gh-106498: Fix an extreme case in colorsys.rgb_to_hls

Lib/colorsys.py

@@ -78,7 +78,13 @@ def rgb_to_hls(r, g, b):
     sumc = (maxc+minc)
     rangec = (maxc-minc)
     l = sumc/2.0
-    if minc == maxc:
+    # gh-106498
+    # Due to a degree of float precision error, it's possible that maxc != minc
+    # but maxc + minc == 2.0. Which will make (2.0 - sumc) zero, causing the
+    # ZeroDivisionError.
+    # e.g. colorsys.rgb_to_hls(1, 1, 0.9999999999999999)
+    # In this case, it's an extreme near white color so we can just return white
+    if minc == maxc or sumc == 2.0:
         return 0.0, l, 0.0
     if l <= 0.5:
         s = rangec / sumc

Lib/test/test_colorsys.py

@@ -69,6 +69,12 @@ def test_hls_values(self):
             self.assertTripleEqual(hls, colorsys.rgb_to_hls(*rgb))
             self.assertTripleEqual(rgb, colorsys.hls_to_rgb(*hls))
 
+    def test_hls_corner_case(self):
+        try:
+            colorsys.rgb_to_hls(1, 1, 0.9999999999999999)
+        except Exception:
+            self.fail("rgb_to_hls(1, 1, 0.9999999999999999) raised an exception!")
+
     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):

Misc/NEWS.d/next/Library/2023-07-07-19-46-27.gh-issue-106498.ijJ-eH.rst

@@ -0,0 +1 @@
+Fixed :func:`colorsys.rgb_to_hls` under an extreme input.