Description
According to Python's Document Type aliases, type aliases can be created through simple assignment or using the type statement. The two ways should be equivalent.
However, as the example code below, If a field's annotation is a type aliases defined by using the type statement, the json decoding doesn't trigger (settings B and C).
This is very similar issue to the #715 which was recently fixed in 2.13, but in this case the ForceDecode is not working in both cases: defined explicite and implicite (by deafult). While the code sample from #715) passed and seems to be fixed, the below code does not.
Example Code
from typing import Annotated, TypeVar
from annotated_types import Len
from pydantic_settings import BaseSettings, ForceDecode
T = TypeVar("T")
MaxLenA = Annotated[T, Len(max_length=4)]
type MaxLenB[_T] = Annotated[_T, Len(max_length=4)]
type MaxLenC[_T] = Annotated[_T, Len(max_length=4), ForceDecode]
class MySettingsA(BaseSettings):
SIMPLE_LIST: MaxLenA[list[str]]
class MySettingsB(BaseSettings):
SIMPLE_LIST: MaxLenB[list[str]]
class MySettingsC(BaseSettings):
SIMPLE_LIST: MaxLenC[list[str]]
if __name__ == "__main__":
import os
os.environ["SIMPLE_LIST"] = r'["a", "b", "c"]'
my_settings_a = MySettingsA()
print(my_settings_a)
# The below settings will raise a validation error:
# Input should be a valid list [type=list_type, input_value='["a", "b", "c"]', input_type=str]
my_settings_b = MySettingsB()
print(my_settings_b)
my_settings_c = MySettingsC()
print(my_settings_c)The Result
SIMPLE_LIST=[1, 2, 3]
Traceback (most recent call last):
File ".../pydantic-bug.py", line 36, in <module>
my_settings_b = MySettingsB()
^^^^^^^^^^^^^
File ".../.venv/lib/python3.12/site-packages/pydantic_settings/main.py", line 242, in __init__
super().__init__(**__pydantic_self__.__class__._settings_build_values(sources, init_kwargs))
File ".../.venv/lib/python3.12/site-packages/pydantic/main.py", line 250, in __init__
validated_self = self.__pydantic_validator__.validate_python(data, self_instance=self)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
pydantic_core._pydantic_core.ValidationError: 1 validation error for MySettingsB
SIMPLE_LIST
Input should be a valid list [type=list_type, input_value='["a", "b", "c"]', input_type=str]
For further information visit https://errors.pydantic.dev/2.12/v/list_type
Version
pydantic version: 2.12.5
pydantic-core version: 2.41.5
pydantic-core build: profile=release pgo=false
python version: 3.12.11 (main, Jun 3 2025, 15:41:47) [Clang 17.0.0 (clang-1700.0.13.3)]
platform: macOS-26.3-arm64-arm-64bit
related packages: email-validator-2.3.0 typing_extensions-4.15.0 pydantic-settings-2.13.0
commit: unknown
Description
According to Python's Document Type aliases, type aliases can be created through simple assignment or using the type statement. The two ways should be equivalent.
However, as the example code below, If a field's annotation is a type aliases defined by using the type statement, the json decoding doesn't trigger (settings B and C).
This is very similar issue to the #715 which was recently fixed in 2.13, but in this case the
ForceDecodeis not working in both cases: defined explicite and implicite (by deafult). While the code sample from #715) passed and seems to be fixed, the below code does not.Example Code
The Result
Version