aiohttp_session not required

aiohttp_security/session_identity.py

@@ -6,7 +6,11 @@ to configure aiohttp_session properly.
 
 import asyncio
 
+try:
     from aiohttp_session import get_session
+    has_aiohttp_session = True
+except ImportError:
+    has_aiohttp_session = False
 
 from .abc import AbstractIdentityPolicy
 
@@ -16,6 +20,10 @@ class SessionIdentityPolicy(AbstractIdentityPolicy):
     def __init__(self, session_key='AIOHTTP_SECURITY'):
         self._session_key = session_key
 
+        if not has_aiohttp_session:
+            raise ImportError(
+                'SessionIdentityPolicy requires aiohttp_session')
+
     @asyncio.coroutine
     def identify(self, request):
         session = yield from get_session(request)