Tables injected into the step.

        Returns
        -------
        tables : set of str

def _tables_used(self):
        """
        Tables injected into the step.

        Returns
        -------
        tables : set of str

        """
        args = list(self._argspec.args)
        if self._argspec.defaults:
            default_args = list(self._argspec.defaults)
        else:
            default_args = []
        # Combine names from argument names and argument default values.
        names = args[:len(args) - len(default_args)] + default_args
        tables = set()
        for name in names:
            parent_name = name.split('.')[0]
            if is_table(parent_name):
                tables.add(parent_name)
        return tables