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

[hyperkai]

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

[picnixz]

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

[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.)

[terryjreedy]

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'.

[skirpichev]

Ah, I thought my patch was backported: #132155

[skirpichev]

Backport for 3.13: #136649

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