Stop disabling declaring in _create_subnode

_create_subnode temporarily disables _context.declaring to avoid
triggering the altered `__getattribute__` behaviour in
TemplateInvocatorSubNodeMixin in two situations:

 - Pydantic's implementation of `!=`, which is attempting to read field
   values, and will raise a validation error due to the altered field
   names being of incorrect type.
 - Accessing `__class__` on a Pydantic subnode object with no build_obj
   set.

Instead, fix this by:

 - Using `is not` instead of `!=`, which is correct where we use it,
   and also faster.
 - Fixing `__getattribute__` to no longer fail if used on an object with
   no build_obj set.

Signed-off-by: Alice Purcell <[email protected]>

src/hera/workflows/_meta_mixins.py

@@ -539,8 +539,7 @@ def _create_subnode(
         assert _context.pieces
 
         template_ref = None
-        _context.declaring = False
-        if _context.pieces[0] != self and isinstance(self, WorkflowTemplate):
+        if _context.pieces[0] is not self and isinstance(self, WorkflowTemplate):
             # Using None for cluster_scope means it won't appear in the YAML spec (saving some bytes),
             # as cluster_scope=False is the default value
             template_ref = TemplateRef(
@@ -571,7 +570,6 @@ def _create_subnode(
             _context.pieces[-1]._current_task_depends.clear()
 
         subnode._build_obj = HeraBuildObj(subnode._subtype, output_class)
-        _context.declaring = True
         return subnode
 
     @_add_type_hints(Script)  # type: ignore

src/hera/workflows/_mixins.py

@@ -715,11 +715,13 @@ class TemplateInvocatorSubNodeMixin(BaseMixin):
     _build_obj: Optional[HeraBuildObj] = PrivateAttr(None)
 
     def __getattribute__(self, name: str) -> Any:
-        if _context.declaring:
+        try:
             # Use object's __getattribute__ to avoid infinite recursion
             build_obj = object.__getattribute__(self, "_build_obj")
-            assert build_obj  # Assertions to fix type checking
+        except AttributeError:
+            build_obj = None
 
+        if build_obj and _context.declaring:
             fields = get_fields(build_obj.output_class)
             annotations = get_field_annotations(build_obj.output_class)
             if name in fields: