> In those cases, I would assume that 'x://y', x:/y','x:/','/' 
> as valid URLS, but not the two examples you mentioned.

Only 2 comments about this:
1) Although the urlparse documentation does mention the relevant RFCs, on a quick read-through I don't see that it actually requires its input to be a valid URL.
2) Obviously, some code is using it for invalid URLs.

> Michael, RDM: If you have any comments on msg102737, 
> let me know. Otherwise we can close this issue as wont-fix.

I don't have a problem with this. I reported the issue to bring attention to the fact that the change affected real code and to see if anyone had suggestions for dealing with the problem in a better way (at the moment, I don't).

> By the way, I’ve been meaning to file a bug against the 
> Mercurial schemes extension...

Please do. I don't have anything to do with the Schemes extension except that I found this issue while investigating a unit test failure, but if you file an issue probably I will look into it--or if I don't, then someone else probably will.