string formatting: normalize negative zero

[belm0]

BPO	45995
Nosy	@mdickinson, @belm0, @ericvsmith, @stevendaprano, @serhiy-storchaka, @belm0
PRs	bpo-45995: add "z" format specifer to coerce negative 0 to zero #30049

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/mdickinson'
closed_at = None
created_at = <Date 2021-12-06.12:03:46.943>
labels = ['type-feature', 'library', '3.11']
title = 'string formatting: normalize negative zero'
updated_at = <Date 2022-03-13.12:04:51.159>
user = 'https://github.com/belm0'

bugs.python.org fields:

activity = <Date 2022-03-13.12:04:51.159>
actor = 'mark.dickinson'
assignee = 'mark.dickinson'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-12-06.12:03:46.943>
creator = 'John Belmonte'
dependencies = []
files = []
hgrepos = []
issue_num = 45995
keywords = ['patch']
message_count = 23.0
messages = ['407792', '407793', '407796', '407810', '407822', '407857', '407905', '407928', '407929', '407930', '407934', '407973', '407990', '408367', '408413', '408529', '408800', '410838', '411243', '411295', '411302', '412360', '415034']
nosy_count = 6.0
nosy_names = ['mark.dickinson', 'jbelmonte', 'eric.smith', 'steven.daprano', 'serhiy.storchaka', 'John Belmonte']
pr_nums = ['30049']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue45995'
versions = ['Python 3.11']

[belm0]

proposal: add a string formatting option to normalize negative 0 values to 0

use case: rounded display of a float that is nominally 0, where the distraction of a flashing minus sign from minute changes around 0 is unwanted

example:
>>> '%~5.1f' % -.00001
' 0.0'

format spec before:
format_spec ::= [[fill]align][sign][#][0][width][grouping_option][.precision][type]
after:
format_spec ::= [[fill]align][sign][~][#][0][width][grouping_option][.precision][type]
where '~' is only allowed for number types

implementation: if '~' is present in the spec, add 0 to the value after applying precision

[serhiy-storchaka]

To normalize negative 0.0 to 0.0 you can just add 0.0. It will work with any method of converting floats to string, there is no need to change all formatting specifications.

>>> x = -0.0
>>> x
-0.0
>>> x + 0.0
0.0

[belm0]

To normalize negative 0.0 to 0.0 you can just add 0.0.

yes, I'm aware-- see "implementation" in the original post

there is no need to change all formatting specifications

For adding 0 to work, it must be done after the rounding. That means if you want make use of anything in the current formatting spec regarding precision, normalizing negative zero would need to be a proper option of the formatting spec.

[stevendaprano]

It was decided long ago that % formatting would not be enhanced with new features. I think that it is supposed to match the standard C formatting codes, and nothing else.

So this is unlikely to be approved for % formatting.

It *might* be approved for the format method and f-strings. (I suspect that unless you get immediate and uncontroversial agreement from multiple core developers, you may need to take it for further discussion and perhaps even a PEP.)

What you call a "distraction" I consider to be critical part of the display. The numbers you are displaying actually are negative, and rounding them for display does not change that. So they ought to show the minus sign. So I'm not really very sympathetic to this feature request.

[belm0]

Here is the same proposal made for C++ std::format:

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1496r2.pdf

It makes fair arguments for the feature's use, and explains why the problem is hard to work around.

It was withdrawn by the author for C++20, but a consensus proposal is promised for C++23.

[mdickinson]

I'd support having this functionality available for format and for f-strings. (As Steven says, changing %-formatting doesn't seem viable.) It really is awkward to do this in any other way, and I'm reliably informed that normal people don't expect to see negative zeros in formatted numeric output.

It did take me a few minutes to get my head around the idea that f"{-0.01:+.1f}" would return "+0.0" rather than "-0.0" or " 0.0" or just plain "0.0" under this proposal, but I agree that it seems like the only thing that can be consistent and make sense.

I'm not 100% convinced by the particular spelling proposed, but I don't have anything better to suggest. If C++ might be going with a "z", would it make sense to do the same for Python?

I don't forsee any implementation difficulties for float and complex types. For Decimal, we'd need to "own" the string formatting, taking that responsibility away from mpdecimal, but there are already other reasons to do that. Once we've done that, again the implementation doesn't seem onerous.

[serhiy-storchaka]

Well, it makes sense for negative zero produced by rounding.

But if we add a special support for this case, it would be useful to have some control on the type of rounding. Currently floats are rounded to the nearest decimal number, but in some cases it would be better to round up, down, toward zero or infinity (seed for example bpo-44884).

You can round explicitly before formatting, but this solution is also applicable for this issue:

>>> '%5.1f' % (round(-.00001, 1) + 0.0)
' 0.0'

[belm0]

changing %-formatting doesn't seem viable

I'm concerned about treating %-formatting specially. As far as float/complex, the logical and efficient place to put this change seems to be PyOS_double_to_string(), which affects all three formatting options.

For example, the dtoa case is as simple as this change to format_float_short():

/* coerce negative zero to positive */
if (sign == 1 && ((digits_len == 0 && decpt == -1) ||
 (digits_len == 1 && digits[0] == '0'))) {
 sign = 0;
}

[stevendaprano]

Sorry John, I don't understand your comment about "treating %-formatting
specifically". Isn't the point here not to change %-formatting at all?