Avoid weird `pydantic` runtime warning

piker/data/_sampling.py

@@ -227,7 +227,7 @@ async def sample_and_broadcast(
             # end up triggering backpressure which which will
             # eventually block this producer end of the feed and
             # thus other consumers still attached.
-            subs = bus.subscribers[sym]
+            subs = bus._subscribers[sym]
             for ctx in subs:
                 # print(f'sub is {ctx.chan.uid}')
                 try:

piker/data/feed.py

@@ -67,11 +67,19 @@ class _FeedsBus(BaseModel):
     brokername: str
     nursery: trio.Nursery
     feeds: Dict[str, trio.CancelScope] = {}
-    subscribers: Dict[str, List[tractor.Context]] = {}
+
     task_lock: trio.StrictFIFOLock = trio.StrictFIFOLock()
 
+    # XXX: so weird but, apparently without this being `._` private
+    # pydantic will complain about private `tractor.Context` instance
+    # vars (namely `._portal` and `._cancel_scope`) at import time.
+    # Reported this bug:
+    # https://github.com/samuelcolvin/pydantic/issues/2816
+    _subscribers: Dict[str, List[tractor.Context]] = {}
+
     class Config:
         arbitrary_types_allowed = True
+        underscore_attrs_are_private = False
 
     async def cancel_all(self) -> None:
         for sym, (cs, msg, quote) in self.feeds.items():
@@ -256,7 +264,7 @@ async def attach_feed_bus(
                     loglevel=loglevel,
                 )
             )
-            bus.subscribers.setdefault(symbol, []).append(ctx)
+            bus._subscribers.setdefault(symbol, []).append(ctx)
         else:
             sub_only = True
 
@@ -269,12 +277,12 @@ async def attach_feed_bus(
     await ctx.send_yield(first_quote)
 
     if sub_only:
-        bus.subscribers[symbol].append(ctx)
+        bus._subscribers[symbol].append(ctx)
 
     try:
         await trio.sleep_forever()
     finally:
-        bus.subscribers[symbol].remove(ctx)
+        bus._subscribers[symbol].remove(ctx)
 
 
 @dataclass