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On Sun, Apr 1, 2018 at 7:11 PM, Eric V. Smith email@example.com wrote:
Interestingly, in 2.7 'ur' is a valid prefix, but not in 3.6. I don't recall if that was deliberate or not. And 'ru' isn't valid in either version.
I believe it was. The 'ur' string literal in Py2 was a bizarre hybrid of raw-but-allowing-Unicode-escapes, which makes no sense in the Py3 world.
$ python3 Python 3.8.0a0 (heads/literal_eval-exception:ddcb2eb331, Feb 21 2018, 04:32:23) [GCC 6.3.0 20170516] on linux Type "help", "copyright", "credits" or "license" for more information.
print(u"\ \u005c \") \ \ \ print(r"\ \u005c \") \ \u005c \
$ python2 Python 2.7.13 (default, Nov 24 2017, 17:33:09) [GCC 6.3.0 20170516] on linux2 Type "help", "copyright", "credits" or "license" for more information.
print(u"\ \u005c \") \ \ \ print(r"\ \u005c \") \ \u005c \ print(ur"\ \u005c \") \ \ \
In Py3, a normal Unicode literal (with or without the 'u' prefix, which has no meaning) will interpret "\u005c" as a backslash. A raw literal will treat it as a backslash followed by five other characters. In Py2, the same semantics hold for normal Unicode literals, and for bytes literals, the "\u" escape code has no meaning (and is parsed as "\u"). But in a raw Unicode literal, the backslashes are treated literally... unless they're starting a "\u" sequence, in which case they're parsed.
So if you use a raw string literal in Python 3 to store a Windows path name, you're fine as long as it doesn't end with a backslash (a wart in the design that probably can't be done any other way). But in Py2, a raw Unicode literal will occasionally misbehave in the exact same way as a non-raw string literal will - complete with it being data-dependent.
Since the entire point of the Py3 u"..." prefix is compatibility with Py2, the semantics have to be retained. There's no point supporting ur"..." in Py3 if it's not going to produce the same result as in Py2.