Wrapper for exception/|None| output handling of X_iter methods.

        Attempts to pass `tup` as arguments to `fxn`.  If the call is
        successful, returns the value produced. If
        :exc:`~exceptions.IndexError` or
        :exc:`~exceptions.ValueError`
        is raised, indicating an invalid index value, re-raise if
        `invalid_error` is |True| or return |None| if |False|.

        Other exceptions are left uncaught.

        Parameters
        ----------
        tup
            |tuple| --
            Input arguments to unpack as arguments to `fxn`

        fxn
            callable --
            Function/method to call to calculate the return value

        invalid_error
            |bool| --
            Flag for whether to return |None| on an
            :exc:`~exceptions.IndexError` or
            :exc:`~exceptions.ValueError`, or to re-raise.

        Returns
        -------
        val
            |npfloat_| or |None| --
            Calculated value from fxn(*tup) call, or 'None' value indicating
            IndexError / ValueError

def _iter_return(tup, fxn, invalid_error):
        """ Wrapper for exception/|None| output handling of X_iter methods.

        Attempts to pass `tup` as arguments to `fxn`.  If the call is
        successful, returns the value produced. If
        :exc:`~exceptions.IndexError` or
        :exc:`~exceptions.ValueError`
        is raised, indicating an invalid index value, re-raise if
        `invalid_error` is |True| or return |None| if |False|.

        Other exceptions are left uncaught.

        Parameters
        ----------
        tup
            |tuple| --
            Input arguments to unpack as arguments to `fxn`

        fxn
            callable --
            Function/method to call to calculate the return value

        invalid_error
            |bool| --
            Flag for whether to return |None| on an
            :exc:`~exceptions.IndexError` or
            :exc:`~exceptions.ValueError`, or to re-raise.

        Returns
        -------
        val
            |npfloat_| or |None| --
            Calculated value from fxn(*tup) call, or 'None' value indicating
            IndexError / ValueError

        """

        try:
            val = fxn(*tup)
        except (IndexError, ValueError):
            if invalid_error:
                # Raise the exception if invalid_error indicates
                raise
            else:
                # Otherwise, just return a 'None' value
                return None
            ## end if
        else:
            # Good value; just generate it
            return val