Import supported backends

This ensures that any syntax issues are caught early (by type checkers
and tests). Backends that are missing dependencies are skipped. By
importing, this exposed an issue where the redis type annotations
raised an exception, which has been fixed by using forward annotations.

To help avoid import dependency hell, the Backend ABC has been moved to
`fastapi_cache.types`. In the process, it has been made an actual ABC.

fastapi_cache/__init__.py

@@ -1,9 +1,8 @@
 from typing import ClassVar, Optional, Type
 
-from fastapi_cache.backends import Backend
 from fastapi_cache.coder import Coder, JsonCoder
 from fastapi_cache.key_builder import default_key_builder
-from fastapi_cache.types import KeyBuilder
+from fastapi_cache.types import Backend, KeyBuilder
 
 
 __all__ = [

fastapi_cache/backends/__init__.py

@@ -1,20 +1,29 @@
-import abc
-from typing import Optional, Tuple
+from fastapi_cache.types import Backend
+from fastapi_cache.backends import inmemory
 
 
-class Backend:
-    @abc.abstractmethod
-    async def get_with_ttl(self, key: str) -> Tuple[int, Optional[bytes]]:
-        raise NotImplementedError
+__all__ = ["Backend", "inmemory"]
 
-    @abc.abstractmethod
-    async def get(self, key: str) -> Optional[bytes]:
-        raise NotImplementedError
+# import each backend in turn and add to __all__. This syntax
+# is explicitly supported by type checkers, while more dynamic
+# syntax would not be recognised.
+try:
+    from fastapi_cache.backends import dynamodb
+except ImportError:
+    pass
+else:
+    __all__ += ["dynamodb"]
 
-    @abc.abstractmethod
-    async def set(self, key: str, value: bytes, expire: Optional[int] = None) -> None:
-        raise NotImplementedError
+try:
+    from fastapi_cache.backends import memcached
+except ImportError:
+    pass
+else:
+    __all__ += ["memcached"]
 
-    @abc.abstractmethod
-    async def clear(self, namespace: Optional[str] = None, key: Optional[str] = None) -> int:
-        raise NotImplementedError
+try:
+    from fastapi_cache.backends import redis
+except ImportError:
+    pass
+else:
+    __all__ += ["redis"]

fastapi_cache/backends/dynamodb.py

@@ -4,7 +4,7 @@ from typing import TYPE_CHECKING, Optional, Tuple
 from aiobotocore.client import AioBaseClient
 from aiobotocore.session import AioSession, get_session
 
-from fastapi_cache.backends import Backend
+from fastapi_cache.types import Backend
 
 if TYPE_CHECKING:
     from types_aiobotocore_dynamodb import DynamoDBClient

fastapi_cache/backends/inmemory.py

@@ -3,7 +3,7 @@ from asyncio import Lock
 from dataclasses import dataclass
 from typing import Dict, Optional, Tuple
 
-from fastapi_cache.backends import Backend
+from fastapi_cache.types import Backend
 
 
 @dataclass

fastapi_cache/backends/memcached.py

@@ -2,7 +2,7 @@ from typing import Optional, Tuple
 
 from aiomcache import Client
 
-from fastapi_cache.backends import Backend
+from fastapi_cache.types import Backend
 
 
 class MemcachedBackend(Backend):

fastapi_cache/backends/redis.py

@@ -3,11 +3,11 @@ from typing import Optional, Tuple, Union
 from redis.asyncio.client import Redis
 from redis.asyncio.cluster import RedisCluster
 
-from fastapi_cache.backends import Backend
+from fastapi_cache.types import Backend
 
 
 class RedisBackend(Backend):
-    def __init__(self, redis: Union[Redis[bytes], RedisCluster[bytes]]):
+    def __init__(self, redis: Union["Redis[bytes]", "RedisCluster[bytes]"]):
         self.redis = redis
         self.is_cluster: bool = isinstance(redis, RedisCluster)
 

fastapi_cache/types.py

@@ -1,3 +1,4 @@
+import abc
 from typing import Any, Awaitable, Callable, Dict, Optional, Tuple, Union
 
 from starlette.requests import Request
@@ -20,3 +21,21 @@ class KeyBuilder(Protocol):
         kwargs: Dict[str, Any],
     ) -> Union[Awaitable[str], str]:
         ...
+
+
+class Backend(abc.ABC):
+    @abc.abstractmethod
+    async def get_with_ttl(self, key: str) -> Tuple[int, Optional[bytes]]:
+        raise NotImplementedError
+
+    @abc.abstractmethod
+    async def get(self, key: str) -> Optional[bytes]:
+        raise NotImplementedError
+
+    @abc.abstractmethod
+    async def set(self, key: str, value: bytes, expire: Optional[int] = None) -> None:
+        raise NotImplementedError
+
+    @abc.abstractmethod
+    async def clear(self, namespace: Optional[str] = None, key: Optional[str] = None) -> int:
+        raise NotImplementedError