Workaround tornado+py38+windows compatibility issue (Same as ipython#456

). Set eventloop policy to the old default while tornado is not compatible with the new one.

ipykernel/ipkernel.py

@@ -57,7 +57,25 @@ def _user_ns_changed(self, name, old, new):
     _sys_raw_input = Any()
     _sys_eval_input = Any()
 
+    def _init_asyncio_patch(self):
+        if sys.platform.startswith("win") and sys.version_info >= (3, 8):
+            import asyncio
+            try:
+                from asyncio import (
+                    WindowsProactorEventLoopPolicy,
+                    WindowsSelectorEventLoopPolicy,
+                )
+            except ImportError:
+                pass
+                # not affected
+            else:
+                if type(asyncio.get_event_loop_policy()) is WindowsProactorEventLoopPolicy:
+                    # WindowsProactorEventLoopPolicy is not compatible with tornado 6
+                    # fallback to the pre-3.8 default of Selector
+                    asyncio.set_event_loop_policy(WindowsSelectorEventLoopPolicy())
+
     def __init__(self, **kwargs):
+        self._init_asyncio_patch()
         super(IPythonKernel, self).__init__(**kwargs)
 
         # Initialize the InteractiveShell subclass