Remove auto unroll for dict values from taskflow

dict return values from functions were automatically unrolled
in separate values. This meant that the default of `multiple_outputs`
was changing based on the return type of the function.

This is confusing as it requires the user to be aware of this and
rather un-pythonic (explicit over implicit) and makes conversion from
plain python functions to Airflow tasks harder as unit tests would suddenly
fail.

Closes: apache#27819

airflow/decorators/base.py

@@ -286,22 +286,13 @@ class _TaskDecorator(ExpandableFactory, Generic[FParams, FReturn, OperatorSubcla
 
     function: Callable[FParams, FReturn] = attr.ib(validator=attr.validators.is_callable())
     operator_class: type[OperatorSubclass]
-    multiple_outputs: bool = attr.ib()
+    multiple_outputs: bool = attr.ib(default=False)
     kwargs: dict[str, Any] = attr.ib(factory=dict)
 
     decorator_name: str = attr.ib(repr=False, default="task")
 
     _airflow_is_task_decorator: ClassVar[bool] = True
 
-    @multiple_outputs.default
-    def _infer_multiple_outputs(self):
-        try:
-            return_type = typing_extensions.get_type_hints(self.function).get("return", Any)
-        except TypeError:  # Can't evaluate return type.
-            return False
-        ttype = getattr(return_type, "__origin__", return_type)
-        return ttype == dict or ttype == Dict
-
     def __attrs_post_init__(self):
         if "self" in self.function_signature.parameters:
             raise TypeError(f"@{self.decorator_name} does not support methods")

docs/apache-airflow/tutorial/taskflow.rst

@@ -377,19 +377,21 @@ section "Having sensors return XOM values" of :doc:`apache-airflow-providers:how
 
 Multiple outputs inference
 --------------------------
-Tasks can also infer multiple outputs by using dict Python typing.
+In case a task has multiple return values, Airflow can unroll these in separate values if using
+Python dict typing. The values then become available as separate arguments to downstream tasks.
 
 .. code-block:: python
 
-   @task
+   @task(multiple_outputs=True)
    def identity_dict(x: int, y: int) -> dict[str, int]:
        return {"x": x, "y": y}
 
 By using the typing ``Dict`` for the function return type, the ``multiple_outputs`` parameter
 is automatically set to true.
 
-Note, If you manually set the ``multiple_outputs`` parameter the inference is disabled and
-the parameter value is used.
+Note, before Airflow 2.5, Airflow would automatically unroll ``dict`` return values and you would need to set
+``multiple_outputs=False` explicitly. This was deemed confusing and now always defaults to ``False``.
+
 
 Adding dependencies between decorated and traditional tasks
 -----------------------------------------------------------

newsfragments/27819.significant.rst

@@ -0,0 +1,2 @@
+Airflow no longer automatically unrolls dict values into separate arguments requiring an explicit
+``multiple_outputs=True`` if unrolling is required.