fixed error message for cache proxy creation (#795)

Author: ar0ne

aiocache/factory.py

@@ -60,7 +60,7 @@ class Cache:
     MEMCACHED = AIOCACHE_CACHES.get("memcached")
 
     def __new__(cls, cache_class=MEMORY, **kwargs):
-        if not issubclass(cache_class, BaseCache):
+        if not (cache_class and issubclass(cache_class, BaseCache)):
             raise InvalidCacheType(
                 "Invalid cache type, you can only use {}".format(list(AIOCACHE_CACHES.keys()))
             )

tests/ut/test_factory.py

@@ -74,9 +74,10 @@ async def test_new(self, cache_type):
     def test_new_defaults_to_memory(self):
         assert isinstance(Cache(), Cache.MEMORY)
 
-    def test_new_invalid_cache_raises(self):
+    @pytest.mark.parametrize("cache_type", (None, object))
+    def test_new_invalid_cache_raises(self, cache_type):
         with pytest.raises(InvalidCacheType) as e:
-            Cache(object)
+            Cache(cache_type)
         assert str(e.value) == "Invalid cache type, you can only use {}".format(
             list(AIOCACHE_CACHES.keys())
         )