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Add underscore as a decimal separator for string formatting #87790Description
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assignee = None
closed_at = None
created_at = <Date 2021-03-25.17:19:07.475>
labels = ['interpreter-core', 'type-feature', '3.11']
title = 'Add underscore as a decimal separator for string formatting'
updated_at = <Date 2021-05-04.15:37:03.108>
user = 'https://bugs.python.org/TerryDavis' bugs.python.org fields:
activity = <Date 2021-05-04.15:37:03.108>
actor = 'Terry Davis'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2021-03-25.17:19:07.475>
creator = 'Terry Davis'
dependencies = []
files = []
hgrepos = []
issue_num = 43624
keywords = []
message_count = 17.0
messages = ['389508', '389512', '389517', '389529', '389534', '389546', '389547', '389574', '389687', '389708', '389709', '389735', '389736', '389744', '389754', '389762', '392911']
nosy_count = 7.0
nosy_names = ['rhettinger', 'mark.dickinson', 'vstinner', 'eric.smith', 'serhiy.storchaka', 'Terry Davis', 'domdfcoding']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue43624'
versions = ['Python 3.11'] Linked PRs
Reacted by Kostas Mouratidis and Sergey B Kirpichev ActivityTerryDavis commented on Mar 25, 2021 MannequinAuthor More actions rhettinger commented on Mar 25, 2021 IIRC there is ISO recommending that after the decimal point, digits be arranged in groups of five. I think is also how printed reference tables are typically formatted. rhettinger commented on Mar 25, 2021 domdfcoding commented on Mar 25, 2021 ISO 80000-1:2009 recommends groups of three digits either side of the decimal sign. ericvsmith commented on Mar 26, 2021 If we do anything for float, we should do the same for decimal.Decimal. vstinner commented on Mar 26, 2021
If we do anything for float, we should do the same for decimal.Decimal.
and complex ;-) vstinner commented on Mar 26, 2021 How backward incompatible and annoying would it be to modify the behavior of the existing "_f" format?
Do you see use cases which only want to group digits in the integer part but not the fractional part?
According to https://discuss.python.org/t/add-underscore-as-a-thousandths-separator-for-string-formatting/7407 discussion, grouping digits was first designed for integers, and the fractional part of floats was simply ignored/forgotten. I mean, it doesn't sound like a deliberate choice to not group digits in the fractional part.
The advantage of changing "_f" format is to keep backward compatibility: Python 3.9 and older would not group digits in the fractional part, but at least they don't fail with an error. If you write code with "_._f" format, you need a fallback code path for Python 3.9 and older:
if sys.version_info >= (3, 10):
text = f"my {...} very {...} long {...} and {...} complex {...} format string: x={x:_._f}"
else:
text = f"my {...} very {...} long {...} and {...} complex {...} format string: x={x:_f}"Or:
text = f"my {...} very {...} long {...} and {...} complex {...} format string:" + (f"x={x:_f}" if sys.version_info >= (3, 10) else "x={x:_f}")Or many other variants.
The main drawback is the risk to break tests relying on the exact output.
About the separator character and the number of digits per group, IMO there is no standard working in all countries and all languages. But since we have a strict rule of 3 digits with "_" separator, I am fine with doing the same for the fractional part. It's an "arbitrary" choice, but at least, it's consistent.
People wanting a different format per locale/language should write their own function. Once enough people will agree on such API, we can consider to add it to the stdlib. But for now, IMO 3 digits with "_" is good enough.
By the way, I agree that it's hard to read numbers with many digits in the decimal part ;-)
>>> f"{1/7:_.30f}"
'0.142857142857142849212692681249'
>>> f"{10**10+1/7:_.10f}"
'10_000_000_000.1428565979'TerryDavis commented on Mar 26, 2021 MannequinAuthor More actions Good point Victor, though I wonder how likely it is that a person using 3.10 would only use this particular new feature, and have an otherwise backwards-compatible codebase.
This isn't something that I asked about out of necessity, and there hasn't been any other discussion of this idea that anyone can remember.
On the other hand, I suppose it would be possible to have a feature flag that can be used to disable decimal underscores in 3.10 to prevent test failures. Just spitballing... vstinner commented on Mar 29, 2021
On the other hand, I suppose it would be possible to have a feature flag that can be used to disable decimal underscores in 3.10 to prevent test failures. Just spitballing...
I wrote PEP-606 -- Python Compatibility Version https://www.python.org/dev/peps/pep-0606/ and it was rejected. skirpichev commented on May 8, 2026
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