Fix: allow empty string aliases with AliasGenerator (#8810)

pydantic/_internal/_generate_schema.py

@@ -296,6 +296,15 @@ def push(self, for_type: type[Any]):
             self._types_namespace_stack.pop()
 
 
+def _get_first_non_null(a: Any, b: Any) -> Any:
+    """Return the first argument if it is not None, otherwise return the second argument.
+
+    Use case: serialization_alias (argument a) and alias (argument b) are both defined, and serialization_alias is ''.
+    This function will return serialization_alias, which is the first argument, even though it is an empty string.
+    """
+    return a if a is not None else b
+
+
 class GenerateSchema:
     """Generate core schema for a Pydantic model, dataclass and types like `str`, `datetime`, ... ."""
 
@@ -997,17 +1006,17 @@ def _apply_alias_generator_to_field_info(
 
             # if the priority is 1, then we set the aliases to the generated alias
             if field_info.alias_priority == 1:
-                field_info.serialization_alias = serialization_alias or alias
-                field_info.validation_alias = validation_alias or alias
+                field_info.serialization_alias = _get_first_non_null(serialization_alias, alias)
+                field_info.validation_alias = _get_first_non_null(validation_alias, alias)
                 field_info.alias = alias
 
             # if any of the aliases are not set, then we set them to the corresponding generated alias
             if field_info.alias is None:
                 field_info.alias = alias
             if field_info.serialization_alias is None:
-                field_info.serialization_alias = serialization_alias or alias
+                field_info.serialization_alias = _get_first_non_null(serialization_alias, alias)
             if field_info.validation_alias is None:
-                field_info.validation_alias = validation_alias or alias
+                field_info.validation_alias = _get_first_non_null(validation_alias, alias)
 
     @staticmethod
     def _apply_alias_generator_to_computed_field_info(
@@ -1050,7 +1059,7 @@ def _apply_alias_generator_to_computed_field_info(
             # note that we use the serialization_alias with priority over alias, as computed_field
             # aliases are used for serialization only (not validation)
             if computed_field_info.alias_priority == 1:
-                computed_field_info.alias = serialization_alias or alias
+                computed_field_info.alias = _get_first_non_null(serialization_alias, alias)
 
     def _common_field_schema(  # C901
         self, name: str, field_info: FieldInfo, decorators: DecoratorInfos

tests/test_aliases.py

@@ -725,3 +725,35 @@ def area(self) -> int:
 
     r = Rectangle(width_val_alias=10, height_val_alias=20)
     assert r.model_dump(by_alias=True) == {'width_ser_alias': 10, 'height_ser_alias': 20, 'area_ser_alias': 200}
+
+
+empty_str_alias_generator = AliasGenerator(
+    validation_alias=lambda x: '', alias=lambda x: f'{x}_alias', serialization_alias=lambda x: ''
+)
+
+
+def test_alias_gen_with_empty_string() -> None:
+    class Model(BaseModel):
+        a: str
+
+        model_config = ConfigDict(alias_generator=empty_str_alias_generator)
+
+    assert Model.model_fields['a'].validation_alias == ''
+    assert Model.model_fields['a'].serialization_alias == ''
+    assert Model.model_fields['a'].alias == 'a_alias'
+
+
+def test_alias_gen_with_empty_string_and_computed_field() -> None:
+    class Model(BaseModel):
+        model_config = ConfigDict(alias_generator=empty_str_alias_generator)
+
+        a: str
+
+        @computed_field
+        def b(self) -> str:
+            return self.a
+
+    assert Model.model_fields['a'].validation_alias == ''
+    assert Model.model_fields['a'].serialization_alias == ''
+    assert Model.model_fields['a'].alias == 'a_alias'
+    assert Model.model_computed_fields['b'].alias == ''