While asking this question, I realized I didn't know much about raw strings. For somebody claiming to be a Django trainer, this sucks.
I know what an encoding is, and I know what u'' alone does since I get what is Unicode.
But what does r'' do exactly? What kind of string does it result in?
And above all, what the heck does ur'' do?
Finally, is there any reliable way to go back from a Unicode string to a simple raw string?
Ah, and by the way, if your system and your text editor charset are set to UTF-8, does u'' actually do anything?
There's not really any "raw string"; there are raw string literals, which are exactly the string literals marked by an r before the opening quote.
A "raw string literal" is a slightly different syntax for a string literal, in which a backslash, \, is taken as meaning "just a backslash" (except when it comes right before a quote that would otherwise terminate the literal) -- no "escape sequences" to represent newlines, tabs, backspaces, form-feeds, and so on. In normal string literals, each backslash must be doubled up to avoid being taken as the start of an escape sequence.
This syntax variant exists mostly because the syntax of regular expression patterns is heavy with backslashes (but never at the end, so the "except" clause above doesn't matter) and it looks a bit better when you avoid doubling up each of them -- that's all. It also gained some popularity to express native Windows file paths (with backslashes instead of regular slashes like on other platforms), but that's very rarely needed (since normal slashes mostly work fine on Windows too) and imperfect (due to the "except" clause above).
r'...' is a byte string (in Python 2.*), ur'...' is a Unicode string (again, in Python 2.*), and any of the other three kinds of quoting also produces exactly the same types of strings (so for example r'...', r'''...''', r"...", r"""...""" are all byte strings, and so on).
Not sure what you mean by "going back" - there is no intrinsically back and forward directions, because there's no raw string type, it's just an alternative syntax to express perfectly normal string objects, byte or unicode as they may be.
And yes, in Python 2.*, u'...' is of course always distinct from just '...' -- the former is a unicode string, the latter is a byte string. What encoding the literal might be expressed in is a completely orthogonal issue.
E.g., consider (Python 2.6):
>>> sys.getsizeof('ciao')
28
>>> sys.getsizeof(u'ciao')
34
The Unicode object of course takes more memory space (very small difference for a very short string, obviously ;-).