Bug report

In Python 3.10 and 3.11, the colorsys.rgb_to_hls function divides by zero when given a valid color which is very close to but not exactly pure white.

Python 3.11.1 (main, Feb  8 2023, 15:39:14) [Clang 14.0.0 (clang-1400.0.29.202)] on darwin
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>>> import colorsys
>>> colorsys.rgb_to_hls(1, 1, 0.9999999999999999)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/dfremont/.pyenv/versions/3.11.1/lib/python3.11/colorsys.py", line 86, in rgb_to_hls
    s = rangec / (2.0-sumc)
        ~~~~~~~^~~~~~~~~~~~
ZeroDivisionError: float division by zero

The current code assumes that, given 3 numbers between 0 and 1, if their minimum and maximum are not equal, then the sum of the minimum and maximum must be strictly less than 2. That claim is true for real numbers but not for floating-point arithmetic. The example above fails because 1 + 0.9999999999999999 == 2.0.

Your environment

• CPython versions tested on: 3.8.8 (works); 3.9.13 (works); 3.10.3 (buggy); 3.11.1 (buggy)
• Operating system and architecture: macOS 12.6.6; x86_64

Linked PRs

• gh-106498: Fix an extreme case in colorsys.rgb_to_hls #106530
• gh-106498: Revert incorrect colorsys.rgb_to_hls change #106627
• [3.12] gh-106498: Revert incorrect colorsys.rgb_to_hls change (GH-106627) #106632
• [3.11] gh-106498: Revert incorrect colorsys.rgb_to_hls change (GH-106627) #106633