Mailing List Archive: f-string error message

I am currently using Python 3.11.4.
First I want to say: f-strings are great! I use them all the time,
mostly but by no means exclusively for debug messages. And in 3.12 they
will get even better.
And the improved error messages in Python (since 3.9) are great too!
Keep up the good work.
However the following error message confused me for a while when it
happened in real code:

>>> import decimal
>>> x=42
>>> f"{x:3d}"
' 42'
>>> x=decimal.Decimal('42')
>>> f"{x:3d}"
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: invalid format string

I understand that this is an error: I'm telling the f-string to expect
an integer when in fact I'm giving it a Decimal.
And indeed f"{x:3}" gives ' 42' whether x is an int or a Decimal.
However, to my mind it is not the format string that is invalid, but the
value supplied to it.
Would it be possible to have a different error message, something like

ValueError: int expected in format string but decimal.Decimal found

Or am I missing something?
Best wishes
Rob Cliffe

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On Sun, Aug 27, 2023, at 17:19, Rob Cliffe via Python-list wrote:
> I understand that this is an error: I'm telling the f-string to expect
> an integer when in fact I'm giving it a Decimal.
> And indeed f"{x:3}" gives ' 42' whether x is an int or a Decimal.
> However, to my mind it is not the format string that is invalid, but the
> value supplied to it.
> Would it be possible to have a different error message, something like
>
> ValueError: int expected in format string but decimal.Decimal found
>
> Or am I missing something?

It's up to the type what format strings are valid for it, so you can't really go "int expected". However, a more detailed error string like "invalid format string '3d' for object Decimal('42')" might be useful.

right now we have some inconsistencies:
- float object [same for str, int, etc]
ValueError: Unknown format code 'd' for object of type 'float' [.if it thinks it's identified a single-letter 'code' in the usual microlanguage]
ValueError: Invalid format specifier '???' for object of type '[type]'
- arbitrary object that doesn't override __format__, ipaddress
TypeError: unsupported format string passed to [type].__format__
- datetime, decimal
ValueError: Invalid format string

neither shows the value of the offending object, only its type. incidentally, ipaddress and object don't support the usual field width, padding, etc specifiers

[.int supports code 'f' just fine, by the way, but has the same message as float if you give it 's']

Going beyond that, it *might* be viable to have some sort of "guess what numeric type the format string was intended for", shared across at least all numeric types of objects. Alternatively, we could require all numeric types to support all numeric formats, even ones that don't make a lot of sense.
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