Helper for _enlarge_axis, compute common labels and extra labels.

        Returns:
             nan_labels: The labels needs to be added

def _compute_enlarge_labels(self, locator, base_index):
        """Helper for _enlarge_axis, compute common labels and extra labels.

        Returns:
             nan_labels: The labels needs to be added
        """
        # base_index_type can be pd.Index or pd.DatetimeIndex
        # depending on user input and pandas behavior
        # See issue #2264
        base_index_type = type(base_index)
        locator_as_index = base_index_type(locator)

        nan_labels = locator_as_index.difference(base_index)
        common_labels = locator_as_index.intersection(base_index)

        if len(common_labels) == 0:
            raise KeyError(
                "None of [{labels}] are in the [{base_index_name}]".format(
                    labels=list(locator_as_index), base_index_name=base_index
                )
            )
        return nan_labels