only store the parent header in parent_headers

the old parent_header was actually the parent *message*, but we don't need to keep doing that

Author: minrk

ipykernel/inprocess/ipkernel.py

@@ -97,7 +97,7 @@ def _input_request(self, prompt, ident, parent, password=False):
         content = json_clean(dict(prompt=prompt, password=password))
         msg = self.session.msg('input_request', content, parent)
         for frontend in self.frontends:
-            if frontend.session.session == parent['header']['session']:
+            if frontend.session.session == parent['session']:
                 frontend.stdin_channel.call_handlers(msg)
                 break
         else:

ipykernel/kernelbase.py

@@ -142,7 +142,9 @@ def _parent_header(self):
             DeprecationWarning,
             stacklevel=2,
         )
-        return self.get_parent_header(channel="shell")
+        # _parent_header wasn't actually the header, it was the *message*
+        # but only promised access to the header
+        return {"header": self.get_parent_header(channel="shell")}
 
     # Time to sleep after flushing the stdout/err buffers in each execute
     # cycle.  While this introduces a hard limit on the minimal latency of the
@@ -538,6 +540,8 @@ def set_parent(self, ident, parent, channel='shell'):
         on the stdin channel.
         """
         self._parent_ident[channel] = ident
+        if parent and 'header' in parent:
+            parent = parent['header']
         self._parent_headers[channel] = parent
 
     def send_response(self, stream, msg_or_type, content=None, ident=None,