Fix pydantic discriminated unions handled incorrectly (#667)

Author: mauvilsa

CHANGELOG.rst

@@ -25,6 +25,8 @@ Fixed
 ^^^^^
 - Incorrect instantiation order when instantiation targets share a parent (`#662
   <https://github.com/omni-us/jsonargparse/pull/662>`__).
+- Pydantic discriminated unions handled incorrectly (`#667
+  <https://github.com/omni-us/jsonargparse/pull/667>`__).
 
 
 v4.36.0 (2025-01-17)

jsonargparse/_optionals.py

@@ -5,7 +5,7 @@
 from contextlib import contextmanager
 from importlib.metadata import version
 from importlib.util import find_spec
-from typing import Optional
+from typing import Optional, Union
 
 __all__ = [
     "get_config_read_mode",
@@ -348,10 +348,13 @@ def get_module(value):
 
 
 def is_annotated_validator(typehint: type) -> bool:
+    from ._util import get_typehint_origin
+
     return (
         pydantic_support > 1
         and is_annotated(typehint)
         and any(get_module(m) in {"pydantic", "annotated_types"} for m in typehint.__metadata__)  # type: ignore[attr-defined]
+        and get_typehint_origin(typehint.__origin__) != Union  # type: ignore[attr-defined]
     )
 
 

jsonargparse_tests/test_dataclass_like.py

@@ -2,7 +2,7 @@
 
 import dataclasses
 import json
-from typing import Any, Dict, Generic, List, Optional, Tuple, TypeVar, Union
+from typing import Any, Dict, Generic, List, Literal, Optional, Tuple, TypeVar, Union
 from unittest.mock import patch
 
 import pytest
@@ -713,6 +713,18 @@ def test_pydantic_annotated_nested_annotated_dataclass_with_default_factory(pars
         cfg = parser.parse_args(["--n", "{}"])
         assert cfg.n == Namespace(a1=Namespace(a2=1))
 
+    class PingTask(pydantic.BaseModel):
+        type: Literal["ping"] = "ping"
+        attr: str = ""
+
+    class PongTask(pydantic.BaseModel):
+        type: Literal["pong"] = "pong"
+
+    PingPongTask = annotated[
+        Union[PingTask, PongTask],
+        pydantic.Field(discriminator="type"),
+    ]
+
 
 length = "length"
 if pydantic_support:
@@ -806,6 +818,8 @@ def test_subclass(self, parser):
         parser.add_argument("--model", type=PydanticSubModel, default=PydanticSubModel(p1="a"))
         cfg = parser.parse_args(["--model.p3=0.2"])
         assert Namespace(p1="a", p2=3, p3=0.2) == cfg.model
+        init = parser.instantiate_classes(cfg)
+        assert isinstance(init.model, PydanticSubModel)
 
     def test_field_default_factory(self, parser):
         parser.add_argument("--model", type=PydanticFieldFactory)
@@ -831,6 +845,18 @@ def test_annotated_field(self, parser):
             parser.parse_args(["--model.p1=0"])
         ctx.match("model.p1")
 
+    @pytest.mark.skipif(not (annotated and pydantic_support > 1), reason="Annotated is required")
+    def test_field_union_discriminator_dot_syntax(self, parser):
+        parser.add_argument("--model", type=PingPongTask)
+        cfg = parser.parse_args(["--model.type=pong"])
+        assert cfg.model == Namespace(type="pong")
+        init = parser.instantiate_classes(cfg)
+        assert isinstance(init.model, PongTask)
+        cfg = parser.parse_args(["--model.type=ping", "--model.attr=abc"])
+        assert cfg.model == Namespace(type="ping", attr="abc")
+        init = parser.instantiate_classes(cfg)
+        assert isinstance(init.model, PingTask)
+
     @pytest.mark.parametrize(
         ["valid_value", "invalid_value", "cast", "type_str"],
         [