def _compress_existing_file(self, filepath):
<EOL>if os.path.exists(filepath):<EOL><INDENT>compressed_filepath = filepath+'<STR_LIT>'<EOL>f_in = open(filepath, '<STR_LIT:rb>')<EOL>f_out = gzip.open(compressed_filepath, '<STR_LIT:wb>')<EOL>f_out.writelines(f_in)<EOL>f_out.close()<EOL>f_in.close()<EOL>return compressed_filepath<EOL><DEDENT>else:<EOL><INDENT>logger.error('<STR_LIT>' + repr(filepath)  +'<STR_LIT>'<EOL>'<STR_LIT>')<EOL>sys.exit(<NUM_LIT:1>)<EOL><DEDENT>
[Helper]Compresses file 'filepath' and returns file path of
the compresses file.