A user pointed me to a problem when reading and writing gpc files. GPC understands two kinds of files, with or without hole-flags. Polygon supports both formats, but this was not yet documented. When using the Polygon() constructor with a filename or the read() or write() methods you may specify an optional  second boolean argument. It defaults to True (with hole-flags), with False you may read and write the old-style formats without hole-flags.

Caution: the file format will not be checked automatically! When using the wrong settings the coordinates read from file may be incorrect!

The next release of Polygon will contain updated documentation and a function to manually check the format of a gpc file.