The current patch should be fine, although it is IMHO not the long-term solution. For installed python's get_config_var('srcdir') still lies, but now at least returns slightly more sane path.

As I noted before the real problem is that it is not clear which config_vars are usable in which situations. One example is 'srcdir': this value is only useful in the build tree and there currently is no clean way to signal that you are retrieving a value that is not useful.  Another example is 'RUNSHARED', that's also only useable when building python.

I'm not sure what the right solution for the larger issue is. It might be good enough to document generally useful config_vars and warn that other's might exist but might not be safe to use, on the other hand it might be better to explicitly reduce the set of config_vars to something sane (and document which those are and when they can be used)