Handle the empty string as a group name (#5131)

holoviews/core/util.py

@@ -382,6 +382,8 @@ def tree_attribute(identifier):
     These custom attributes start with a capitalized character when
     applicable (not applicable to underscore or certain unicode characters)
     """
+    if identifier == '':
+        return True
     if identifier[0].upper().isupper() is False and identifier[0] != '_':
         return True
     else:
@@ -1826,7 +1828,10 @@ def capitalize(string):
     """
     Capitalizes the first letter of a string.
     """
-    return string[0].upper() + string[1:]
+    if string:
+        return string[0].upper() + string[1:]
+    else:
+        return string
 
 
 def get_path(item):

holoviews/tests/core/testoptions.py

@@ -4,7 +4,8 @@
 from unittest import SkipTest
 
 import numpy as np
-from holoviews import Store, Histogram, Image, Curve, Points, DynamicMap, opts
+
+from holoviews import Store, Histogram, Image, Curve, Points, DynamicMap, opts, util
 from holoviews.core.options import (
     OptionError, Cycle, Options, OptionTree, StoreOptions, options_policy
 )
@@ -309,6 +310,12 @@ def initialize_option_tree(self):
         options.Image = Options('style', cmap='hot', interpolation='nearest')
         return options
 
+    def test_empty_group(self):
+        "Test to prevent regression of issue fixed in #5131"
+        ls = np.linspace(0, 10, 200)
+        xx, yy = np.meshgrid(ls, ls)
+        util.render(Image(np.sin(xx)*np.cos(yy), group="").opts(cmap="greys"))
+
     def test_merge_keywords(self):
         options = self.initialize_option_tree()
         options.Image = Options('style', clims=(0, 0.5))