[Python-ideas] Briefer string format

[Eric V. Smith]

On 8/1/2015 2:25 PM, Steven D'Aprano wrote:
> On Sat, Aug 01, 2015 at 01:43:49PM -0400, Eric V. Smith wrote:

>> With str.format, !s and !r are needed because you can't put the call to
>> repr in str.format's very limited expression syntax. But since f-strings
>> support arbitrary expressions, it's not needed.
> 
> Wait, did I miss something? Does this mean that f-strings will 
> essentially be syntactic sugar for str(eval(s))?
> 
> f"[i**2 for i in sequence]"
> 
> f = lambda s: str(eval(s))
> f("[i**2 for i in sequence]")

Well, it's somewhat more complex.

It's true that:
>>> sequence=[1,2,3]
>>> f"{[i**2 for i in sequence]}"
'[1, 4, 9]'

But it's more complex when there are format specifiers and literals
involved.

Basically, the idea is that:
f'a{expr1:spec1}b{expr2:spec2}c'

is shorthand for:
''.join(['a', expr1.__format__(spec1), 'b',
         expr2.__format__(spec2), 'c'])

The expressions can indeed be arbitrarily complex expressions. Because
only string literals are supported, it just the same as if you'd written
the expressions not inside of a string (as shown above). It's not like
you're eval-ing user supplied strings.

Eric.