On 23Apr2021 18:25, Stephen J. Turnbull <[email protected]>
wrote:
>Cameron Simpson writes:
> > I would _frequently_ like to be able to provide custom
> > conversions. At present I'm using elaborate hacks based on
> > __getattr__ etc to recognise things like this:
> >
> > '{x} is {x_lc} in lowercase'
> >
> > where the _lc suffix is caught and a value computed from "x".
> >
> > Custom conversions would let me use this:
> >
> > '{x} is {x!lc} in lowercase'
>
>I don't understand how this is supposed to work. It looks to me like
>!code is a preprocessor: [...]
>If so,
>
> '{x} is {x!lc:foo} in lowercase'
>
>will fail because str doesn't implement the 'foo' format code.
Maybe we're talking about different things. In the example above, I'm
talking about "lc", not "foo".
>Do we
>really need to extend format() rather than using
>
> def lc(x): return str(x).lower()
>
> '{x} is {lc(x)} in lowercase'
I'm not talking about f'' strings, but str.format_map. They use the same
{} placeholder syntax, but for obvious injection related reasons you
can't call an arbitrary function from str.format_map. (You can play
games with magic __getattr__ based attributes, like:
{z.some_attr}
My use case is presupplied strings, eg a command line supplied format
string. So _not_ an f'blah {lc(x)}' string, but a plain string using
format_map. I'm enjoying using Python style string formats, and who
wants to write yet another macro syntax?
Contrast:
Python 3.9.2 (default, Feb 24 2021, 13:30:36)
[Clang 12.0.0 (clang-1200.0.32.29)] on darwin
Type "help", "copyright", "credits" or "license" for more
information.
>>> def lc(x): return x.lower()
...
>>> z='ABC'
>>> f'{lc(z)}'
'abc'
>>> fmt='{lc(z)}'
>>> fmt.format_map({z:"ABC"})
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
KeyError: 'lc(z)'
If I could provide additional formatters to format_map to extend beyond
'r' for repr and friends, I could go:
fmt.format_map({z:"ABC"}, conversion_map={'lc':lambda str.lower})
and use:
fmt = '{z!lc}'
Cheers,
Cameron Simpson <[email protected]>
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