'Adds a :ref:`formatter` to the :class:`Dataset`.
.. versionadded:: 0.9.5
:param col: column to. Accepts index int or header str.
:param handler: reference to callback function to execute
against each cell value.'

def add_formatter(self, col, handler):

    if isinstance(col, str):
        if (col in self.headers):
            col = self.headers.index(col)
        else:
            raise KeyError
    if (not (col > self.width)):
        self._formatters.append((col, handler))
    else:
        raise InvalidDatasetIndex
    return True
