Read the key/values if the file exists.

        returns -- a dictionary with key/values, or empty dictionary if the
                   file does not exist.

def _read_keyvals(filename):
        """Read the key/values if the file exists.

        returns -- a dictionary with key/values, or empty dictionary if the
                   file does not exist.

        """
        assert isinstance(filename, str)
        if os.path.exists(filename):
            return _KeyValuePersister._actually_populate_keyvals(filename)
        else:
            return {}