Unhandled exception in event loop: in ipython

Exception
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Unhandled exception in event loop:
File "/usr/local/lib/python3.6/site-packages/prompt_toolkit/eventloop/posix.py", line 154, in _run_task
t()
File "/usr/local/lib/python3.6/site-packages/prompt_toolkit/eventloop/context.py", line 115, in new_func
return func(*a, **kw)
File "/usr/local/lib/python3.6/site-packages/prompt_toolkit/key_binding/key_processor.py", line 404, in flush_keys
self.process_keys()
File "/usr/local/lib/python3.6/site-packages/prompt_toolkit/key_binding/key_processor.py", line 273, in process_keys
self._process_coroutine.send(key_press)
File "/usr/local/lib/python3.6/site-packages/prompt_toolkit/key_binding/key_processor.py", line 180, in _process
self._call_handler(matches[-1], key_sequence=buffer[:])
File "/usr/local/lib/python3.6/site-packages/prompt_toolkit/key_binding/key_processor.py", line 323, in _call_handler
handler.call(event)
File "/usr/local/lib/python3.6/site-packages/prompt_toolkit/key_binding/key_bindings.py", line 78, in call
return self.handler(event)
File "/usr/local/lib/python3.6/site-packages/IPython/terminal/shortcuts.py", line 157, in reset_buffer
b.cancel_completion()
File "/usr/local/lib/python3.6/site-packages/prompt_toolkit/buffer.py", line 788, in cancel_completion
self.go_to_completion(None)
File "/usr/local/lib/python3.6/site-packages/prompt_toolkit/buffer.py", line 853, in go_to_completion
new_text, new_cursor_position = state.new_text_and_position()
File "/usr/local/lib/python3.6/site-packages/prompt_toolkit/buffer.py", line 93, in new_text_and_position
c = self.completions[self.complete_index]

Exception list index out of range
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Unhandled exception in event loop:
File "/usr/local/lib/python3.6/site-packages/prompt_toolkit/eventloop/coroutine.py", line 90, in step_next
new_f = coroutine.throw(exc)
File "/usr/local/lib/python3.6/site-packages/prompt_toolkit/application/run_in_terminal.py", line 95, in _run_in_t
result = yield From(async_func())
File "/usr/local/lib/python3.6/site-packages/prompt_toolkit/eventloop/coroutine.py", line 90, in step_next
new_f = coroutine.throw(exc)
File "/usr/local/lib/python3.6/site-packages/prompt_toolkit/application/application.py", line 701, in print_exception
yield From(_do_wait_for_enter('Press ENTER to continue...'))
File "/usr/local/lib/python3.6/site-packages/prompt_toolkit/eventloop/coroutine.py", line 90, in step_next
new_f = coroutine.throw(exc)
File "/usr/local/lib/python3.6/site-packages/prompt_toolkit/application/application.py", line 971, in _do_wait_for_enter
yield From(session.app.run_async())
File "/usr/local/lib/python3.6/site-packages/prompt_toolkit/eventloop/coroutine.py", line 90, in step_next
new_f = coroutine.throw(exc)
File "/usr/local/lib/python3.6/site-packages/prompt_toolkit/application/application.py", line 658, in _run_async2
result = yield f
File "/usr/local/lib/python3.6/site-packages/prompt_toolkit/eventloop/coroutine.py", line 92, in step_next
new_f = coroutine.send(f.result())
File "/usr/local/lib/python3.6/site-packages/prompt_toolkit/application/application.py", line 610, in _run_async
result = yield From(f)

Exception
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Start by reproducing the IPython interaction that produces the traceback, then read prompt_toolkit/buffer.py around go_to_completion and prompt_toolkit/key_binding/key_processor.py around process_keys. Trace how IPython/terminal/shortcuts.py reset_buffer reaches the completion state, and consider the issue done when the same interaction no longer raises the unhandled exception.