[Python-ideas] Briefer string format

Steven D'Aprano steve at pearwood.info
Sat Jul 25 04:37:29 CEST 2015

On Fri, Jul 24, 2015 at 11:40:49AM +1200, Greg Ewing wrote:
> Steven D'Aprano wrote:
> >I don't think I want this behaviour:
> >
> >    f'{spam}' '{eggs}'
> >    => format(spam) + '{eggs}'
> What do you think should happen in this case:
>    '{spam}' f'{eggs}'

As I stated before, I think that should at least raise a warning, if not 
a syntax error. I think we're in uncharted territory here, because f 
strings aren't really a literal string, they're actually a runtime 
function call, and we haven't got much in the way of prior-art for 
implicit concatenation of a literal with a function call. So we ought to 
be cautious when dealing with anything the least bit ambiguous, and 
avoid baking in a mistake that we can't easily change.

There's no ambiguity with concat'ing f strings only, or non-f strings 
only, but the more we discuss this, the more inclined I am to say that 
implicit concatenation between f strings and non-f strings *in any 
order* should be a syntax error.

> It would seem very strange to me if the f infected
> strings *before* it as well as after it.

It's consistent with Python 2:

py> "abc" u"ßŮƕΩж" "def"

The unicodeness of the middle term turns the entire concatenation into 
unicode. I think it is informative that Python 3 no longer allows this 

py> b"abc" u"ßŮƕΩж" b"def"
  File "<stdin>", line 1
SyntaxError: cannot mix bytes and nonbytes literals


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