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From: Ian Kelly <ian.g.kelly@gmail.com>
Newsgroups: comp.lang.python
Subject: Re: Curious Omission In New-Style Formats
Date: Mon, 11 Jul 2016 12:38:36 -0600
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On Mon, Jul 11, 2016 at 10:28 AM, Steven D'Aprano <steve@pearwood.info> wrote:
> On Tue, 12 Jul 2016 01:04 am, Ian Kelly wrote:
>> Your example showed a 3-digit number being formatted with a requested
>> precision of 5 digits. The way this was done was by left-padding the
>> number with 0s until there were 5 digits,
>
> Right.
>
>> but still only 3 digits of precision.
>
> Er, what? I count *five* digits in "00123", not three.
>
> You seem to be assuming that "precision" can only refer to digits after the
> decimal place, but that's a dubious proposition. That might be the way
> printf interprets the precision for *floats*, but its not the way it
> interprets the precision for *ints*, and who is to say that one way is
> right and the other is wrong?

In what way do the leading zeroes in "00123" add to the precision of
the number? 00123 is the same quantity as 123 and represents no more
precise a measurement. Neither does 0000000123 for "ten" digits of
precision. We could just keep adding zeroes ad nauseam and then claim
that 123 has infinite precision.

Clearly, that's wrong.

>> If you truly wanted to format the number with a precision
>> of 5 digits, it would look like this:
>>
>>     0x123.00
>
> Er, no, because its an integer.

Which is why if you actually want to do this, you should convert it to
a decimal or a float first (of course, those don't support hexadecimal
output, so if you actually want hexadecimal output *and* digits after
the (hexa)decimal point, then I think you would just have to roll your
own formatting at that point).

>> It may happen to do what you want in the printf-style format, but
>> calling the field "precision" is at best misleading, and there are
>> other ways to accomplish the same result.
>
> Naturally. So why bother to have .format() or % in the first place? There's
> always other ways to accomplish the same result.

I think it's just a case of purity winning out over practicality. As I
said before, I don't really know why the decision was made to not
support or allow it. What I've written above was my best guess.