Make InitSettingsSource resolution deterministic (#681)

Co-authored-by: Enrico Stauss <enrico.stauss@pace.de>
Co-authored-by: Enrico Stauss <enrico_sta@web.de>

Author: 3 people

pydantic_settings/sources/base.py

@@ -268,8 +268,9 @@ def __init__(
             init_kwarg_name = init_kwarg_names & set(alias_names)
             if init_kwarg_name:
                 preferred_alias = alias_names[0]
+                preferred_set_alias = next(alias for alias in alias_names if alias in init_kwarg_name)
                 init_kwarg_names -= init_kwarg_name
-                self.init_kwargs[preferred_alias] = init_kwargs[init_kwarg_name.pop()]
+                self.init_kwargs[preferred_alias] = init_kwargs[preferred_set_alias]
         self.init_kwargs.update({key: val for key, val in init_kwargs.items() if key in init_kwarg_names})
 
         super().__init__(settings_cls)

tests/test_settings.py

@@ -699,6 +699,16 @@ class Example(BaseSettings):
     assert Example(name='john', PREFIX_SURNAME='doe').model_dump() == {'name': 'john', 'last_name': 'doe'}
 
 
+def test_init_kwargs_alias_resolution_deterministic():
+    class Example(BaseSettings):
+        name: str
+        last_name: str = Field(validation_alias=AliasChoices('surname', 'last_name'))
+
+    result = Example(name='john', surname='doe', last_name='smith').model_dump()
+
+    assert result == {'name': 'john', 'last_name': 'doe'}
+
+
 def test_alias_nested_model_default_partial_update():
     class SubModel(BaseModel):
         v1: str = 'default'