Add warning for creation a session outside of coroutine

CHANGES.rst

@@ -45,7 +45,7 @@ CHANGES
 
 - Allow to raise web exceptions on router resolving stage #1460
 
--
+- Add a warning for session creation outside of coroutine #1468
 
 -
 

aiohttp/client.py

@@ -54,6 +54,17 @@ def __init__(self, *, connector=None, loop=None, cookies=None,
         if loop.get_debug():
             self._source_traceback = traceback.extract_stack(sys._getframe(1))
 
+        if not loop.is_running():
+            warnings.warn("Creating a client session outside of coroutine is "
+                          "a very dangerous idea", ResourceWarning,
+                          stacklevel=2)
+            context = {'client_session': self,
+                       'message': 'Creating a client session outside '
+                       'of coroutine'}
+            if self._source_traceback is not None:
+                context['source_traceback'] = self._source_traceback
+            loop.call_exception_handler(context)
+
         if cookie_jar is None:
             cookie_jar = CookieJar(loop=loop)
         self._cookie_jar = cookie_jar

tests/test_client_session.py

@@ -435,3 +435,9 @@ def test_proxy_str(session, params):
                                            allow_redirects=True,
                                            proxy='http://proxy.com',
                                            **params)]
+
+
+def test_create_session_outside_of_coroutine(loop):
+    with pytest.warns(ResourceWarning):
+        sess = ClientSession(loop=loop)
+    sess.close()