close()

Conclusion

Django's approach to async testing prioritizes isolation, which can make sharing resources challenging. The SerializeMixin approach is likely your best option if you need to share a Redis client across test methods, though it will force your tests to run sequentially rather than in parallel.