Fix apply method on HoloMap (#3989)

holoviews/core/accessors.py

@@ -81,7 +81,7 @@ def function(object, **kwargs):
                                          method_name)
                 return method(*args, **kwargs)
 
-        applies = isinstance(self._obj, (ViewableElement, HoloMap))
+        applies = isinstance(self._obj, ViewableElement)
         params = {p: val for p, val in kwargs.items()
                   if isinstance(val, param.Parameter)
                   and isinstance(val.owner, param.Parameterized)}
@@ -96,7 +96,7 @@ def function(object, **kwargs):
                        util.is_param_method(function, has_deps=True) or
                        params or dependent_kws)
 
-        if applies and dynamic:
+        if (applies or isinstance(self._obj, HoloMap)) and dynamic:
             return Dynamic(self._obj, operation=function, streams=streams,
                            kwargs=kwargs, link_inputs=link_inputs)
         elif applies:

holoviews/tests/core/testapply.py

@@ -1,7 +1,8 @@
+import numpy as np
 import param
 
 from holoviews.core.spaces import DynamicMap, HoloMap
-from holoviews.element import Curve
+from holoviews.element import Image, Curve
 from holoviews.element.comparison import ComparisonTestCase
 from holoviews.streams import Params, ParamMethod
 
@@ -158,6 +159,12 @@ def test_element_apply_dynamic_with_param_method(self):
         pinst.label = 'Another label'
         self.assertEqual(applied[()], self.element.relabel('Another label!'))
 
+    def test_holomap_apply_with_method(self):
+        hmap = HoloMap({i: Image(np.array([[i, 2], [3, 4]])) for i in range(3)})
+        reduced = hmap.apply.reduce(x=np.min)
+
+        expected = HoloMap({i: Curve([(-0.25, 3), (0.25, i)], 'y', 'z') for i in range(3)})
+        self.assertEqual(reduced, expected)