fix: round-trip serialization of Ellipsis in variadic tuple and Callable types (#12062)

Author: winklemad

haystack/utils/type_serialization.py

@@ -22,6 +22,12 @@
 
 _import_lock = Lock()
 
+# Literal singletons that are not types and must be resolved before the generic/module-path handling in
+# deserialize_type: the None singleton ("None"), NoneType, and Ellipsis ("..."). Ellipsis appears in
+# variadic tuples (tuple[int, ...]) and Callable[..., X]. "Ellipsis" is also accepted so that types
+# serialized by older Haystack versions (which emitted the literal "Ellipsis") can still be read back.
+_SPECIAL_LITERALS: dict[str, Any] = {"None": None, "NoneType": NoneType, "...": ..., "Ellipsis": ...}
+
 
 def _is_union_type(target: Any) -> bool:
     """
@@ -59,6 +65,13 @@ def serialize_type(target: Any) -> str:
     if target is NoneType:
         return "None"
 
+    # `...` (Ellipsis) shows up as an argument in variadic tuples (tuple[int, ...]) and in
+    # Callable[..., X]. It is a singleton constant rather than a type, so serialize it to the same
+    # literal Python uses when rendering these types. Without this it would fall through to
+    # str(Ellipsis) == "Ellipsis", which deserialize_type then rejects as a non-type builtin.
+    if target is Ellipsis:
+        return "..."
+
     args = get_args(target)
 
     if isinstance(target, UnionType):
@@ -171,6 +184,12 @@ def deserialize_type(type_str: str) -> Any:
         If the module is not on the deserialization allowlist, or if the type cannot be
         deserialized due to a missing module or type.
     """
+    # Resolve the non-type literal singletons (see _SPECIAL_LITERALS) up front, before the generic and
+    # module-path handling below: the dots in "..." would otherwise be read as a module path, and the
+    # None/Ellipsis singletons would be refused by the builtin type gate.
+    if type_str in _SPECIAL_LITERALS:
+        return _SPECIAL_LITERALS[type_str]
+
     # Handle PEP 604 union syntax at the top level (e.g., "str | int", "str | None")
     pep604_union_args = _parse_pep604_union_args(type_str)
     if len(pep604_union_args) > 1:
@@ -199,15 +218,9 @@ def deserialize_type(type_str: str) -> Any:
         except ImportError as e:
             raise DeserializationError(str(e)) from e
 
-    # No module prefix, check builtins and typing
-    # Special cases for None / NoneType first: `getattr(builtins, "None")` returns the `None`
-    # singleton (not a type), so these must be handled before the builtins type gate below.
-    if type_str == "None":
-        return None
-    if type_str == "NoneType":
-        return NoneType
-
-    # Then check builtins
+    # No module prefix, check builtins and typing.
+    # (None / NoneType / Ellipsis are handled at the top of this function, before they can reach the
+    # builtin type gate below which would refuse them for not being types.)
     if hasattr(builtins, type_str):
         resolved = getattr(builtins, type_str)
         # This bare-name path never consults the allowlist. A type annotation must resolve to an

releasenotes/notes/fix-variadic-tuple-type-serialization-5fea4406404a597f.yaml

@@ -0,0 +1,10 @@
+---
+fixes:
+  - |
+    Fixed serialization of types that contain ``...`` (``Ellipsis``), such as variadic tuples
+    (``tuple[int, ...]``) and ``Callable[..., X]``. Previously ``serialize_type`` rendered the ``...``
+    as the literal string ``"Ellipsis"``, which ``deserialize_type`` then rejected as a non-type
+    builtin, so a component using such a type (for example ``OutputAdapter(output_type=tuple[int, ...])``)
+    could be serialized but not deserialized, breaking ``Pipeline.loads()`` / ``Pipeline.load()``.
+    These types now round-trip correctly, and pipelines serialized by older versions (which emitted
+    ``"Ellipsis"``) can still be loaded.

test/utils/test_type_serialization.py

@@ -7,7 +7,7 @@
 import typing
 from collections import deque
 from types import UnionType
-from typing import Any, Deque, Dict, FrozenSet, List, Optional, Set, Tuple, Union
+from typing import Any, Callable, Deque, Dict, FrozenSet, List, Optional, Set, Tuple, Union
 
 import pytest
 
@@ -80,13 +80,21 @@
     pytest.param("tuple[dict]", tuple[dict]),
     pytest.param("tuple[float]", tuple[float]),
     pytest.param("tuple[bool]", tuple[bool]),
+    # variadic tuple (the `...` is the Ellipsis singleton, not a type)
+    pytest.param("tuple[int, ...]", tuple[int, ...]),
+    pytest.param("tuple[str, ...]", tuple[str, ...]),
+    pytest.param("tuple[dict[str, int], ...]", tuple[dict[str, int], ...]),
     # typing Tuple
     pytest.param("typing.Tuple", Tuple),
     pytest.param("typing.Tuple[int]", Tuple[int]),
     pytest.param("typing.Tuple[str]", Tuple[str]),
     pytest.param("typing.Tuple[dict]", Tuple[dict]),
     pytest.param("typing.Tuple[float]", Tuple[float]),
     pytest.param("typing.Tuple[bool]", Tuple[bool]),
+    pytest.param("typing.Tuple[int, ...]", Tuple[int, ...]),
+    # callable (the `...` is the Ellipsis singleton, not a type)
+    pytest.param("typing.Callable[..., int]", Callable[..., int]),
+    pytest.param("typing.Callable[..., str]", Callable[..., str]),
     # PEP 604 X | Y
     pytest.param("str | int", str | int),
     pytest.param("int | float", int | float),
@@ -245,6 +253,12 @@ def test_output_type_round_trip_typing_generic_with_nonetype():
         assert deserialize_type(serialize_type(type_)) == type_
 
 
+def test_output_type_deserialization_legacy_ellipsis_literal():
+    # Types serialized by older versions emitted the literal "Ellipsis"; make sure they still load.
+    assert deserialize_type("tuple[int, Ellipsis]") == tuple[int, ...]
+    assert deserialize_type("typing.Callable[Ellipsis, int]") == Callable[..., int]
+
+
 def test_output_type_serialization_haystack_dataclasses():
     # typing
     # Answer