docs: def write_downloadable(self, organism, export_type='FASTA',
                             seq_type='peptide', export_format='text',
                             export_gff3_fasta=False, sequences=[], region=None):
          """
          Prepare a download for an organism
  
          :type organism: str
          :param organism: organism common name
  
          :type sequences: str
          :param sequences: Names of references sequences to add (default is all)
  
          :type export_type: str
          :param export_type: Export type. Choices: FASTA, GFF3, VCF
  
          :type seq_type: str
          :param seq_type: Export selection. Choices: peptide, cds, cdna, genomic
  
          :type export_format: str
          :param export_format: Export format, either gzip or text
  
          :type export_gff3_fasta: bool
          :param export_gff3_fasta: Export reference sequence when exporting GFF3 annotations.
  
          :type region: str
          :param region: Region to export in form sequence:min..max e.g., chr3:1001..1034
  
          :rtype: dict
          :return: a dictionary containing download information
          """
  
          if export_format.lower() not in ('gzip', 'text'):
              raise Exception("export_format must be one of file, text")
  
          if export_type.lower() not in ('fasta', 'gff3', 'vcf'):
              raise Exception("export_type must be one of FASTA, GFF3, VCF")
  
          data = {
              'type': export_type,
              'seq_type': seq_type,
              'format': export_format,
              'sequences': sequences,
              'organism': organism,
              'output': 'file',
              'exportAllSequences': True if not sequences else len(sequences) == 0,
              'exportGff3Fasta': export_gff3_fasta,
          }
  
          if region:
              data['region'] = region
  
          return self.post('write', data)