Creates a PDBML object by using a cached copy of the files if they exists or by retrieving the files from the RCSB.

def retrieve(pdb_id, cache_dir = None):
        '''Creates a PDBML object by using a cached copy of the files if they exists or by retrieving the files from the RCSB.'''

        pdb_contents = None
        xml_contents = None
        pdb_id = pdb_id.upper()

        if cache_dir:
            # Check to see whether we have a cached copy of the PDB file
            filename = os.path.join(cache_dir, "%s.pdb" % pdb_id)
            if os.path.exists(filename):
                pdb_contents = read_file(filename)

            # Check to see whether we have a cached copy of the XML file
            filename = os.path.join(cache_dir, "%s.xml" % pdb_id)
            if os.path.exists(filename):
                xml_contents = read_file(filename)

        # Get any missing files from the RCSB and create cached copies if appropriate
        if not pdb_contents:
            pdb_contents = rcsb.retrieve_pdb(pdb_id)
            if cache_dir:
                write_file(os.path.join(cache_dir, "%s.pdb" % pdb_id), pdb_contents)

        if not xml_contents:
            xml_contents = rcsb.retrieve_xml(pdb_id)
            if cache_dir:
                write_file(os.path.join(cache_dir, "%s.xml" % pdb_id), xml_contents)

        # Return the object
        return PDBML_slow(xml_contents, pdb_contents)