I’m not really an expert on non-ASCII URLs / IRIs. Maybe it is obvious to other people that this is a good general implementation, but for me to thoroughly review it I would need time to research the relevant RFCs, other implementations, suitability for the URL schemes listed at <https://docs.python.org/dev/library/urllib.parse.html>, security implications, etc.

One problem problem with using urlunsplit() is it would strip empty URL components, e.g. quote_iri("http://example/file#") -> "http://example/file". See Issue 22852. This is highlighted by the file:///[. . .] → file:/[. . .] test case.

FYI Martin Panter and vadmium are both just me, no need to get too excited. :) I just updated my settings for Rietveld (code review), so hopefully that is more obvious now.