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Showing content from https://mail.python.org/pipermail/python-dev/2015-October/142094.html below:

[Python-Dev] Should PEP 498 specify if rf'...' is valid?

[Python-Dev] Should PEP 498 specify if rf'...' is valid?Terry Reedy tjreedy at udel.edu
Sat Oct 31 22:32:38 EDT 2015
On 10/31/2015 8:48 PM, Nick Coghlan wrote:

> Given that "f" is standing for a runtime transformation (unlike the
> purely declarative "b" and "r"), it makes sense to me to mentally
> translate it as
> "magic_format_call_that_needs_compiler_assistance(<normal string
> literal>)", so requiring the "f" to be first isn't arbitrary, it's
> slotting in where the function name would be for a call to a builtin.
>
> I'd also like to leave the door open to i-strings in the future, so my
> answer to Eric's "What would the docs say?" question is that string
> prefixes can contain imperative runtime flags (which appear first, are
> mutually exclusive, are always lowercase, and cause a runtime
> transformation by changing the actual code generated at compile time)
> and declarative compile time flags (which can appear in any order
> after the imperative flag, may be in upper or lower case,

I think either order for b|u versus r is ok, even though a nuisance to 
specify in a grammer that assumes order significance.  But given that 
Python is case-sensitive, I think the exception here was a mistake that 
need not be copied.

 > and only
> cause a compile time transformation in the stored constant without
> changing the code to load that constant at runtime)

It makes sense to me that f should be kept logically distinct from the 
other two.

> Currently the only imperative prefix we have is "f", while "b", "u",
> and "r" are available as declarative prefixes. "i" has been proposed
> as a second imperative prefix, but is currently deferred until 3.7 at
> the earliest (after we have a release worth's of experience with "f").

-- 
Terry Jan Reedy

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