The lexical analysis documentation says this:

http://docs.python.org/py3k/reference/lexical_analysis.html?highlight=raw%20bytes

"Bytes literals are always prefixed with 'b' or 'B';..."

"Both string and bytes literals may optionally be prefixed with a letter 'r' or 'R';..."

But that would lead you to believe that to get raw byte strings you should use rb"foo".  In fact, that's a SyntaxError in Python 2.6+ and Python 3.  What *does* work though is br"foo".

Either Python should accept both spellings (harder) or the documentation should make it clear that the 'b' must preceded the 'r'.