Allow options for non-enabled backends (#3587)

holoviews/core/options.py

@@ -1117,6 +1117,13 @@ def apply(self, value, input_ranges, backend=None):
         return transformed
 
 
+class _OptionsDict(dict):
+    def __missing__(self, backend):
+        options = OptionTree([], groups=Options._option_groups, backend=backend)
+        self[backend] = options
+        return options
+
+
 class Store:
     """
     The Store is what links up HoloViews objects to their
@@ -1143,7 +1150,7 @@ class Store:
 
     # Once register_plotting_classes is called, this OptionTree is
     # populated for the given backend.
-    _options = {}
+    _options = _OptionsDict()
 
     # Weakrefs to record objects per id
     _weakrefs = {}
@@ -1156,7 +1163,7 @@ class Store:
     option_setters = []
 
     # A dictionary of custom OptionTree by custom object id by backend
-    _custom_options = {'matplotlib': {}}
+    _custom_options = defaultdict(dict)
     load_counter_offset = None
     save_option_state = False
 

holoviews/util/__init__.py

@@ -338,16 +338,16 @@ def _expand_options(cls, options, backend=None):
 
         try:
             backend_options = Store.options(backend=backend or current_backend)
-        except KeyError as e:
-            raise Exception(f'The {e} backend is not loaded. Please load the backend using hv.extension.') from None
+        except KeyError:
+            return {}
         expanded = {}
         if isinstance(options, list):
             options = merge_options_to_dict(options)
 
         for objspec, option_values in options.items():
             objtype = objspec.split('.')[0]
             if objtype not in backend_options:
-                raise ValueError(f'{objtype} type not found, could not apply options.')
+                continue
             obj_options = backend_options[objtype]
             expanded[objspec] = {g: {} for g in obj_options.groups}
             for opt, value in option_values.items():
@@ -450,8 +450,9 @@ def builder(cls, spec=None, **kws):
             backend = kws.get('backend', None)
             keys = set(kws.keys())
             if backend:
-                allowed_kws = cls._element_keywords(backend,
-                                                    elements=[element])[element]
+                keywords = cls._element_keywords(backend,
+                                                    elements=[element])
+                allowed_kws = keywords.get(element, keys)
                 invalid = keys - set(allowed_kws)
             else:
                 mismatched = {}
@@ -506,11 +507,15 @@ def _element_keywords(cls, backend, elements=None):
             if '.' in element: continue
             element = element if isinstance(element, tuple) else (element,)
             element_keywords = []
-            options = backend_options['.'.join(element)]
-            for group in Options._option_groups:
-                element_keywords.extend(options[group].allowed_keywords)
+            try:
+                options = backend_options['.'.join(element)]
+            except KeyError:
+                pass
+            else:
+                for group in Options._option_groups:
+                    element_keywords.extend(options[group].allowed_keywords)
 
-            mapping[element[0]] = element_keywords
+                mapping[element[0]] = element_keywords
         return mapping