Simplify backend import process/missing dependency handling

aiohttp_client_cache/backends/__init__.py

@@ -1,47 +1,36 @@
 from logging import getLogger
 from importlib import import_module
-from typing import Optional
+from typing import Optional, Type
 
 from aiohttp_client_cache.backends.base import BaseCache
 
 PICKLE_PROTOCOL = 4
+BACKEND_QUALNAMES = {
+    'dynamodb': 'aiohttp_client_cache.backends.dynamodb.DynamoDbCache',
+    'gridfs': 'aiohttp_client_cache.backends.gridfs.GridFSCache',
+    'memory': 'aiohttp_client_cache.backends.base.BaseCache',
+    'mongodb': 'aiohttp_client_cache.backends.mongo.MongoCache',
+    'redis': 'aiohttp_client_cache.backends.redis.RedisCache',
+    'sqlite': 'aiohttp_client_cache.backends.sqlite.DbCache',
+}
 logger = getLogger(__name__)
 
+
+def import_member(qualname: str) -> Optional[Type]:
+    """Attempt to import a class or other module member by qualified name"""
+    try:
+        module, member = qualname.rsplit('.', 1)
+        return getattr(import_module(module), member)
+    except (AttributeError, ImportError) as e:
+        logger.debug(f'Could not load {qualname}: {str(e)}')
+        return None
+
+
 # Import all backends for which dependencies have been installed
-try:
-    # Heroku doesn't allow the SQLite3 module to be installed
-    from aiohttp_client_cache.backends.sqlite import DbCache
-except ImportError:
-    DbCache = None
-try:
-    from aiohttp_client_cache.backends.mongo import MongoCache, MongoDict
-except ImportError:
-    MongoCache, MongoDict = None, None
-try:
-    from aiohttp_client_cache.backends.gridfs import GridFSCache
-except ImportError:
-    GridFSCache = None
-try:
-    from aiohttp_client_cache.backends.redis import RedisCache
-except ImportError:
-    RedisCache = None
-try:
-    from aiohttp_client_cache.backends.dynamodb import DynamoDbCache
-except ImportError:
-    DynamoDbCache = None
-
-BACKEND_CLASSES = {
-    'dynamodb': DynamoDbCache,
-    'gridfs': GridFSCache,
-    'memory': BaseCache,
-    'mongo': MongoCache,
-    'mongodb': MongoCache,
-    'redis': RedisCache,
-    'sqlite': DbCache,
-}
+BACKEND_CLASSES = {name: import_member(qualname) for name, qualname in BACKEND_QUALNAMES.items()}
 
 
-def create_backend(backend_name: Optional[str], *args, **kwargs):
+def create_backend(backend_name: Optional[str], *args, **kwargs) -> BaseCache:
     """Initialize a backend by name"""
     logger.info(f'Initializing backend: {backend_name}')
     if isinstance(backend_name, BaseCache):

aiohttp_client_cache/backends/gridfs.py

@@ -4,7 +4,8 @@
 from gridfs import GridFS
 from pymongo import MongoClient
 
-from aiohttp_client_cache.backends import PICKLE_PROTOCOL, BaseCache, MongoDict
+from aiohttp_client_cache.backends import PICKLE_PROTOCOL, BaseCache
+from aiohttp_client_cache.backends.mongo import MongoDict
 
 
 class GridFSCache(BaseCache):