code: def _extract_colnums(self, columns=None):
          
          if columns is None:
              return numpy.arange(self._ncol, dtype='i8')
  
          if not isinstance(columns, (tuple, list, numpy.ndarray)):
              # is a scalar
              return self._extract_colnum(columns)
  
          colnums = numpy.zeros(len(columns), dtype='i8')
          for i in xrange(colnums.size):
              colnums[i] = self._extract_colnum(columns[i])
  
          # returns unique sorted
          colnums = numpy.unique(colnums)
          return colnums
docs: Extract an array of columns from the input