code: def add_section(self, alias, section):
          
          if not isinstance(alias, six.string_types):
              raise TypeError('Section name must be a string, got a {!r}'.format(type(alias)))
  
          self._tree[alias] = section
  
          if self.settings.str_path_separator in alias:
              raise ValueError(
                  'Section alias must not contain str_path_separator which is configured for this Config -- {!r} -- '
                  'but {!r} does.'.format(self.settings.str_path_separator, alias)
              )
  
          section._section = self
          section._section_alias = alias
  
          self.dispatch_event(self.hooks.section_added_to_section, alias=alias, section=self, subject=section)
docs: Add a sub-section to this section.