Fix/informative error message

haystack/core/pipeline/base.py

@@ -417,7 +417,7 @@ def remove_component(self, name: str) -> Component:
 
         return instance
 
-    def connect(self, sender: str, receiver: str) -> "PipelineBase":  # noqa: PLR0915 PLR0912
+    def connect(self, sender: str, receiver: str) -> "PipelineBase":  # noqa: PLR0915 PLR0912 C901 pylint: disable=too-many-branches
         """
         Connects two components together.
 
@@ -456,13 +456,20 @@ def connect(self, sender: str, receiver: str) -> "PipelineBase":  # noqa: PLR091
         except KeyError as exc:
             raise ValueError(f"Component named {receiver_component_name} not found in the pipeline.") from exc
 
+        if not sender_sockets:
+            raise PipelineConnectError(
+                f"'{sender_component_name}' does not have any output connections. "
+                f"Please check that the output types of '{sender_component_name}.run' are set, "
+                f"for example by using the '@component.output_types' decorator."
+            )
+
         # If the name of either socket is given, get the socket
         sender_socket: Optional[OutputSocket] = None
         if sender_socket_name:
             sender_socket = sender_sockets.get(sender_socket_name)
             if not sender_socket:
                 raise PipelineConnectError(
-                    f"'{sender} does not exist. "
+                    f"'{sender}' does not exist. "
                     f"Output connections of {sender_component_name} are: "
                     + ", ".join([f"{name} (type {_type_name(socket.type)})" for name, socket in sender_sockets.items()])
                 )

releasenotes/notes/informative-error-message-on-two-components-connection-0cc535409855b06e.yaml

@@ -0,0 +1,3 @@
+fixes:
+  - |
+    Return a more informative error message when attempting to connect two components and the sender component does not have any OutputSockets defined.

test/core/pipeline/test_pipeline_base.py

@@ -259,6 +259,40 @@ def test_remove_component_allows_you_to_reuse_the_component(self):
         # instance = pipe2.get_component("some")
         # assert instance == component
 
+    def test_connect_with_nonexistent_output_socket_name(self):
+        """Test connecting using a non-existent output socket name."""
+        comp1 = component_class("Comp1", output_types={"output": int})()
+        comp2 = component_class("Comp2", input_types={"value": int})()
+        pipe = PipelineBase()
+        pipe.add_component("comp1", comp1)
+        pipe.add_component("comp2", comp2)
+
+        with pytest.raises(PipelineConnectError) as excinfo:
+            pipe.connect("comp1.value", "comp2.value")
+
+        assert "'comp1.value' does not exist" in str(excinfo.value)
+        assert "Output connections of comp1 are: output (type int)" in str(excinfo.value)
+
+    def test_connect_with_no_output_sockets(self):
+        """Test connecting from a component that has no output sockets at all."""
+
+        @component
+        class NoOutputComponent:
+            def run(self):
+                pass
+
+        comp1 = NoOutputComponent()
+        comp2 = component_class("Comp2", input_types={"value": int})()
+        pipe = PipelineBase()
+        pipe.add_component("comp1", comp1)
+        pipe.add_component("comp2", comp2)
+
+        with pytest.raises(PipelineConnectError) as excinfo:
+            pipe.connect("comp1.value", "comp2.value")
+
+        assert "'comp1' does not have any output connections" in str(excinfo.value)
+        assert "@component.output_types" in str(excinfo.value)
+
     # UNIT
     def test_get_component_name(self):
         pipe = PipelineBase()