Python: Create default node tree for new materials and worlds

The motivation is to keep backward compatibility after deprecating
 `material.use_nodes()` and `world.use_nodes`. For example the
following script would behave the same way in 4.5 and 5.0:
```python
mat = bpy.data.materials.new("My new mat")
mat.use_nodes = True
```

Pull Request: https://projects.blender.org/blender/blender/pulls/147052

scripts/addons_core/io_curve_svg/import_svg.py

@@ -207,6 +207,7 @@ def SVGGetMaterial(color, context):
         return materials[color]
 
     mat = bpy.data.materials.new(name='SVGMat')
+    mat.node_tree.nodes.clear()
     node_tree = mat.node_tree
     bsdf = node_tree.nodes.new("ShaderNodeBsdfDiffuse")
     output = node_tree.nodes.new("ShaderNodeOutputMaterial")

scripts/startup/bl_operators/image_as_planes.py

@@ -424,6 +424,7 @@ def create_cycles_material(self, context, img_spec, name):
         material = bpy.data.materials.get((name, None))
     if material is None:
         material = bpy.data.materials.new(name=name)
+        material.node_tree.nodes.clear()
 
     material.surface_render_method = self.render_method
     material.use_backface_culling = self.use_backface_culling

scripts/startup/bl_operators/object_quick_effects.py

@@ -27,6 +27,7 @@ def object_ensure_material(obj, mat_name):
             break
     if mat is None:
         mat = bpy.data.materials.new(mat_name)
+        mat.node_tree.nodes.clear()
         if mat_slot:
             mat_slot.material = mat
         else:

scripts/startup/bl_operators/world.py

@@ -54,6 +54,7 @@ class WORLD_OT_convert_volume_to_mesh(bpy.types.Operator):
         material = bpy.data.materials.new(name)
         mesh.materials.append(material)
         volume_tree = material.node_tree
+        volume_tree.nodes.clear()
         volume_tree.nodes.new("ShaderNodeOutputMaterial")
         volume_output = volume_tree.get_output_node('EEVEE')
 

source/blender/blenkernel/intern/lib_query_test.cc

@@ -127,7 +127,7 @@ class IDSubDataTestData : public WholeIDTestData {
     /* Add a material that contains an embedded nodetree and assign a custom property to one of
      * its nodes. */
     this->material = BKE_material_add(this->bmain, "Material");
-    ED_node_shader_default(this->C, &this->material->id);
+    ED_node_shader_default(this->C, this->bmain, &this->material->id);
 
     BKE_object_material_assign(
         this->bmain, this->object, this->material, this->object->actcol, BKE_MAT_ASSIGN_OBJECT);

source/blender/blenkernel/intern/lib_remap_test.cc

@@ -100,7 +100,7 @@ class MaterialTestData : public TestData {
   MaterialTestData()
   {
     material = BKE_material_add(this->bmain, "Material");
-    ED_node_shader_default(this->C, &this->material->id);
+    ED_node_shader_default(this->C, this->bmain, &this->material->id);
     this->material_nodetree = this->material->nodetree;
   }
 };

source/blender/editors/include/ED_node_c.hh

@@ -78,7 +78,7 @@ bool ED_node_supports_preview(SpaceNode *snode);
  * Assumes nothing being done in ntree yet, sets the default in/out node.
  * Called from shading buttons or header.
  */
-void ED_node_shader_default(const bContext *C, ID *id);
+void ED_node_shader_default(const bContext *C, Main *bmain, ID *id);
 
 /**
  * Initializes an empty compositing node tree with default nodes.

source/blender/editors/render/render_shading.cc

@@ -853,7 +853,7 @@ static wmOperatorStatus new_material_exec(bContext *C, wmOperator * /*op*/)
     else {
       ma = BKE_gpencil_material_add(bmain, name);
     }
-    ED_node_shader_default(C, &ma->id);
+    ED_node_shader_default(C, bmain, &ma->id);
   }
 
   if (prop) {
@@ -982,7 +982,7 @@ static wmOperatorStatus new_world_exec(bContext *C, wmOperator * /*op*/)
   }
   else {
     wo = BKE_world_add(bmain, CTX_DATA_(BLT_I18NCONTEXT_ID_WORLD, "World"));
-    ED_node_shader_default(C, &wo->id);
+    ED_node_shader_default(C, bmain, &wo->id);
   }
 
   /* hook into UI */

source/blender/editors/sculpt_paint/paint_image_proj.cc

@@ -6747,7 +6747,7 @@ static bool proj_paint_add_slot(bContext *C, wmOperator *op)
     bNodeTree *ntree = ma->nodetree;
 
     if (!ntree) {
-      ED_node_shader_default(C, &ma->id);
+      ED_node_shader_default(C, bmain, &ma->id);
       ntree = ma->nodetree;
     }
 

source/blender/editors/space_node/node_edit.cc

@@ -510,10 +510,8 @@ bool ED_node_supports_preview(SpaceNode *snode)
          (USER_EXPERIMENTAL_TEST(&U, use_shader_node_previews) && ED_node_is_shader(snode));
 }
 
-void ED_node_shader_default(const bContext *C, ID *id)
+void ED_node_shader_default(const bContext *C, Main *bmain, ID *id)
 {
-  Main *bmain = CTX_data_main(C);
-
   if (GS(id->name) == ID_MA) {
     /* Materials */
     Object *ob = CTX_data_active_object(C);

source/blender/makesrna/intern/rna_light.cc

@@ -76,9 +76,10 @@ static void rna_Light_draw_update(Main * /*bmain*/, Scene * /*scene*/, PointerRN
 static void rna_Light_use_nodes_update(bContext *C, PointerRNA *ptr)
 {
   Light *la = (Light *)ptr->data;
+  Main *bmain = CTX_data_main(C);
 
   if (la->use_nodes && la->nodetree == nullptr) {
-    ED_node_shader_default(C, &la->id);
+    ED_node_shader_default(C, bmain, &la->id);
   }
 
   rna_Light_update(CTX_data_main(C), CTX_data_scene(C), ptr);

source/blender/makesrna/intern/rna_main_api.cc

@@ -288,8 +288,7 @@ static Material *rna_Main_materials_new(Main *bmain, const char *name)
   Material *material = BKE_material_add(bmain, safe_name);
   id_us_min(&material->id);
 
-  material->nodetree = blender::bke::node_tree_add_tree_embedded(
+  ED_node_shader_default(nullptr, bmain, &material->id);
-      bmain, &material->id, "Material Node Tree", "ShaderNodeTree");
 
   WM_main_add_notifier(NC_ID | NA_ADDED, nullptr);
 
@@ -561,8 +560,7 @@ static World *rna_Main_worlds_new(Main *bmain, const char *name)
   World *world = BKE_world_add(bmain, safe_name);
   id_us_min(&world->id);
 
-  world->nodetree = blender::bke::node_tree_add_tree_embedded(
+  ED_node_shader_default(nullptr, bmain, &world->id);
-      bmain, &world->id, "World Node Tree", "ShaderNodeTree");
 
   WM_main_add_notifier(NC_ID | NA_ADDED, nullptr);
 

source/blender/nodes/intern/node_iterator_tests.cc

@@ -118,7 +118,7 @@ TEST_F(NodeTest, tree_iterator_1_mat)
   TestData context;
 
   Material *material = BKE_material_add(context.bmain, "Material");
-  ED_node_shader_default(context.C, &material->id);
+  ED_node_shader_default(context.C, context.bmain, &material->id);
 
   IteratorResult iter_result = this->get_node_trees(context.bmain);
 
@@ -133,7 +133,7 @@ TEST_F(NodeTest, tree_iterator_scene_no_tree)
   TestData context;
 
   Material *material = BKE_material_add(context.bmain, "Material");
-  ED_node_shader_default(context.C, &material->id);
+  ED_node_shader_default(context.C, context.bmain, &material->id);
 
   BKE_scene_add(context.bmain, "Scene");
 
@@ -151,7 +151,7 @@ TEST_F(NodeTest, tree_iterator_1mat_1scene)
   const char SCENE_NAME[MAX_ID_NAME] = "Scene for testing";
 
   Material *material = BKE_material_add(context.bmain, "Material");
-  ED_node_shader_default(context.C, &material->id);
+  ED_node_shader_default(context.C, context.bmain, &material->id);
 
   Scene *scene = BKE_scene_add(context.bmain, SCENE_NAME);
   /* Embedded compositing trees are deprecated, but still relevant for versioning/backward
@@ -186,7 +186,7 @@ TEST_F(NodeTest, tree_iterator_1mat_3scenes)
   const char MATERIAL_NTREE_NAME[MAX_NAME] = "Shader Nodetree";
 
   Material *material = BKE_material_add(context.bmain, "Material");
-  ED_node_shader_default(context.C, &material->id);
+  ED_node_shader_default(context.C, context.bmain, &material->id);
 
   BKE_scene_add(context.bmain, SCENE_NAME_1);
   /* Note: no node tree for scene 1. */
@@ -227,7 +227,7 @@ TEST_F(NodeTest, tree_iterator_1mat_1scene_2compositing_trees)
   const char MATERIAL_NTREE_NAME[MAX_NAME] = "Shader Nodetree";
 
   Material *material = BKE_material_add(context.bmain, "Material");
-  ED_node_shader_default(context.C, &material->id);
+  ED_node_shader_default(context.C, context.bmain, &material->id);
 
   BKE_scene_add(context.bmain, SCENE_NAME_1);
 

tests/python/bl_usd_export_test.py

@@ -1258,6 +1258,7 @@ class USDExportTest(AbstractUSDTest):
         bpy.ops.wm.open_mainfile(filepath=str(self.testdir / "empty.blend"))
         material = bpy.data.materials.new(name="test_material")
         node_tree = material.node_tree
+        node_tree.nodes.clear()
         bsdf = node_tree.nodes.new("ShaderNodeBsdfPrincipled")
         output = node_tree.nodes.new("ShaderNodeOutputMaterial")
         node_tree.links.new(bsdf.outputs["BSDF"], output.inputs["Surface"])

tools/utils_doc/code_layout_diagram.py

@@ -934,6 +934,7 @@ def render_output(scene, bounds, filepath):
     scene.render.filepath = filepath
 
     world = bpy.data.worlds.new(name_gen)
+    world.node_tree.nodes.clear()
     output = world.node_tree.nodes.new("ShaderNodeOutputWorld")
     background = world.node_tree.nodes.new("ShaderNodeBackground")
     world.node_tree.links.new(output.outputs["Surface"], background.outputs["Surface"])