, works with Decimal() against the doc of **Format Specification Mini-Language**

Bug report

Bug description:

The doc of Format Specification Mini-Language doesn't say that , works with Decimal() as shown below:

',': Inserts a comma every 3 digits for integer presentation type 'd' and floating-point presentation types, excluding 'n'. For other presentation types, this option is not supported.

But , works with Decimal() against the doc as shown below:

from decimal import Decimal

v = Decimal(value='1234.5555555555')

print('"{:,.20f}"'.format(v))
print('"{:,.20F}"'.format(v))
# "1,234.55555555550000000000"
#        |        20        |

print('"{:,f}"'.format(v))
print('"{:,F}"'.format(v))
# "1,234.5555555555"
#        |   10   |

from decimal import Decimal

v = Decimal('123456.78912')

print('"{:,.20g}"'.format(v))
print('"{:,.20G}"'.format(v))
print('"{:,.20}"'.format(v))
print('"{:,g}"'.format(v))
print('"{:,G}"'.format(v))
print('"{:,}"'.format(v))
# "123,456.78912"
#  |    11     |

In addition, the doc doesn't say that _ works with Decimal() as shown below:

'_': Inserts an underscore every 3 digits for integer presentation type 'd' and floating-point presentation types, excluding 'n'. For integer presentation types 'b', 'o', 'x', and 'X', underscores are inserted every 4 digits. For other presentation types, this option is not supported.

So, _ doesn't work with Decimal() as the doc doesn't say so as shown below:

from decimal import Decimal

v = Decimal(value='1234.5555555555')

print('"{:_.20f}"'.format(v))
print('"{:_.20F}"'.format(v))
print('"{:_f}"'.format(v))
print('"{:_F}"'.format(v))
# ValueError: invalid format string

from decimal import Decimal

v = Decimal('123456.78912')

print('"{:_.20g}"'.format(v))
print('"{:_.20G}"'.format(v))
print('"{:_.20}"'.format(v))
print('"{:_g}"'.format(v))
print('"{:_G}"'.format(v))
print('"{:_}"'.format(v))
# ValueError: invalid format string

CPython versions tested on:

3.12

Operating systems tested on:

Windows

@skirpichev

I'm not sure I understand the first issue namely with ,. The grammar says:

grouping ::= "," | "_"

So , is supported as you observed (both for integer and floating-point presentations). As for _, do you mean that it doesn't work for any of the floating-point presentation types? I don't have access to a 3.13+ installation so I can't check. Also, in the future, please check the bugs against the stable versions, and not those that are security-only.

cc @skirpichev

This is certainly not a bug. The Decimal formatting is documented on the quoted page, as it says that available presentation types for floats and Decimals are same.

On the other hand, the Decimal is not a builtin type, maybe it's better to document it's formatting separately, as for Fractions, in the module page. (Right now it even has no examples of formatted output, see #128698.)

The "floating-point presentation types" are the implicit or explict f,F,g,G, not the object type. I don't see why you are getting the error messages. Maybe a fixed bug. In installed 3.14.0b4 and main:

>>> '{:_.20}'.format(d)
'123_456.123456'
>>> '"{:_.20}"'.format(d)
'"123_456.123456"'

Same with f-strings.

EDIT: Mismatches between presentation type and object type are ValueErrors.

>>> '{:_.20}'.format('453455')
Traceback (most recent call last):
  File "<python-input-4>", line 1, in <module>
    '{:_.20}'.format('453455')
    ~~~~~~~~~~~~~~~~^^^^^^^^^^
ValueError: Cannot specify '_' with 's'.

Ah, I thought my patch was backported: #132155

Backport for 3.13: #136649

3.12 is security-only, so we can't do anything here.

Backport has been merged, should this not be classified as a duplicate of its issue?

Yes, it would be a duplicate now.

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`,` works with `Decimal()` against the doc of Format Specification Mini-Language #136636

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, works with Decimal() against the doc of **Format Specification Mini-Language** #136636

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picnixz commented on Jul 13, 2025

skirpichev commented on Jul 14, 2025

terryjreedy commented on Jul 14, 2025

skirpichev commented on Jul 14, 2025

skirpichev commented on Jul 14, 2025

StanFromIreland commented on Jul 14, 2025

picnixz commented on Jul 14, 2025

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`,` works with `Decimal()` against the doc of Format Specification Mini-Language #136636