Fix single-color palette generation

CHANGELOG.md

@@ -5,6 +5,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 ## Unreleased
 
 ### Fixed
+- Fix single-color requests in `n_colors` and `sample_colorscale` raising `ZeroDivisionError` [[#5805](https://github.com/plotly/plotly.py/issues/5805)].
 - Fix `mpl_to_plotly` tick marker mirroring, axis positioning, and trace visibility for twinned and overlaid subplots (such as `twinx` and `twiny`) [[#5310](https://github.com/plotly/plotly.py/pull/5310)], with thanks to @robertoffmoura for the contribution!
 - Fix `mpl_to_plotly` losing matplotlib axis line colors by exporting the color of each axis's displayed spine to the plotly x and y axis `linecolor` [[#5311](https://github.com/plotly/plotly.py/pull/5311)], with thanks to @robertoffmoura for the contribution!
 - Fix concurrent first access to lazily initialized graph object properties, which could raise `ValueError("Invalid value")` [[#5691](https://github.com/plotly/plotly.py/pull/5691)], with thanks to @hb1915 for the contribution!

_plotly_utils/colors/__init__.py

@@ -677,18 +677,20 @@ def n_colors(lowcolor, highcolor, n_colors, colortype="tuple"):
     which form the intermediate colors between lowcolor and highcolor
     from linearly interpolating through RGB space. If colortype is 'rgb'
     the function will return a list of colors in the same form.
+    When one color is requested, the result contains only lowcolor.
     """
     if colortype == "rgb":
         # convert to tuple
         lowcolor = unlabel_rgb(lowcolor)
         highcolor = unlabel_rgb(highcolor)
 
+    intervals = max(n_colors - 1, 1)
     diff_0 = float(highcolor[0] - lowcolor[0])
-    incr_0 = diff_0 / (n_colors - 1)
+    incr_0 = diff_0 / intervals
     diff_1 = float(highcolor[1] - lowcolor[1])
-    incr_1 = diff_1 / (n_colors - 1)
+    incr_1 = diff_1 / intervals
     diff_2 = float(highcolor[2] - lowcolor[2])
-    incr_2 = diff_2 / (n_colors - 1)
+    incr_2 = diff_2 / intervals
     list_of_colors = []
 
     def _constrain_color(c):
@@ -874,6 +876,7 @@ def sample_colorscale(colorscale, samplepoints, low=0.0, high=1.0, colortype="rg
     be spaced equally between the low value (default 0.0) and the high value
     (default 1.0). The output is a list of colors, formatted according to the
     specified colortype.
+    When one point is requested, the color at low is returned.
     """
     from bisect import bisect_left
 
@@ -891,8 +894,9 @@ def sample_colorscale(colorscale, samplepoints, low=0.0, high=1.0, colortype="rg
     colors = validate_colors(colors, colortype="tuple")
 
     if isinstance(samplepoints, int):
+        intervals = max(samplepoints - 1, 1)
         samplepoints = [
-            low + idx / (samplepoints - 1) * (high - low) for idx in range(samplepoints)
+            low + idx / intervals * (high - low) for idx in range(samplepoints)
         ]
     elif isinstance(samplepoints, float):
         samplepoints = [samplepoints]

tests/test_core/test_colors/test_colors.py

@@ -1,5 +1,7 @@
 from unittest import TestCase
 
+import pytest
+
 from plotly.exceptions import PlotlyError
 import plotly.colors as colors
 
@@ -224,3 +226,65 @@ def test_n_colors(self):
         ]
 
         self.assertEqual(generated_colorscale, expected_colorscale)
+
+
+@pytest.mark.parametrize(
+    "colorscale,low,high,colortype,expected",
+    [
+        ("Viridis", 0.0, 1.0, "rgb", ["rgb(68, 1, 84)"]),
+        ("Viridis_r", 0.0, 1.0, "rgb", ["rgb(253, 231, 37)"]),
+        (["#000000", "#ffffff"], 0.0, 1.0, "tuple", [(0.0, 0.0, 0.0)]),
+        (
+            [[0.0, "#ff0000"], [1.0, "#0000ff"]],
+            0.25,
+            0.75,
+            "tuple",
+            [(0.75, 0.0, 0.25)],
+        ),
+        (["#000000", "#ffffff"], 0.5, 0.5, "tuple", [(0.5, 0.5, 0.5)]),
+        (["#000000", "#ffffff"], 1.0, 0.0, "rgb", ["rgb(255, 255, 255)"]),
+    ],
+)
+def test_sample_colorscale_single_color(colorscale, low, high, colortype, expected):
+    assert (
+        colors.sample_colorscale(colorscale, 1, low=low, high=high, colortype=colortype)
+        == expected
+    )
+
+
+@pytest.mark.parametrize(
+    "lowcolor,highcolor,colortype,expected",
+    [
+        ((0, 0, 0), (1, 1, 1), "tuple", [(0.0, 0.0, 0.0)]),
+        ("rgb(255,0,0)", "rgb(0,255,0)", "rgb", ["rgb(255.0, 0.0, 0.0)"]),
+    ],
+)
+def test_n_colors_single_color(lowcolor, highcolor, colortype, expected):
+    assert colors.n_colors(lowcolor, highcolor, 1, colortype=colortype) == expected
+
+
+@pytest.mark.parametrize(
+    "count,expected",
+    [
+        (0, []),
+        (2, [(0.0, 0.0, 0.0), (1.0, 1.0, 1.0)]),
+        (3, [(0.0, 0.0, 0.0), (0.5, 0.5, 0.5), (1.0, 1.0, 1.0)]),
+    ],
+)
+def test_sample_colorscale_empty_and_multiple_colors(count, expected):
+    assert (
+        colors.sample_colorscale(["#000000", "#ffffff"], count, colortype="tuple")
+        == expected
+    )
+
+
+@pytest.mark.parametrize(
+    "count,expected",
+    [
+        (0, []),
+        (2, [(0.0, 0.0, 0.0), (1.0, 1.0, 1.0)]),
+        (3, [(0.0, 0.0, 0.0), (0.5, 0.5, 0.5), (1.0, 1.0, 1.0)]),
+    ],
+)
+def test_n_colors_empty_and_multiple_colors(count, expected):
+    assert colors.n_colors((0, 0, 0), (1, 1, 1), count) == expected