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[Python-Dev] PEP 498 f-string: is it a preprocessor?

Eric V. Smith eric at trueblade.com
Tue Aug 11 00:59:29 CEST 2015 
On 8/10/2015 6:22 PM, Victor Stinner wrote:
> Le lundi 10 août 2015, Eric V. Smith <eric at trueblade.com
> <mailto:eric at trueblade.com>> a écrit :
> 
>     On 08/10/2015 10:18 AM, Victor Stinner wrote:
>     > Hi,
>     >
>     > I read the PEP but I don't understand how it is implemented. For
>     me, it
>     > should be a simple preprocessor:
>     >
>     > - f'x={x}' is replaced with 'x={0}'.format(x) by the compiler
>     > - f'x={1+1}' is replaced with 'x={0}'.format(1+1)
>     > - f'x={foo()!r}' is replaced with 'x={0!r}'.format(foo())
>     > - ...
>     >
>     > That's all. No new language, no new function or method.
> 
>     There is no new function or method being proposed. The "pre-processor"
>     is being implemented as the ast is being built. As the PEP says, though,
>     the expressions supported aren't exactly the same, so a simple
>     conversion to str.format syntax isn't possible.
> 
> 
> Can you please provide example(s) of f-string(s) which cannot be
> replaced by a call to .format() like I did?

Oops, I was thinking of going the other way (str.format -> f''). Yes, I
think you're correct.

But in any event, I don't see the distinction between calling
str.format(), and calling each object's __format__ method. Both are
compliant with the PEP, which doesn't specify exactly how the
transformation is done.

Eric.
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