FIX: avoid error when prop_cycle has no color (#1992)

* Rework get_color_cycle

* No longer fail when cycler has no color (fixes #1977)

* Improves docstring

* Adds tests

* default to dark gray

* remove redundant test

(cherry picked from commit 34f02d8)

seaborn/tests/test_palettes.py

@@ -5,7 +5,6 @@
 import pytest
 import nose.tools as nt
 import numpy.testing as npt
-import matplotlib.pyplot as plt
 
 from .. import palettes, utils, rcmod
 from ..external import husl
@@ -354,11 +353,3 @@ def test_preserved_palette_length(self):
         pal_in = palettes.color_palette("Set1", 10)
         pal_out = palettes.color_palette(pal_in)
         nt.assert_equal(pal_in, pal_out)
-
-    def test_get_color_cycle(self):
-
-        colors = [(1., 0., 0.), (0, 1., 0.)]
-        prop_cycle = plt.cycler(color=colors)
-        with plt.rc_context({"axes.prop_cycle": prop_cycle}):
-            result = utils.get_color_cycle()
-            assert result == colors

seaborn/tests/test_utils.py

@@ -5,6 +5,7 @@
 import pandas as pd
 import matplotlib as mpl
 import matplotlib.pyplot as plt
+from cycler import cycler
 import pytest
 import nose
 import nose.tools as nt
@@ -374,6 +375,23 @@ def test_locator_to_legend_entries():
         assert str_levels == ['1e-07', '1e-05', '1e-03', '1e-01', '10']
 
 
+@pytest.mark.parametrize(
+    "cycler,result",
+    [
+        (cycler(color=["y"]), ["y"]),
+        (cycler(color=["k"]), ["k"]),
+        (cycler(color=["k", "y"]), ["k", "y"]),
+        (cycler(color=["y", "k"]), ["y", "k"]),
+        (cycler(color=["b", "r"]), ["b", "r"]),
+        (cycler(color=["r", "b"]), ["r", "b"]),
+        (cycler(lw=[1, 2]), [".15"]),  # no color in cycle
+    ],
+)
+def test_get_color_cycle(cycler, result):
+    with mpl.rc_context(rc={"axes.prop_cycle": cycler}):
+        assert utils.get_color_cycle() == result
+
+
 def check_load_dataset(name):
     ds = load_dataset(name, cache=False)
     assert(isinstance(ds, pd.DataFrame))

seaborn/utils.py

@@ -555,8 +555,20 @@ def get_view_interval(self):
 
 
 def get_color_cycle():
-    """Return the list of colors in the current matplotlib color cycle."""
-    return [x['color'] for x in mpl.rcParams['axes.prop_cycle']]
+    """Return the list of colors in the current matplotlib color cycle
+
+    Parameters
+    ----------
+    None
+
+    Returns
+    -------
+    colors : list
+        List of matplotlib colors in the current cycle, or dark gray if
+        the current color cycle is empty.
+    """
+    cycler = mpl.rcParams['axes.prop_cycle']
+    return cycler.by_key()['color'] if 'color' in cycler.keys else [".15"]
 
 
 def relative_luminance(color):