def set(self, section, option, value=None):
        """Set an option."""
        if value:
            value = self._interpolation.before_set(self, section, option,
                                                   value)
        if not section or section == self.default_section:
            sectdict = self._defaults
        else:
            try:
                sectdict = self._sections[section]
            except KeyError:
                raise from_none(NoSectionError(section))
        sectdict[self.optionxform(option)] = value
Set an option.