[Python-Dev] Raw string syntax inconsistency (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Sun Jun 17 22:11:27 CEST 2012

On Mon, Jun 18, 2012 at 3:54 AM, Terry Reedy <tjreedy at udel.edu> wrote:

The premise of the discussion of adding 'u', and of Guido's acceptance, was that "it's about as harmless as they come". I do not remember any discussion of 'ur' and what it really means in 2.x, and that supporting it meant adding back 2.x's interaction effect. Indeed, Nick's version goes on to say "This PEP was originally written by Armin Ronacher, and Guido's approval was given based on that version." Armin's original version (and subsequent edit) only proposed adding 'u' (and 'U') and made no mention of 'ur'. Nick's seemingly innocuous addition of also adding 'ur' came after Guido's approval, and as discovered, is not so innocuous.

Right, that matches my recollection as well - we (or least I) thought mapping "ur" to the Python 3 "r" prefix was sufficient, but it turns out doing so means there are some 2.x string literals that will silently behave differently in 3.x.

Martin's right that that part of the PEP should definitely be amended (along with the relevant section in What's New)

I do not think he needs to discuss adding and deleting support, but merely state that 'ur' support is not added because 'ur' has a special meaning that would require changing literal handling. The sentence about supporting 'ur' could be negated and moved after the sentence about not changing Unicode handling. A possibility:

"Combination of the unicode prefix with the raw string prefix will not be supported because in Python 2, the combination 'ur' has a special meaning that would require changing the handling of unicode literals"

In addition to changing the proposal section to only cover "u" and "U", I'll actually add a new subsection along the lines of the following:

Exclusion of Raw Unicode Strings

Python 2.x includes a concept of "raw Unicode" strings. These are partially raw string literals that still support the "\u" and "\U" escape codes for Unicode character entry, but otherwise treat "" as a literal backslash character. As 3.x has no such concept of a partially raw string literal, explicit raw Unicode literals are still not supported. Such literals in Python 2 code will need to be converted to ordinary Unicode literals for forward compatibility with Python 3.

Cheers, Nick.

-- Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia

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