code: def _sanitize_usecols(usecols):
      
  
      if usecols is None:
          return None
  
      try:
          pats = usecols.split(',')
          pats = [p.strip() for p in pats if p]
      except AttributeError:
          usecols = [int(c) for c in usecols]  # Make error if mix.
          usecols.sort()
          return tuple(usecols)   # Assume sane sequence of integers.
  
      cols = []
      for pat in pats:
          if ':' in pat:
              c1, c2 = pat.split(':')
              n1 = letter2num(c1, zbase=True)
              n2 = letter2num(c2, zbase=False)
              cols += range(n1, n2)
          else:
              cols += [letter2num(pat, zbase=True)]
  
      # Remove duplicates:
      cols = list(set(cols))
      cols.sort()
      return tuple(cols)
docs: Make a tuple of sorted integers and return it. Return None if
      usecols is None