code: def _addDefaultSkyKW(imageObjList):
      
      skyKW = "MDRIZSKY"
      Value = 0.0
      for imageSet in imageObjList:
          fname = imageSet._filename
          numchips=imageSet._numchips
          sciExt=imageSet.scienceExt
          fobj = fileutil.openImage(fname, mode='update', memmap=False)
          for chip in range(1,numchips+1,1):
              ext = (sciExt,chip)
              if not imageSet[ext].group_member:
                  # skip over extensions not used in processing
                  continue
              if skyKW not in fobj[ext].header:
                  fobj[ext].header[skyKW] = (Value, 'Sky value computed by AstroDrizzle')
                  log.info("MDRIZSKY keyword not found in the %s[%s,%d] header."%(
                              fname,sciExt,chip))
                  log.info("    Adding MDRIZSKY to header with default value of 0.")
          fobj.close()
docs: Add MDRIZSKY keyword to "commanded" SCI headers of all input images,
          if that keyword does not already exist.