[issue45739] The Python implementation of Decimal does not support the "N" format

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

You are right about an uppercase converter. I forget this. I withdraw this 
proposition.

It seems that even if we add support of "N" to all numeric formatting it will 
not cover all use cases. "n" is a locale specific variant of "g", but other 
formats could have locale specific variants too. In C all numeric formatting is 
locale specific. Maybe add a flag which makes any numeric formatting locale 
specific? Then "n" will became obsolete.

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[issue45739] The Python implementation of Decimal does not support the "N" format

[Eric V. Smith]

Eric V. Smith  added the comment:

While I'd prefer -1 overall, I would also prefer 3 over 2. If we were adding 
the feature from scratch, we wouldn't have decimal differ from int, float, and 
complex.

And I'm not in favor of an uppercase converter, no matter what we do here. The 
other converters work like:

f'{obj!s}' -> format(str(obj))
f'{obj!r}' -> format(repr(obj))

but the proposed !u would be

f'{obj!u}' -> format(obj).upper()

That is, it operates on the result of __format__, not its input.

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[issue45739] The Python implementation of Decimal does not support the "N" format

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

I have no strong opinion. I found the discrepancy when reviewed one of Mark's 
PRs. I was surprised because I did not know that "N" is supported.

On other hand, there are reasons for supporting upper-case variant of "n" if we 
support upper-case variants of "e", "g" and "x" (formats which produce output 
containing letters).

There is an alternative solution of the problem solved by upper-case variations 
of formats: add a special converter for converting a string to upper case, so 
for example f'{x:12.8E}' could be written as f'{x:12.8e!u}'.

I prefer -1 or 2. If we choose deprecation I prefer adding an upper-case 
converter.

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[issue45739] The Python implementation of Decimal does not support the "N" format

[Eric V. Smith]

Eric V. Smith  added the comment:

I'd support -1.

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[issue45739] The Python implementation of Decimal does not support the "N" format

[Raymond Hettinger]

Raymond Hettinger  added the comment:

I support deprecating "N".

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[issue45739] The Python implementation of Decimal does not support the "N" format

[Mark Dickinson]

Mark Dickinson  added the comment:

I could be persuaded for any of options -1, 1 and 2. I don't much like option 0.

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[issue45739] The Python implementation of Decimal does not support the "N" format

[Mark Dickinson]

Mark Dickinson  added the comment:

Eric, Serhiy: do you have opinions on the right way forward?

Here are 6 options, on a spectrum of increasing level of acceptance of "N".

-2. Remove "N" support for cdecimal right now (i.e., for Python 3.11), on the 
basis that there's no need for deprecation warnings, because it was never 
officially a feature.
-1. Deprecate "N" support for cdecimal, remove it in Python 3.13.
0. Do nothing (the default), leaving _pydecimal and cdecimal inconsistent.
1. Add "N" support to the Python implementation for parity with cdecimal, but 
don't document it - leave it as an undocumented feature.
2. Officially add "N" support to decimal formatting - add documentation, tests, 
and fix the Python implementation.
3. Officially add "N" support to all numeric formatting ...

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[issue45739] The Python implementation of Decimal does not support the "N" format

[Eric V. Smith]

Eric V. Smith  added the comment:

I don’t think “N” should be supported. It was never the intention to have it 
work.

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[issue45739] The Python implementation of Decimal does not support the "N" format

[Mark Dickinson]

Mark Dickinson  added the comment:

Interesting. I think the behaviour of the Python implementation behaviour is 
actually more correct here: neither `int` nor `float` supports 'N', and I'm not 
seeing any indication in tests or documentation that 'N' should be supported. 
So is this a bug in libmpdec, or a missing feature in the Python 
implementation? (Either way, it's definitely a bug that the two aren't aligned.)

>>> format(123, 'n')
'123'
>>> format(123, 'N')
Traceback (most recent call last):
  File "", line 1, in 
ValueError: Unknown format code 'N' for object of type 'int'

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[issue45739] The Python implementation of Decimal does not support the "N" format

[Serhiy Storchaka]

New submission from Serhiy Storchaka :

The C implementation supports both formats "n" and "N". The Python 
implementation only supports format "n".

>>> from decimal import Decimal
>>> format(Decimal('1e100'), 'n')
'1e+100'
>>> format(Decimal('1e100'), 'N')
'1E+100'
>>> from _pydecimal import Decimal
>>> format(Decimal('1e100'), 'n')
'1e+100'
>>> format(Decimal('1e100'), 'N')
Traceback (most recent call last):
  File "", line 1, in 
  File "/home/serhiy/py/cpython/Lib/_pydecimal.py", line 3766, in __format__
spec = _parse_format_specifier(specifier, _localeconv=_localeconv)
   ^^^
  File "/home/serhiy/py/cpython/Lib/_pydecimal.py", line 6194, in 
_parse_format_specifier
raise ValueError("Invalid format specifier: " + format_spec)

ValueError: Invalid format specifier: N

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components: Library (Lib)
messages: 405861
nosy: facundobatista, mark.dickinson, rhettinger, serhiy.storchaka
priority: normal
severity: normal
status: open
title: The Python implementation of Decimal does not support the "N" format
type: behavior
versions: Python 3.10, Python 3.11, Python 3.9

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