code: def horz_dpi(self):
          
          pHYs = self._chunks.pHYs
          if pHYs is None:
              return 72
          return self._dpi(pHYs.units_specifier, pHYs.horz_px_per_unit)
docs: Integer dots per inch for the width of this image. Defaults to 72
          when not present in the file, as is often the case.