[3.12] gh-106498: Revert incorrect colorsys.rgb_to_hls change (GH-106627) (#106632)

miss-islington · terryjreedy · web-flow · commit c594e25cd76b · 2023-07-11T15:47:15.000Z
gh-106498: Revert incorrect colorsys.rgb_to_hls change (GH-106627) gh-86618 assumed a-b-c = a-(b+c) = a-d where d = b+d. For floats 2.0, 1.0, and 0.9999999999999999, this assumption is false. The net change of 1.1102230246251565e-16 to 0.0 results in division by 0. Revert the replacement. Add test. (cherry picked from commit a2d54d4) Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>
diff --git a/Lib/colorsys.py b/Lib/colorsys.py
@@ -83,7 +83,7 @@ def rgb_to_hls(r, g, b):
     if l <= 0.5:
         s = rangec / sumc
     else:
-        s = rangec / (2.0-sumc)
+        s = rangec / (2.0-maxc-minc)  # Not always 2.0-sumc: gh-106498.
     rc = (maxc-r) / rangec
     gc = (maxc-g) / rangec
     bc = (maxc-b) / rangec
diff --git a/Lib/test/test_colorsys.py b/Lib/test/test_colorsys.py
@@ -69,6 +69,16 @@ def test_hls_values(self):
             self.assertTripleEqual(hls, colorsys.rgb_to_hls(*rgb))
             self.assertTripleEqual(rgb, colorsys.hls_to_rgb(*hls))
 
+    def test_hls_nearwhite(self):  # gh-106498
+        values = (
+            # rgb, hls: these do not work in reverse
+            ((0.9999999999999999, 1, 1), (0.5, 1.0, 1.0)),
+            ((1, 0.9999999999999999, 0.9999999999999999), (0.0, 1.0, 1.0)),
+        )
+        for rgb, hls in values:
+            self.assertTripleEqual(hls, colorsys.rgb_to_hls(*rgb))
+            self.assertTripleEqual((1.0, 1.0, 1.0), colorsys.hls_to_rgb(*hls))
+
     def test_yiq_roundtrip(self):
         for r in frange(0.0, 1.0, 0.2):
             for g in frange(0.0, 1.0, 0.2):
diff --git a/Misc/NEWS.d/next/Library/2023-07-11-09-25-40.gh-issue-106530.VgXrMx.rst b/Misc/NEWS.d/next/Library/2023-07-11-09-25-40.gh-issue-106530.VgXrMx.rst
@@ -0,0 +1,2 @@
+Revert a change to :func:`colorsys.rgb_to_hls` that caused division by zero
+for certain almost-white inputs.  Patch by Terry Jan Reedy.

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,2 @@`
	`1`	+Revert a change to :func:`colorsys.rgb_to_hls` that caused division by zero
	`2`	`+for certain almost-white inputs. Patch by Terry Jan Reedy.`