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Fix serialization of unions with nested PEP 695 type aliases #331

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Fix serialization of unions with nested PEP 695 type aliases #331

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006c237
Fix serialization of unions with nested PEP 695 type aliases. Fixes #330
PraneelBhatia Jun 5, 2026
829f7c8
Guard resolve_type_alias_type against recursive aliases and wrong sub…
PraneelBhatia Jun 6, 2026
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15 changes: 15 additions & 0 deletions mashumaro/core/meta/helpers.py
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Original file line number Diff line number Diff line change
Expand Up @@ -43,6 +43,7 @@
"is_dataclass_dict_mixin_subclass",
"collect_type_params",
"resolve_type_params",
"resolve_type_alias_type",
"substitute_type_params",
"get_generic_name",
"get_name_error_name",
Expand Down Expand Up @@ -643,6 +644,20 @@ def substitute_type_params(typ: Type, substitutions: dict[Type, Type]) -> Type:
return typ


def resolve_type_alias_type(typ: Type) -> Type:
while True:
if is_type_alias_type(typ):
typ = typ.__value__
elif is_type_alias_type(get_type_origin(typ)):
origin = get_type_origin(typ)
type_params = getattr(origin, "__type_params__", ())
args = get_args(typ)
param_map = dict(zip(type_params, args))
typ = substitute_type_params(origin.__value__, param_map)
Comment on lines +661 to +683

@PraneelBhatia PraneelBhatia Jun 6, 2026

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Replaced the silent truncation in 829f7c8: missing trailing parameters now fall back to their PEP 696 defaults when defined (TypeAliasType subscription doesn't auto-fill defaults, so Pair[int] with TypeVar("V", default=str) previously leaked an unsubstituted ~V into the resolved type), and a genuine mismatch raises TypeError: Too few/Too many arguments for ...; actual N, expected M in the same format as the existing generics arity check. The check is skipped for TypeVarTuple/ParamSpec parameters since their subscription arity is flexible. One behavior note: invalid annotations like Pair[int] without a default used to serialize with a half-substituted value and now fail at class creation — happy to relax that if you'd rather keep them lenient.

else:
return typ
Comment on lines +647 to +685

@PraneelBhatia PraneelBhatia Jun 6, 2026

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Added cycle protection in 829f7c8: the resolver tracks seen values and bounds the number of resolution steps, raising TypeError: Cannot resolve recursive type alias ... for both exact cycles (type A = B; type B = A) and non-repeating chains (type G[T] = G[list[T]]). The seen set tracks values rather than id()s because each substitution builds a new (equal) object for parameterized cycles; is_hashable() guards unhashable values and the step bound is the backstop. Raising a targeted error seemed more useful than returning the unresolved alias, which would just push the non-termination into the packer/unpacker dispatch. Tests cover both shapes, including at dataclass creation time.



def get_name_error_name(e: NameError) -> str:
return e.name # type: ignore

Expand Down
17 changes: 8 additions & 9 deletions mashumaro/core/meta/types/pack.py
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Original file line number Diff line number Diff line change
Expand Up @@ -47,6 +47,7 @@
is_union,
is_unpack,
not_none_type_arg,
resolve_type_alias_type,
resolve_type_params,
substitute_type_params,
type_name,
Expand Down Expand Up @@ -325,6 +326,7 @@ def pack_union(
packers: list[str] = []
packer_arg_types: dict[str, list[type]] = {}
for type_arg in args:
type_arg = resolve_type_alias_type(type_arg)
packer = PackerRegistry.get(
spec.copy(type=type_arg, expression="value", owner=spec.type)
)
Expand Down Expand Up @@ -549,15 +551,12 @@ def pack_special_typing_primitive(spec: ValueSpec) -> Expression | None:
evaluated = evaluate_forward_ref(spec.type)
if evaluated is not None:
return PackerRegistry.get(spec.copy(type=evaluated))
elif is_type_alias_type(spec.type):
return PackerRegistry.get(spec.copy(type=spec.type.__value__))
elif is_type_alias_type(get_type_origin(spec.type)):
origin = get_type_origin(spec.type)
type_params = getattr(origin, "__type_params__", ())
args = get_args(spec.type)
param_map = dict(zip(type_params, args))
resolved = substitute_type_params(origin.__value__, param_map)
return PackerRegistry.get(spec.copy(type=resolved))
elif is_type_alias_type(spec.type) or is_type_alias_type(
get_type_origin(spec.type)
):
return PackerRegistry.get(
spec.copy(type=resolve_type_alias_type(spec.type))
)
elif is_readonly(spec.type):
return PackerRegistry.get(spec.copy(type=get_args(spec.type)[0]))
raise UnserializableDataError(
Expand Down
17 changes: 8 additions & 9 deletions mashumaro/core/meta/types/unpack.py
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Original file line number Diff line number Diff line change
Expand Up @@ -60,6 +60,7 @@
is_unpack,
iter_all_subclasses,
not_none_type_arg,
resolve_type_alias_type,
resolve_type_params,
substitute_type_params,
type_name,
Expand Down Expand Up @@ -195,6 +196,7 @@ def _add_body(self, spec: ValueSpec, lines: CodeLines) -> None:
type_arg_unpackers = []
type_match_statements = 0
for type_arg in self.union_args:
type_arg = resolve_type_alias_type(type_arg)
unpacker = UnpackerRegistry.get(
spec.copy(type=type_arg, expression="value", owner=spec.type)
)
Expand Down Expand Up @@ -874,15 +876,12 @@ def unpack_special_typing_primitive(spec: ValueSpec) -> Expression | None:
evaluated = evaluate_forward_ref(spec.type)
if evaluated is not None:
return UnpackerRegistry.get(spec.copy(type=evaluated))
elif is_type_alias_type(spec.type):
return UnpackerRegistry.get(spec.copy(type=spec.type.__value__))
elif is_type_alias_type(get_type_origin(spec.type)):
origin = get_type_origin(spec.type)
type_params = getattr(origin, "__type_params__", ())
args = get_args(spec.type)
param_map = dict(zip(type_params, args))
resolved = substitute_type_params(origin.__value__, param_map)
return UnpackerRegistry.get(spec.copy(type=resolved))
elif is_type_alias_type(spec.type) or is_type_alias_type(
get_type_origin(spec.type)
):
return UnpackerRegistry.get(
spec.copy(type=resolve_type_alias_type(spec.type))
)
elif is_readonly(spec.type):
return UnpackerRegistry.get(spec.copy(type=get_args(spec.type)[0]))
raise UnserializableDataError(
Expand Down
71 changes: 71 additions & 0 deletions tests/test_pep_695.py
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Expand Up @@ -5,6 +5,7 @@

from mashumaro import DataClassDictMixin
from mashumaro.codecs import BasicDecoder, BasicEncoder
from mashumaro.config import TO_DICT_ADD_OMIT_NONE_FLAG, BaseConfig
from mashumaro.exceptions import MissingField
from tests.entities_pep_695 import (
Boxed,
Expand Down Expand Up @@ -144,3 +145,73 @@ def test_recursive_generic_alias_with_serializable_type():
DataClassWithRecursiveGenericAlias.from_dict({"x": ("hello", 7)})
== obj2
)


def test_type_alias_type_nested_in_union():
# https://github.com/Fatal1ty/mashumaro/issues/330
type UniqueIdentifier = str
type UniqueIdentifierList = list[UniqueIdentifier]
type UniqueIdentifierOrList = UniqueIdentifier | UniqueIdentifierList

@dataclass
class MyClass(DataClassDictMixin):
x: UniqueIdentifierOrList | None = None

assert MyClass(x="a").to_dict() == {"x": "a"}
assert MyClass(x=["a", "b"]).to_dict() == {"x": ["a", "b"]}
assert MyClass().to_dict() == {"x": None}
assert MyClass.from_dict({"x": "a"}) == MyClass(x="a")
assert MyClass.from_dict({"x": ["a", "b"]}) == MyClass(x=["a", "b"])
assert MyClass.from_dict({"x": None}) == MyClass()


@pytest.mark.parametrize("lazy", [False, True])
def test_type_alias_type_nested_in_union_with_omit_none_flag(lazy):
type UniqueIdentifier = str
type UniqueIdentifierList = list[UniqueIdentifier]
type UniqueIdentifierOrList = UniqueIdentifier | UniqueIdentifierList

@dataclass
class MyClass(DataClassDictMixin):
x: UniqueIdentifierOrList | None = None

class Config(BaseConfig):
code_generation_options = [TO_DICT_ADD_OMIT_NONE_FLAG]
lazy_compilation = lazy

assert MyClass(x="a").to_dict(omit_none=True) == {"x": "a"}
assert MyClass(x=["a", "b"]).to_dict(omit_none=True) == {"x": ["a", "b"]}
assert MyClass().to_dict(omit_none=True) == {}
assert MyClass.from_dict({"x": ["a", "b"]}) == MyClass(x=["a", "b"])


def test_type_alias_type_nested_in_union_with_codecs():
type UniqueIdentifier = str
type UniqueIdentifierList = list[UniqueIdentifier]
type UniqueIdentifierOrList = UniqueIdentifier | UniqueIdentifierList

decoder = BasicDecoder(UniqueIdentifierOrList)
encoder = BasicEncoder(UniqueIdentifierOrList)

assert decoder.decode("a") == "a"
assert decoder.decode(["a", "b"]) == ["a", "b"]
assert encoder.encode("a") == "a"
assert encoder.encode(["a", "b"]) == ["a", "b"]


def test_parameterized_type_alias_type_in_union():
type Identity[T] = T
type ListOf[T] = list[T]

@dataclass
class MyClass(DataClassDictMixin):
x: date | ListOf[int]
y: int | Identity[str]

obj1 = MyClass(x=date(2024, 4, 15), y=42)
assert obj1.to_dict() == {"x": "2024-04-15", "y": 42}
assert MyClass.from_dict({"x": "2024-04-15", "y": 42}) == obj1

obj2 = MyClass(x=[1, 2, 3], y="a")
assert obj2.to_dict() == {"x": [1, 2, 3], "y": "a"}
assert MyClass.from_dict({"x": [1, 2, 3], "y": "a"}) == obj2