[MAL]
Wait... the -U option was added in order to be able to see how well the 8-bit string / Unicode integration works. It's a know fact that the Python standard lib is not Unicode compatible yet and that's exactly what the -U option allows you to test (in a very simple way).
I don't object to testing hacks provided they don't trip up the innocent; it would help to remove -U from the user-visible docs (which I'll do). Note that, by coincidence, Andreas Jung (at Zope Corp) pissed away time worrying about -U breakage yesterday independent of our thread here: it's doing harm. If you're the only one who tries -U on purpose (anyone? it's clear that I don't ...), it would be better done via a preprocessor define. How often is this used even by you? If it's once per release just to make sure it's still broken <wink>, a variant build wouldn't be a real burden.
... The -U option is currently the only application of such a flag. We will definitely have a need for these options in the future to make the runtime aware of certain assumptions which have been made in the compiled byte code, e.g. byte code using special opcodes, byte code compiled for a different Python virtual machine (once we get pluggable Python compiler / VM combos), byte code which was compiled using special literal interpretations (such as in the -U case or when compiling the source code with a different source code encoding assumption).
There remains no current use for any of these things. When a real use appears, "magic number" abuse won't be appropriate: imp.get_magic() doesn't return a vector; we're not doing the Unixish /etc/magic database any favors by *ever* changing it; and needing to register umpteen distinct magic numbers per release for Linux binfmt would make Python even more irritating to live with there.
I would be more than happy to get rid off the current PYC magic hack for -U and have it replaced with a better and extensible alternative, e.g. a combination of PYC version number and marhsalled option dictionary.
I agree, except that I still think having -U now is a net loss.