def getUmis(self, n):
        ''' return n umis from the random_umis atr.'''
        if n < (self.random_fill_size - self.random_ix):
            barcodes = self.random_umis[self.random_ix: self.random_ix+n]
        else:
            # could use the end of the random_umis but
            # let's just make a new random_umis
            if n > self.random_fill_size:  # ensure random_umis is long enough
                self.random_fill_size = n * 2
            self.refill_random()
            barcodes = self.random_umis[self.random_ix: self.random_ix+n]

        self.random_ix += n
        return barcodes
return n umis from the random_umis atr.