On Thu, Sep 17, 2020 at 1:13 PM Eric V. Smith <e...@trueblade.com> wrote:
> On 9/17/2020 12:24 PM, Chris Angelico wrote:
> > On Fri, Sep 18, 2020 at 1:38 AM Eric V. Smith <e...@trueblade.com>
> wrote:
> >> In general, there's no way to start with a format specifier string and
> >> from it get the type of the object that it should be applied to. For
> >> example, any string without %'s is a valid datetime format specifier (of
> >> dubious value, but such is life). Perhaps a better example is decimal
> >> vs. float. What if I want %.2f to return a decimal? Would that just not
> >> be possible?
> >>
> >> So I think you'd have to limit this to a small set of built-in types.
> >>
> >> In general, I think overloading f-strings as assignment targets would be
> >> confusing. But I've been wrong before.
> >>
> > It doesn't have to be absolutely identical, just as long as it's
> > mostly parallel. Consider C's printf and scanf formats; any whitespace
> > in scanf matches any whitespace, and the integer handlers are always
> > happy to accept a leading plus sign (which has to be specifically
> > requested in printf). Conversely, scanf can be more restrictive, eg
> > %[a-z] which will match some sequence of lowercase ASCII letters.
>
> Sure. And the less it looks like "real" f-strings, the worse it is, IMO.
>
> If someone wanted to play with this in pure Python, string.Formatter's
> parse() method would do all of the parsing for you.
>
Surprisingly it is not *quite* all. The format specification mini language
parser is not exposed for use. This has bummed me out more than once in the
past.
Here is my attempt at recreating it from a couple years ago. Do with it
what you will.
>>> r=r'(([\s\S])?([<>=\^]))?([\+\-
])?([#])?([0])?(\d)*([,])?((\.)(\d)*)?([sbcdoxXneEfFgGn%])?'
>>>
>>> from collections import namedtuple as nt
>>> FormatSpec = nt('FormatSpec', 'fill align sign alt zero_padding width
comma decimal precision type')
>>>
>>> import re
>>> spec =
FormatSpec(*re.fullmatch(r,'x>5.2f').group(2,3,4,5,6,7,8,10,11,12)) # skip
groups not interested in
>>> spec
FormatSpec(fill='x', align='>', sign=None, alt=None, zero_padding=None,
width='5', comma=None, decimal='.', precision='2', type='f')
>>> ''.join(s for s in spec if s is not None) # recreate the input spec
'x>5.2f'
---
Ricky.
"I've never met a Kentucky man who wasn't either thinking about going home
or actually going home." - Happy Chandler
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