code: def set_background(self, fname=None, genome=None, length=200, nseq=10000):
          
          length = int(length)
  
          if genome and fname:
              raise ValueError("Need either genome or filename for background.")
  
          if fname:
              if not os.path.exists(fname):
                  raise IOError("Background file {} does not exist!".format(fname))
  
              self.background = Fasta(fname)
              self.background_hash = file_checksum(fname)
              return
          
          if not genome:
              if self.genome:
                  genome = self.genome
                  logger.info("Using default background: genome {} with length {}".format(
                      genome, length))
              else:
                  raise ValueError("Need either genome or filename for background.")
          
          
          logger.info("Using background: genome {} with length {}".format(genome, length))
          with Cache(CACHE_DIR) as cache:
              self.background_hash = "{}\{}".format(genome, int(length))
              fa = cache.get(self.background_hash)
              if not fa:
                  fa = RandomGenomicFasta(genome, length, nseq)
                  cache.set(self.background_hash, fa)
          self.background = fa
docs: Set the background to use for FPR and z-score calculations.
  
          Background can be specified either as a genome name or as the 
          name of a FASTA file.
          
          Parameters
          ----------
          fname : str, optional
              Name of FASTA file to use as background.
  
          genome : str, optional
              Name of genome to use to retrieve random sequences.
  
          length : int, optional
              Length of genomic sequences to retrieve. The default
              is 200.
  
          nseq : int, optional
              Number of genomic sequences to retrieve.