code: def check_singlekey(self):
          
          if len(self._qsequences) == 0:
              return True
          else:
              keystr = self._qsequences[0]
              valid_single_keys = (EDITOR_SINGLE_KEYS if
                                   self.context == 'editor' else SINGLE_KEYS)
              if any((m in keystr for m in ('Ctrl', 'Alt', 'Shift', 'Meta'))):
                  return True
              else:
                  # This means that the the first subsequence is composed of
                  # a single key with no modifier.
                  valid_single_keys = (EDITOR_SINGLE_KEYS if
                                       self.context == 'editor' else SINGLE_KEYS)
                  if any((k == keystr for k in valid_single_keys)):
                      return True
                  else:
                      return False
docs: Check if the first sub-sequence of the new key sequence is valid.