def upstream_structure(self):
        """Build dictionary with entire upstream pipeline structure
        (with regard to the current Step).

        Returns:
            dict: dictionary describing the upstream pipeline structure. It has two keys:
            ``'edges'`` and ``'nodes'``, where:

            - value of ``'edges'`` is set of tuples ``(input_step.name, self.name)``
            - value of ``'nodes'`` is set of all step names upstream to this Step
        """
        structure_dict = {'edges': set(),
                          'nodes': set()}
        structure_dict = self._build_structure_dict(structure_dict)
        return structure_dict
Build dictionary with entire upstream pipeline structure
        (with regard to the current Step).

        Returns:
            dict: dictionary describing the upstream pipeline structure. It has two keys:
            ``'edges'`` and ``'nodes'``, where:

            - value of ``'edges'`` is set of tuples ``(input_step.name, self.name)``
            - value of ``'nodes'`` is set of all step names upstream to this Step