Suggest to make implementations of some function always return awaita…

…ble.

See discussion in ipython#1272; It is not deprecated, but being able to always
know you can (and must) await should be simpler in the long run.

Deprecating now is not the point, but I want to cover our bases, so
that we are more confident later when and if we want to enforce those await.

In particular many of those branches are not covered in our tests – and
I don't even know wether they were ever taken;

I changed some of the base methods to be async, but I'm happy to move
those back to sync.

A few other things use the `if awaitable(...):` pattern but are a bit
more complicted, and some do not dates from 2021, so those will be dealt
with separately.

ipykernel/kernelbase.py

@@ -57,6 +57,14 @@
 from ._version import kernel_protocol_version
 from .iostream import OutStream
 
+_AWAITABLE_MESSAGE: str = (
+    "For consistency across implementations, it is recommended that `{func_name}`"
+    " either be a coroutine function (`async def`) or return an awaitable object"
+    " (like a Future). It might become a requirement in the future."
+    " Coroutine functions and awaitables have been supported since"
+    " ipykernel 6.0 (2021)."
+)
+
 
 def _accepts_parameters(meth, param_names):
     parameters = inspect.signature(meth).parameters
@@ -742,6 +750,12 @@ async def execute_request(self, socket, ident, parent):
 
         if inspect.isawaitable(reply_content):
             reply_content = await reply_content
+        else:
+            warnings.warn(
+                _AWAITABLE_MESSAGE.format(func_name="execute_request"),
+                PendingDeprecationWarning,
+                stacklevel=1,
+            )
 
         # Flush output before sending the reply.
         if sys.stdout is not None:
@@ -802,6 +816,12 @@ async def complete_request(self, socket, ident, parent):
         matches = self.do_complete(code, cursor_pos)
         if inspect.isawaitable(matches):
             matches = await matches
+        else:
+            warnings.warn(
+                _AWAITABLE_MESSAGE.format(func_name="do_complete"),
+                PendingDeprecationWarning,
+                stacklevel=1,
+            )
 
         matches = json_clean(matches)
         self.session.send(socket, "complete_reply", matches, parent, ident)
@@ -830,6 +850,12 @@ async def inspect_request(self, socket, ident, parent):
         )
         if inspect.isawaitable(reply_content):
             reply_content = await reply_content
+        else:
+            warnings.warn(
+                _AWAITABLE_MESSAGE.format(func_name="do_inspect"),
+                PendingDeprecationWarning,
+                stacklevel=1,
+            )
 
         # Before we send this object over, we scrub it for JSON usage
         reply_content = json_clean(reply_content)
@@ -849,6 +875,12 @@ async def history_request(self, socket, ident, parent):
         reply_content = self.do_history(**content)
         if inspect.isawaitable(reply_content):
             reply_content = await reply_content
+        else:
+            warnings.warn(
+                _AWAITABLE_MESSAGE.format(func_name="do_history"),
+                PendingDeprecationWarning,
+                stacklevel=1,
+            )
 
         reply_content = json_clean(reply_content)
         msg = self.session.send(socket, "history_reply", reply_content, parent, ident)
@@ -966,6 +998,12 @@ async def shutdown_request(self, socket, ident, parent):
         content = self.do_shutdown(parent["content"]["restart"])
         if inspect.isawaitable(content):
             content = await content
+        else:
+            warnings.warn(
+                _AWAITABLE_MESSAGE.format(func_name="do_shutdown"),
+                PendingDeprecationWarning,
+                stacklevel=1,
+            )
         self.session.send(socket, "shutdown_reply", content, parent, ident=ident)
         # same content, but different msg_id for broadcasting on IOPub
         self._shutdown_message = self.session.msg("shutdown_reply", content, parent)
@@ -990,6 +1028,12 @@ async def is_complete_request(self, socket, ident, parent):
         reply_content = self.do_is_complete(code)
         if inspect.isawaitable(reply_content):
             reply_content = await reply_content
+        else:
+            warnings.warn(
+                _AWAITABLE_MESSAGE.format(func_name="do_is_complete"),
+                PendingDeprecationWarning,
+                stacklevel=1,
+            )
         reply_content = json_clean(reply_content)
         reply_msg = self.session.send(socket, "is_complete_reply", reply_content, parent, ident)
         self.log.debug("%s", reply_msg)
@@ -1006,6 +1050,12 @@ async def debug_request(self, socket, ident, parent):
         reply_content = self.do_debug_request(content)
         if inspect.isawaitable(reply_content):
             reply_content = await reply_content
+        else:
+            warnings.warn(
+                _AWAITABLE_MESSAGE.format(func_name="do_debug_request"),
+                PendingDeprecationWarning,
+                stacklevel=1,
+            )
         reply_content = json_clean(reply_content)
         reply_msg = self.session.send(socket, "debug_reply", reply_content, parent, ident)
         self.log.debug("%s", reply_msg)