Retrieve a file from the RCSB.

def retrieve_data_from_rcsb(cls, ligand_code, pdb_id = None, silent = True, cached_dir = None):
        '''Retrieve a file from the RCSB.'''
        if not silent:
            colortext.printf("Retrieving data from RCSB")
        if cached_dir:
            assert(os.path.exists(cached_dir))

        ligand_info_path, ligand_info, pdb_ligand_info, pdb_ligand_info_path = None, None, None, None
        if cached_dir:
            ligand_info_path = os.path.join(cached_dir, '{0}.cif'.format(ligand_code))
            if os.path.exists(ligand_info_path):
                ligand_info = read_file(ligand_info_path)
        if not ligand_info:
            ligand_info = retrieve_ligand_cif(ligand_code)
            if cached_dir:
                write_file(ligand_info_path, ligand_info)

        # Parse .cif
        l = cls(ligand_code)
        l.parse_cif(ligand_info)
        l.pdb_id = pdb_id or l.pdb_id
        has_pdb_id = l.pdb_id and (len(l.pdb_id) == 4) and (l.pdb_id != '?')  # the last case is unnecessary and will be short-cut but I included it to show possible values

        # Parse PDB XML
        if has_pdb_id:
            if cached_dir:
                pdb_ligand_info_path = os.path.join(cached_dir, '{0}.pdb.ligandinfo'.format(l.pdb_id.lower()))
                if os.path.exists(pdb_ligand_info_path):
                    pdb_ligand_info = read_file(pdb_ligand_info_path)
                else:
                    pdb_ligand_info = retrieve_pdb_ligand_info(l.pdb_id)
                    write_file(pdb_ligand_info_path, pdb_ligand_info)
            else:
                pdb_ligand_info = retrieve_pdb_ligand_info(l.pdb_id)
        if pdb_ligand_info:
            l.parse_pdb_ligand_info(pdb_ligand_info)

        # Retrive the diagram image
        l.get_diagram()

        return l