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

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Merged
merged 2 commits into from
Oct 7, 2025
Merged

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

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523fb9e
Make `type.__new__()` raise clear errors instead of returning `None`
emmanuel-ferdman Oct 7, 2025
1688ef9
Make `type.__new__()` raise clear errors instead of returning `None`
emmanuel-ferdman Oct 7, 2025
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34 changes: 26 additions & 8 deletions astroid/bases.py
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Original file line number Diff line number Diff line change
Expand Up @@ -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:
Expand All @@ -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:
Expand All @@ -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:
Expand All @@ -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:
Expand Down Expand Up @@ -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,))
Expand Down