code: def registerIcon(self, fileName, glyph, isOpen=None):
          
          check_class(isOpen, bool, allow_none=True)
  
          if fileName and not os.path.isabs(fileName):
              fileName = os.path.join(self.ICONS_DIRECTORY, fileName)
  
          if isOpen is None:
              # Register both opened and closed variants
              self._registry[(glyph, True)] = fileName
              self._registry[(glyph, False)] = fileName
          else:
              self._registry[(glyph, isOpen)] = fileName
docs: Register an icon SVG file given a glyph, and optionally the open/close state.
  
              :param fileName: filename to the SVG file.
                  If the filename is a relative path, the ICONS_DIRECTORY will be prepended.
              :param glyph: a string describing the glyph (e.g. 'file', 'array')
              :param isOpen: boolean that indicates if the RTI is open or closed.
                  If None, the icon will be registered for open is both True and False
              :return: QIcon