do the same for python tasks

ffa9ec9b1103ffbc9ee5a83f0662effa56c7cdb4
commit ffa9ec9b1103ffbc9ee5a83f0662effa56c7cdb4
@@ -79,6 +79,10 @@ def __init__(self, *, loop=None):
         loop object used by the future. If it's not provided, the future uses
         the default event loop.
         """
+        if self._loop is not None:
+            raise RuntimeError(f'{self.__class__.__name__} object is already '
+                               'initialized.')
+
         if loop is None:
             self._loop = events.get_event_loop()
         else: