PEP: 682 Title: Format Specifier for Signed Zero Author: John Belmonte
<john@neggie.net> Sponsor: Mark Dickinson <dickinsm@gmail.com>
PEP-Delegate: Mark Dickinson Discussions-To:
https://discuss.python.org/t/pep-682-format-specifier-for-signed-zero/13596
Status: Final Type: Standards Track Content-Type: text/x-rst Created:
29-Jan-2022 Python-Version: 3.11 Post-History: 08-Feb-2022 Resolution:
https://discuss.python.org/t/accepting-pep-682-format-specifier-for-signed-zero/14088

Abstract

Though float and Decimal types can represent signed zero, in many fields
of mathematics negative zero is surprising or unwanted -- especially in
the context of displaying an (often rounded) numerical result. This PEP
proposes an extension to the string format specification allowing
negative zero to be normalized to positive zero.

Motivation

Here is negative zero:

    >>> x = -0.
    >>> x
    -0.0

When formatting a number, negative zero can result from rounding.
Assuming the user's intention is truly to discard precision, the
distinction between negative and positive zero of the rounded result
might be considered an unwanted artifact:

    >>> for x in (.002, -.001, .060):
    ...     print(f'{x: .1f}')
     0.0
    -0.0
     0.1

There are various approaches to clearing the sign of a negative zero. It
can be achieved without a conditional by adding positive zero:

    >>> x = -0.
    >>> x + 0.
    0.0

To normalize negative zero when formatting, it is necessary to perform a
redundant (and error-prone) pre-rounding of the input:

    >>> for x in (.002, -.001, .060):
    ...     print(f'{round(x, 1) + 0.: .1f}')
     0.0
     0.0
     0.1

There is ample evidence that, regardless of the language, programmers
are often looking for a way to suppress negative zero, and landing on a
variety of workarounds (pre-round, post-regex, etc.). A sampling:

-   How to have negative zero always formatted as positive zero in a
    python string? (Python, post-regex)
-   (Iron)Python formatting issue with modulo operator & "negative zero"
    (Python, pre-round)
-   Negative sign in case of zero in java (Java, post-regex)
-   Prevent small negative numbers printing as "-0" (Objective-C, custom
    number formatter)

What we would like instead is a first-class option to normalize negative
zero, on top of everything else that numerical string formatting already
offers.

Rationale

There are use cases where negative zero is unwanted in formatted number
output -- arguably, not wanting it is more common. Expanding the format
specification is the best way to support this because number formatting
already incorporates rounding, and the normalization of negative zero
must happen after rounding.

While it is possible to pre-round and normalize a number before
formatting, it's tedious and prone to error if the rounding doesn't
precisely match that of the format spec. Furthermore, functions that
wrap formatting would find themselves having to parse format specs to
extract the precision information. For example, consider how this
utility for formatting one-dimensional numerical arrays would be
complicated by such pre-rounding:

    def format_vector(v, format_spec='8.2f'):
        """Format a vector (any iterable) using given per-term format string."""
        return f"[{','.join(f'{term:{format_spec}}' for term in v)}]"

To date, there doesn't appear to be any other widely-used language or
library providing a formatting option for negative zero. However, the
same z option syntax and semantics specified below have been proposed
for C++ std::format(). While the proposal was withdrawn for C++20, a
consensus proposal is promised for C++23. (The original feature request
prompting this PEP was argued without knowledge of the C++ proposal.)

When Rust developers debated whether to suppress negative zero in print
output, they took a small survey of other languages. Notably, it didn't
mention any language providing an option for negative zero handling.

Specification

An optional, literal z is added to the Format Specification
Mini-Language following sign:

    [[fill]align][sign][z][#][0][width][grouping_option][.precision][type]

where z is allowed for floating-point presentation types (f, g, etc., as
defined by the format specification documentation). Support for z is
provided by the .__format__() method of each numeric type, allowing the
specifier to be used in f-strings, built-in format(), and str.format().

When z is present, negative zero (whether the original value or the
result of rounding) will be normalized to positive zero.

Synopsis:

    >>> x = -.00001
    >>> f'{x:z.1f}'
    '0.0'

    >>> x = decimal.Decimal('-.00001')
    >>> '{:+z.1f}'.format(x)
    '+0.0'

Design Notes

The solution must be opt-in, because we can't change the behavior of
programs that may be expecting or relying on negative zero when
formatting numbers.

The proposed extension is intentionally [sign][z] rather than [sign[z]].
The default for sign (-) is not widely known or explicitly written, so
this avoids everyone having to learn it just to use the z option.

While f-strings, built-in format(), and str.format() can access the new
option, %-formatting cannot. There is already precedent for not
extending %-formatting with new options, as was the case for the ,
option (PEP 378).

C99 printf already uses the z option character for another purpose:
qualifying the unsigned type (u) to match the length of size_t. However,
since the signed zero option specifically disallows z for integer
presentation types, it's possible to disambiguate the two uses, should C
want to adopt this new option.

Backwards Compatibility

The new formatting behavior is opt-in, so numerical formatting of
existing programs will not be affected.

How to Teach This

A typical introductory Python course will not cover string formatting in
full detail. For such a course, no adjustments would need to be made.
For a course that does go into details of the string format
specification, a single example demonstrating the effect of the z option
on a negative value that's rounded to zero by the formatting should be
enough. For an independent developer encountering the feature in someone
else's code, reference to the Format Specification Mini-Language section
of the library reference manual should suffice.

Reference Implementation

A reference implementation exists at pull request #30049.

Copyright

This document is placed in the public domain or under the
CC0-1.0-Universal license, whichever is more permissive.



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