Fix: avoid concealing dialect module exception in _try_load (#4708)

sqlglot/dialects/dialect.py

@@ -129,16 +129,15 @@ def classes(cls):
 
     @classmethod
     def _try_load(cls, key: str | Dialects) -> None:
-        try:
-            if isinstance(key, Dialects):
-                key = key.value
-
-            # This import will lead to a new dialect being loaded, and hence, registered.
-            # We assert that the key is an actual module to avoid blindly importing files.
-            assert key in DIALECT_MODULE_NAMES
+        if isinstance(key, Dialects):
+            key = key.value
+
+        # This import will lead to a new dialect being loaded, and hence, registered.
+        # We check that the key is an actual sqlglot module to avoid blindly importing
+        # files. Custom user dialects need to be imported at the top-level package, in
+        # order for them to be registered as soon as possible.
+        if key in DIALECT_MODULE_NAMES:
             importlib.import_module(f"sqlglot.dialects.{key}")
-        except Exception:
-            pass
 
     @classmethod
     def __getitem__(cls, key: str) -> t.Type[Dialect]:

sqlglot/expressions.py

@@ -637,7 +637,7 @@ def transform(self, fun: t.Callable, *args: t.Any, copy: bool = True, **kwargs)
 
             if not root:
                 root = new_node
-            elif new_node is not node:
+            elif parent and arg_key and new_node is not node:
                 parent.set(arg_key, new_node, index)
 
         assert root

sqlglot/parser.py

@@ -3176,9 +3176,11 @@ def _parse_with(self, skip_with_token: bool = False) -> t.Optional[exp.With]:
         last_comments = None
         expressions = []
         while True:
-            expressions.append(self._parse_cte())
-            if last_comments:
-                expressions[-1].add_comments(last_comments)
+            cte = self._parse_cte()
+            if isinstance(cte, exp.CTE):
+                expressions.append(cte)
+                if last_comments:
+                    cte.add_comments(last_comments)
 
             if not self._match(TokenType.COMMA) and not self._match(TokenType.WITH):
                 break