Don't nest asyncio input hooks.

This fixes the "event loop already running" error.

For prompt_toolkit 2.0: this will still nest the event loops, but that's fine,
because we have the asyncio event loop running an prompt_toolkit's custom loop.
They don't interfere with each other.

For prompt_toolkit 3.0: we only use one asyncio event loop when "%gui asyncio"
is used. The terminal prompt will now use the active asyncio event loop.

IPython/terminal/interactiveshell.py

@@ -580,11 +580,22 b' class TerminalInteractiveShell(InteractiveShell):'
         # For prompt_toolkit 3.0. We have to create an asyncio event loop with
         # this inputhook.
         if PTK3:
-            if self._inputhook:
-                from prompt_toolkit.eventloop import new_eventloop_with_inputhook
+            import asyncio
+            from prompt_toolkit.eventloop import new_eventloop_with_inputhook
+
+            if gui == 'asyncio':
+                # When we integrate the asyncio event loop, run the UI in the
+                # same event loop as the rest of the code. don't use an actual
+                # input hook. (Asyncio is not made for nesting event loops.)
+                self.pt_loop = asyncio.get_event_loop()
+
+            elif self._inputhook:
+                # If an inputhook was set, create a new asyncio event loop with
+                # this inputhook for the prompt.
                 self.pt_loop = new_eventloop_with_inputhook(self._inputhook)
             else:
-                import asyncio
+                # When there's no inputhook, run the prompt in a separate
+                # asyncio event loop.
                 self.pt_loop = asyncio.new_event_loop()
 
     # Run !system commands directly, not through pipes, so terminal programs

IPython/terminal/pt_inputhooks/asyncio.py

@@ -28,6 +28,10 b' prompt_toolkit`s `patch_stdout`)::'
 
 """
 import asyncio
+from prompt_toolkit import __version__ as ptk_version
+
+PTK3 = ptk_version.startswith('3.')
+
 
 # Keep reference to the original asyncio loop, because getting the event loop
 # within the input hook would return the other loop.
@@ -35,6 +39,19 b' loop = asyncio.get_event_loop()'
 
 
 def inputhook(context):
+    """
+    Inputhook for asyncio event loop integration.
+    """
+    # For prompt_toolkit 3.0, this input hook literally doesn't do anything.
+    # The event loop integration here is implemented in `interactiveshell.py`
+    # by running the prompt itself in the current asyncio loop. The main reason
+    # for this is that nesting asyncio event loops is unreliable.
+    if PTK3:
+        return
+
+    # For prompt_toolkit 2.0, we can run the current asyncio event loop,
+    # because prompt_toolkit 2.0 uses a different event loop internally.
+
     def stop():
         loop.stop()
 
@@ -44,3 +61,4 b' def inputhook(context):'
         loop.run_forever()
     finally:
         loop.remove_reader(fileno)
+