Change the behavior of field_is_complex to be more similar to pydantic v1

pydantic_settings/sources.py

@@ -4,9 +4,10 @@
 import os
 import warnings
 from abc import ABC, abstractmethod
+from collections import deque
 from dataclasses import is_dataclass
 from pathlib import Path
-from typing import TYPE_CHECKING, Any, Dict, List, Mapping, Optional, Tuple, Type, Union
+from typing import TYPE_CHECKING, Any, Dict, List, Mapping, Optional, Sequence, Tuple, Type, Union
 
 from pydantic import BaseModel
 from pydantic._internal._typing_extra import origin_is_union
@@ -53,10 +54,16 @@ def get_field_value(self, field: FieldInfo, field_name: str) -> Tuple[Any, str,
         pass
 
     def field_is_complex(self, field: FieldInfo) -> bool:
-        def _annotation_is_complex(annotation: type[Any] | None) -> bool:
-            return lenient_issubclass(annotation, (BaseModel, list, set, frozenset, dict)) or is_dataclass(annotation)
+        """
+        Checks whether a field is complex, in which case it will attempt to be parsed as JSON.
+
+        Args:
+            field (FieldInfo): The field.
 
-        return _annotation_is_complex(field.annotation) or _annotation_is_complex(get_origin(field.annotation))
+        Returns:
+            bool: Whether the field is complex.
+        """
+        return _annotation_is_complex(field.annotation)
 
     def prepare_field_value(self, field_name: str, field: FieldInfo, value: Any, value_is_complex: bool) -> Any:
         """
@@ -433,3 +440,22 @@ def find_case_path(dir_path: Path, file_name: str, case_sensitive: bool) -> Opti
         elif not case_sensitive and f.name.lower() == file_name.lower():
             return f
     return None
+
+
+def _annotation_is_complex(annotation: type[Any] | None) -> bool:
+    origin = get_origin(annotation)
+    return (
+        _annotation_is_complex_inner(annotation)
+        or _annotation_is_complex_inner(origin)
+        or hasattr(origin, '__pydantic_core_schema__')
+        or hasattr(origin, '__get_pydantic_core_schema__')
+    )
+
+
+def _annotation_is_complex_inner(annotation: type[Any] | None) -> bool:
+    if lenient_issubclass(annotation, str):
+        return False
+
+    return lenient_issubclass(annotation, (BaseModel, Mapping, Sequence, tuple, set, frozenset, deque)) or is_dataclass(
+        annotation
+    )

tests/test_settings.py

@@ -622,6 +622,10 @@ class B(BaseSettings):
     class Settings(BaseSettings):
         content: Union[A, B, datetime]
 
+    env.set('content', '{"a": "test"}')
+    s = Settings()
+    assert s.content == A(a='test')
+
     env.set('content', '2020-07-05T00:00:00Z')
     s = Settings()
     assert s.content == datetime(2020, 7, 5, 0, 0, tzinfo=timezone.utc)