Use a dictionary function map

This reduces the complexity even more.

Author: tomschr

src/docbuild/config/merge.py

@@ -28,8 +28,16 @@ def deep_merge(*dcts: Mapping[str, Any]) -> dict[str, Any]:
     if not dcts:
         return {}
 
+    # Initial deep copy of the base dictionary
     result = deepcopy(dict(dcts[0]))
 
+    # Handlers for specific types (excluding Mapping which needs special handling)
+    funcmap = {
+        (list, list): lambda e, v: e + deepcopy(v),
+        (tuple, tuple): lambda e, v: e + deepcopy(v),
+        (set, set): lambda e, v: e | deepcopy(v),
+    }
+
     for src in dcts[1:]:
         stack: list[tuple[dict[str, Any], Mapping[str, Any]]] = [(result, src)]
 
@@ -43,26 +51,19 @@ def deep_merge(*dcts: Mapping[str, Any]) -> dict[str, Any]:
 
                 existing = dest[key]
 
-                # Mapping → merge deeper
+                # Special handling for Mappings (recursive merge)
                 if isinstance(existing, Mapping) and isinstance(value, Mapping):
-                    # Ensure we are working with a dict for the merge target
-                    # This avoids nested specialized types if they support item assignment
                     if type(existing) is not dict:
                         existing = dict(existing)
                         dest[key] = existing
                     stack.append((existing, value))
+                    continue
 
-                # Lists / tuples → concatenate
-                elif (isinstance(existing, list) and isinstance(value, list)) or (
-                    isinstance(existing, tuple) and isinstance(value, tuple)
-                ):
-                    dest[key] = existing + deepcopy(value)  # type: ignore[operator]
-
-                # Sets → union
-                elif isinstance(existing, set) and isinstance(value, set):
-                    dest[key] = existing | deepcopy(value)
-
-                # Fallback → overwrite
+                # Use funcmap for other types
+                # Using get() on dict with tuple key avoids the loop
+                handler = funcmap.get((type(existing), type(value)))
+                if handler:
+                    dest[key] = handler(existing, value)
                 else:
                     dest[key] = deepcopy(value)