Support extending list from override

hydra/_internal/config_loader_impl.py

@@ -7,7 +7,15 @@
 from textwrap import dedent
 from typing import Any, List, MutableSequence, Optional, Tuple
 
-from omegaconf import Container, DictConfig, OmegaConf, flag_override, open_dict
+from omegaconf import (
+    Container,
+    DictConfig,
+    ListConfig,
+    Node,
+    OmegaConf,
+    flag_override,
+    open_dict,
+)
 from omegaconf.errors import (
     ConfigAttributeError,
     ConfigKeyError,
@@ -475,7 +483,8 @@ def _load_single_config(
                     ):
                         hydra = config.pop("hydra")
 
-                    merged = OmegaConf.merge(schema.config, config)
+                    override_cfg = _convert_list_extend_overrides(schema.config, config)
+                    merged = OmegaConf.merge(schema.config, override_cfg)  # TODO
                     assert isinstance(merged, DictConfig)
 
                     if hydra is not None:
@@ -548,7 +557,8 @@ def _compose_config_from_defaults_list(
             for default in defaults:
                 loaded = self._load_single_config(default=default, repo=repo)
                 try:
-                    cfg.merge_with(loaded.config)
+                    override_config = _convert_list_extend_overrides(cfg, loaded.config)
+                    cfg.merge_with(override_config)  # TODO
                 except OmegaConfBaseException as e:
                     raise ConfigCompositionException(
                         f"In '{default.config_path}': {type(e).__name__} raised while"
@@ -594,6 +604,38 @@ def compute_defaults_list(
         return defaults_list
 
 
+def _convert_list_extend_overrides(base_cfg: Node, override_cfg: Node) -> Node:
+    if isinstance(base_cfg, ListConfig) and isinstance(override_cfg, DictConfig):
+        if "_extend_" in override_cfg and isinstance(override_cfg._extend_, ListConfig):
+            new_list = copy.copy(override_cfg._extend_)
+            new_list.extend(base_cfg)
+            return new_list
+        # This is invalid, we don't bother trying to handle it
+        return override_cfg
+
+    if (
+        isinstance(base_cfg, DictConfig)
+        and isinstance(override_cfg, DictConfig)
+        and not override_cfg._is_missing()
+        and not override_cfg._is_none()
+    ):
+        items_to_change = {}
+        for key in override_cfg.keys():
+            value = override_cfg._get_node(key)
+            if not value:
+                continue
+            base_value = base_cfg._get_node(key, False) or ListConfig([])
+            new_value = _convert_list_extend_overrides(base_value, value)
+            if not new_value is value:
+                items_to_change[key] = new_value
+        if len(items_to_change) > 0:
+            new_dict = copy.copy(override_cfg)
+            new_dict.update(items_to_change)
+            return new_dict
+
+    return override_cfg
+
+
 def get_overrides_dirname(
     overrides: List[Override], exclude_keys: List[str], item_sep: str, kv_sep: str
 ) -> str: