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: >>> print(f'{1!a:g}') Traceback (most recent call last): File "<stdin>", line 1, in <module> ValueError: Unknown format code 'g' for object of type 'str' If so, '{x} is {x!lc:foo} in lowercase' will fail because str doesn't implement the 'foo' format code. Do we really need to extend format() rather than using def lc(x): return str(x).lower() '{x} is {lc(x)} in lowercase' ?