No need for the \, since the parens guarantee Python won't get confused.

"by default," (with the comma) and not "by-default"

Seems like the set() math should be part of load_config_files instead.

Once you've done that, you no longer need to convert this all into a dictionary. You'll just have one that you can iterate.

Can you import rbconfig instead, and then call rbconfig.main()?

ouput?

No need for re.split. You can just do str.split(delimiter).

Also, we should avoid map and filter where possible. They're less readable and Python suggests not using them. Instead, this can be:

[s.strip() for s in line.split(":")]
for line in text.splitlines() if line

You also don't really need to turn this into a dictionary. This whole thing can be greatly simplified:

values = [[s.strip() for s in line.split(":")]
          for line in text.splitlines() if line]

for key, value in values:
    self.assertEquals(value, ...)

Actually... What's this even doing? It's just comparing itself with itself. So something is wrong here.

Should be clear that it's testing in rb config.

Should be clear that it's testing in rb config.

I think you can use StringIO instead of this.

ouput?

The "by default," wasn't addressed.

"Test the '-a/--all' option in rb-config"

This needs to be:

try:
    from cStringIO import StringIO
except ImportError:
    from StringIO import StringIO

I meant use StringIO here. We don't need OutputBuffer.

Whatever is done here needs to be heavily commented.

Can you instead just do: builtin = globals()?

Looks good. Made a couple small grammatical fixes.