Handle case where autocompleting empty string crashes

Fixes awslabs#39.

Author: jamesls

awsshell/autocomplete.py

@@ -49,15 +49,15 @@ def autocomplete(self, line):
             # The user has hit backspace.  We'll need to check
             # the current words.
             return self._handle_backspace()
+        elif not line:
+            return []
         elif current_length != self._last_position + 1:
             return self._complete_from_full_parse()
 
         # This position is important.  We only update the _last_position
         # after we've checked the special cases above where that value
         # matters.
         self._last_position = len(line)
-        if not line:
-            return []
         if line and not line.strip():
             # Special case, the user hits a space on a new line so
             # we autocomplete all the top level commands.

tests/unit/test_autocomplete.py

@@ -337,3 +337,20 @@ def test_last_option_is_updated_on_global_options(index_data):
     assert completer.last_option == '--resources'
     completer.autocomplete('ec2 create-tags --resources f --no-sign-request ')
     assert completer.last_option == '--no-sign-request'
+
+
+def test_can_handle_autocompleting_same_string_twice(index_data):
+    index_data['aws']['commands'] = ['first', 'second']
+    completer = AWSCLIModelCompleter(index_data)
+    completer.autocomplete('f')
+    assert completer.autocomplete('f') == ['first']
+
+
+def test_can_handle_autocomplete_empty_string_twice(index_data):
+    # Sometimes prompt_toolkit will try to autocomplete
+    # the empty string multiple times.  We need to handle this
+    # gracefully.
+    index_data['aws']['commands'] = ['first', 'second']
+    completer = AWSCLIModelCompleter(index_data)
+    assert completer.autocomplete('') == []
+    assert completer.autocomplete('') == []