Add overridable is_streaming() to runnables

Adds an `is_streaming()` method to `ParallelExecutionRunnable` that detects whether the runnable produces streaming output by inspecting `run()` and `run_async()` for generator functions.
>
> Subclasses can override this method when streaming detection requires custom logic (e.g., when the runnable delegates to other methods like `predict()` in mlrun).

To enable a cleaner solution to [ML-11878](https://iguazio.atlassian.net/browse/ML-11878).

Author: gtopper

storey/flow.py

@@ -1796,6 +1796,18 @@ async def run_async(self, body: Any, path: str, origin_name: Optional[str] = Non
         """
         return body
 
+    def is_streaming(self) -> bool:
+        """
+        Returns True if this runnable produces streaming output (generator).
+
+        Override this method if your runnable's streaming behavior cannot be detected
+        by inspecting the run()/run_async() methods directly (e.g., when run() delegates
+        to another method that returns a generator).
+
+        :return: True if the runnable produces streaming output, False otherwise.
+        """
+        return inspect.isgeneratorfunction(self.run) or inspect.isasyncgenfunction(self.run_async)
+
     def _run(self, body: Any, path: str, origin_name: Optional[str] = None) -> Any:
         timestamp = datetime.datetime.now(tz=datetime.timezone.utc)
         start = time.monotonic()
@@ -1963,8 +1975,7 @@ def init_runnable(self, runnable: Union[str, ParallelExecutionRunnable]) -> None
         execution_mechanism = self._execution_mechanism_by_runnable_name[runnable.name]
 
         # Record whether this runnable is a streaming runnable (generator function)
-        is_streaming = inspect.isgeneratorfunction(runnable.run) or inspect.isasyncgenfunction(runnable.run_async)
-        self._is_streaming_by_runnable_name[runnable.name] = is_streaming
+        self._is_streaming_by_runnable_name[runnable.name] = runnable.is_streaming()
 
         if execution_mechanism == ParallelExecutionMechanisms.process_pool:
             self.num_processes += 1

tests/test_streaming.py

@@ -62,6 +62,13 @@ def run(self, body, path: str, origin_name: Optional[str] = None) -> Generator:
         raise ValueError("Simulated streaming error")
 
 
+class NonStreamingRunnable(ParallelExecutionRunnable):
+    """A non-streaming runnable that returns a single value."""
+
+    def run(self, body, path: str, origin_name: Optional[str] = None):
+        return f"{body}_result"
+
+
 class TestStreamingPrimitives:
     """Tests for streaming primitive classes."""
 
@@ -120,6 +127,46 @@ async def coro():
             c.close()
 
 
+class TestIsStreamingMethod:
+    """Tests for ParallelExecutionRunnable.is_streaming() method."""
+
+    def test_is_streaming_sync_generator(self):
+        """Test that a runnable with a sync generator run() is detected as streaming."""
+        runnable = StreamingRunnable(name="test")
+        assert runnable.is_streaming() is True
+
+    def test_is_streaming_async_generator(self):
+        """Test that a runnable with an async generator run_async() is detected as streaming."""
+        runnable = AsyncStreamingRunnable(name="test")
+        assert runnable.is_streaming() is True
+
+    def test_is_streaming_non_generator(self):
+        """Test that a runnable with a non-generator run() is not detected as streaming."""
+        runnable = NonStreamingRunnable(name="test")
+        assert runnable.is_streaming() is False
+
+    def test_is_streaming_base_class(self):
+        """Test that the base ParallelExecutionRunnable is not streaming by default."""
+        runnable = ParallelExecutionRunnable(name="test")
+        assert runnable.is_streaming() is False
+
+    def test_is_streaming_override(self):
+        """Test that is_streaming() can be overridden by subclasses."""
+
+        class OverriddenRunnable(ParallelExecutionRunnable):
+            """A runnable that overrides is_streaming() to return True."""
+
+            def is_streaming(self) -> bool:
+                return True
+
+            def run(self, body, path: str, origin_name: Optional[str] = None):
+                # Even though run() is not a generator, is_streaming() returns True
+                return f"{body}_result"
+
+        runnable = OverriddenRunnable(name="test")
+        assert runnable.is_streaming() is True
+
+
 class TestMapStreaming:
     """Tests for Map step streaming support."""