Unhandled exception in event loop in auto_suggest keybindings

Hi. Thanks.

What's issue

I got Unhandled exception in event loop error in this keybinding function.
https://github.com/prompt-toolkit/python-prompt-toolkit/blob/master/prompt_toolkit/key_binding/bindings/auto_suggest.py#L49

Environment

OS X
python 3.6.8
prompt-toolkit == 2.0.9 or 3.0.0

How reproduction

using this sample: https://github.com/prompt-toolkit/python-prompt-toolkit/blob/master/examples/prompts/auto-suggestion.py

Say something: import [[[[escape f]]] path [[[[escape f]]] 

Say something: import path/homebrew/versions/3.6.8/lib/python3.6/site-packages/prompt_toolkit/key_binding/key_processor.py:273: DeprecationWarning: generator 'KeyProcessor._process' raised StopIteration

Unhandled exception in event loop:
  File ".../prompt_toolkit/eventloop/posix.py", line 154, in _run_task
  File ".../prompt_toolkit/eventloop/context.py", line 115, in new_func
  File ".../prompt_toolkit/application/application.py", line 562, in read_from_input
  File ".../prompt_toolkit/key_binding/key_processor.py", line 273, in process_keys
  File ".../prompt_toolkit/key_binding/key_processor.py", line 180, in _process
  File ".../prompt_toolkit/key_binding/key_processor.py", line 323, in _call_handler
  File ".../prompt_toolkit/key_binding/key_bindings.py", line 78, in call
  File ".../prompt_toolkit/key_binding/bindings/auto_suggest.py", line 51, in _

Exception
Press ENTER to continue...

Solution

Because generator raises StopIteration.
We use suggestion_available filter, but app.current_buffer.suggestion becomes None or
Suggestion() . It can't catch exception.
https://github.com/prompt-toolkit/python-prompt-toolkit/blob/master/prompt_toolkit/key_binding/bindings/auto_suggest.py#L31

We can use try/except or Suggestion.__len__.

A: try/except

https://github.com/prompt-toolkit/python-prompt-toolkit/blob/master/prompt_toolkit/key_binding/bindings/auto_suggest.py#L49

    @handle('escape', 'f', filter=suggestion_available & emacs_mode)
    def _(event: E) -> None:
        b = event.current_buffer
        suggestion = b.suggestion
        if suggestion:
            t = re.split(r'(\S+\s+)', suggestion.text)
-            b.insert_text(next(x for x in t if x))
+            try:
+                b.insert_text(next(x for x in t if x))
+            except StopIteration:
+                pass
+            except:
+                raise

B: Suggestion.__len__

https://github.com/prompt-toolkit/python-prompt-toolkit/blob/master/prompt_toolkit/auto_suggest.py#L48

+    def __len__(self) -> int:
+        return len(self.text)

https://github.com/prompt-toolkit/python-prompt-toolkit/blob/master/prompt_toolkit/key_binding/bindings/auto_suggest.py#L31

-        return (app.current_buffer.suggestion is not None and
+        return (app.current_buffer.suggestion and
                app.current_buffer.document.is_cursor_at_the_end)

Comment

I think Solution B is better:)

Reproduce the failure with examples/prompts/auto-suggestion.py using the reported auto_suggest keybinding and inspect prompt_toolkit/key_binding/bindings/auto_suggest.py, especially the handler around line 49. Check the related suggestion handling in prompt_toolkit/auto_suggest.py and the reported key processor path. Done means the reproduction no longer raises an unhandled StopIteration exception.