Make type.__new__() raise clear errors instead of returning None (#2851)

Signed-off-by: Emmanuel Ferdman <[email protected]>

Author: emmanuel-ferdman

ChangeLog

@@ -13,6 +13,7 @@ What's New in astroid 4.0.1?
 ============================
 Release date: TBA
 
+* Make `type.__new__()` raise clear errors instead of returning `None`
 
 
 What's New in astroid 4.0.0?

astroid/bases.py

@@ -571,10 +571,14 @@ def _infer_type_new_call(
             raise InferenceError(context=context) from e
         if not isinstance(mcs, nodes.ClassDef):
             # Not a valid first argument.
-            return None
+            raise InferenceError(
+                "type.__new__() requires a class for metaclass", context=context
+            )
         if not mcs.is_subtype_of("builtins.type"):
             # Not a valid metaclass.
-            return None
+            raise InferenceError(
+                "type.__new__() metaclass must be a subclass of type", context=context
+            )
 
         # Verify the name
         try:
@@ -583,10 +587,14 @@ def _infer_type_new_call(
             raise InferenceError(context=context) from e
         if not isinstance(name, nodes.Const):
             # Not a valid name, needs to be a const.
-            return None
+            raise InferenceError(
+                "type.__new__() requires a constant for name", context=context
+            )
         if not isinstance(name.value, str):
             # Needs to be a string.
-            return None
+            raise InferenceError(
+                "type.__new__() requires a string for name", context=context
+            )
 
         # Verify the bases
         try:
@@ -595,14 +603,18 @@ def _infer_type_new_call(
             raise InferenceError(context=context) from e
         if not isinstance(bases, nodes.Tuple):
             # Needs to be a tuple.
-            return None
+            raise InferenceError(
+                "type.__new__() requires a tuple for bases", context=context
+            )
         try:
             inferred_bases = [next(elt.infer(context=context)) for elt in bases.elts]
         except StopIteration as e:
             raise InferenceError(context=context) from e
         if any(not isinstance(base, nodes.ClassDef) for base in inferred_bases):
             # All the bases needs to be Classes
-            return None
+            raise InferenceError(
+                "type.__new__() requires classes for bases", context=context
+            )
 
         # Verify the attributes.
         try:
@@ -611,7 +623,9 @@ def _infer_type_new_call(
             raise InferenceError(context=context) from e
         if not isinstance(attrs, nodes.Dict):
             # Needs to be a dictionary.
-            return None
+            raise InferenceError(
+                "type.__new__() requires a dict for attrs", context=context
+            )
         cls_locals: dict[str, list[InferenceResult]] = collections.defaultdict(list)
         for key, value in attrs.items:
             try:
@@ -664,9 +678,13 @@ def infer_call_result(
             and self.bound.name == "type"
             and self.name == "__new__"
             and isinstance(caller, nodes.Call)
-            and len(caller.args) == 4
         ):
             # Check if we have a ``type.__new__(mcs, name, bases, attrs)`` call.
+            if len(caller.args) != 4:
+                raise InferenceError(
+                    f"type.__new__() requires 4 arguments, got {len(caller.args)}",
+                    context=context,
+                )
             new_cls = self._infer_type_new_call(caller, context)
             if new_cls:
                 return iter((new_cls,))