Description

hyperkai

opened on Jul 13, 2025

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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

Activity

1. hyperkai

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Jul 13, 2025
2. picnixz commented on Jul 13, 2025

   

   picnixz

   on Jul 13, 2025

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   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
3. picnixz

   added

   pendingThe issue will be closed if no feedback is providedThe issue will be closed if no feedback is provided

   on Jul 13, 2025
4. skirpichev commented on Jul 14, 2025

   

   skirpichev

   on Jul 14, 2025

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   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.)
5. skirpichev

   closed this as not plannedon Jul 14, 2025
6. skirpichev

   added

   invalid

   and removed

   pendingThe issue will be closed if no feedback is providedThe issue will be closed if no feedback is provided

   on Jul 14, 2025
7. terryjreedy commented on Jul 14, 2025

   

   terryjreedy

   on Jul 14, 2025

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   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'.
   
8. skirpichev commented on Jul 14, 2025

   

   skirpichev

   on Jul 14, 2025

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   Ah, I thought my patch was backported: #132155
9. skirpichev

   reopened this on Jul 14, 2025
10. skirpichev

    added

    3.13only security fixesonly security fixes

    and removed

    invalid

    on Jul 14, 2025
11. skirpichev commented on Jul 14, 2025

    

    skirpichev

    on Jul 14, 2025

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    Backport for 3.13: #136649

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

    self-assigned this

    on Jul 14, 2025
13. StanFromIreland

    added

    stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

    extension-modulesC modules in the Modules dirC modules in the Modules dir

    on Jul 14, 2025
14. StanFromIreland commented on Jul 14, 2025

    

    StanFromIreland

    on Jul 14, 2025

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    Backport has been merged, should this not be classified as a duplicate of its issue?
15. picnixz commented on Jul 14, 2025

    

    picnixz

    on Jul 14, 2025

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    Yes, it would be a duplicate now.
16. picnixz

    closed this as a duplicateof Undocumented handling of thousands separator for floats and differences wrt Decimal #130664 on Jul 14, 2025
17. skirpichev

    removed their assignment

    on Jul 14, 2025

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