'Set fill value for each column and then apply that fill value
In the first step it is evaluated with value from ``fill_values`` applies to
which column using ``fill_include_names`` and ``fill_exclude_names``.
In the second step all replacements are done for the appropriate columns.'

def masks(self, cols):

    if self.fill_values:
        self._set_fill_values(cols)
        self._set_masks(cols)
