The request method of Session class does not take a session argument. But api.request does. So, it has to be popped before you can send the whole kwargs dict to Session.request method.

requests/api.py

@@ -34,7 +34,7 @@ def request(method, url, **kwargs):
     :param verify: (optional) if ``True``, the SSL cert will be verified. A CA_BUNDLE path can also be provided.
     """
 
-    s = kwargs.get('session') or sessions.session()
+    s = kwargs.pop('session') if 'session' in kwargs else sessions.session()
     return s.request(method=method, url=url, **kwargs)