feat: Highlight optional connections in Pipeline.draw() (#6724)

* highlight optional connections in Pipeline.draw()

* reno

haystack/core/pipeline/draw/draw.py

@@ -76,7 +76,9 @@ def _prepare_for_drawing(graph: networkx.MultiDiGraph, style_map: Dict[str, str]
 
     # Label the edges
     for inp, outp, key, data in graph.edges(keys=True, data=True):
-        data["label"] = f"{data['from_socket'].name} -> {data['to_socket'].name}"
+        data[
+            "label"
+        ] = f"{data['from_socket'].name} -> {data['to_socket'].name}{' (opt.)' if not data['mandatory'] else ''}"
         graph.add_edge(inp, outp, key=key, **data)
 
     # Draw the inputs

haystack/core/pipeline/draw/mermaid.py

@@ -12,7 +12,10 @@ from haystack.core.type_utils import _type_name
 
 logger = logging.getLogger(__name__)
 
-
+ARROWTAIL_MANDATORY = "--"
+ARROWTAIL_OPTIONAL = "-."
+ARROWHEAD_MANDATORY = "-->"
+ARROWHEAD_OPTIONAL = ".->"
 MERMAID_STYLED_TEMPLATE = """
 %%{{ init: {{'theme': 'neutral' }} }}%%
 
@@ -81,11 +84,15 @@ def _to_mermaid_text(graph: networkx.MultiDiGraph) -> str:
         if comp not in ["input", "output"]
     }
 
-    connections_list = [
-        f"{states[from_comp]} -- \"{conn_data['label']}<br><small><i>{conn_data['conn_type']}</i></small>\" --> {states[to_comp]}"
-        for from_comp, to_comp, conn_data in graph.edges(data=True)
-        if from_comp != "input" and to_comp != "output"
-    ]
+    connections_list = []
+    for from_comp, to_comp, conn_data in graph.edges(data=True):
+        if from_comp != "input" and to_comp != "output":
+            arrowtail = ARROWTAIL_MANDATORY if conn_data["mandatory"] else ARROWTAIL_OPTIONAL
+            arrowhead = ARROWHEAD_MANDATORY if conn_data["mandatory"] else ARROWHEAD_OPTIONAL
+            label = f'"{conn_data["label"]}<br><small><i>{conn_data["conn_type"]}</i></small>"'
+            conn_string = f"{states[from_comp]} {arrowtail} {label} {arrowhead} {states[to_comp]}"
+            connections_list.append(conn_string)
+
     input_connections = [
         f"i{{*}} -- \"{conn_data['label']}<br><small><i>{conn_data['conn_type']}</i></small>\" --> {states[to_comp]}"
         for _, to_comp, conn_data in graph.out_edges("input", data=True)

haystack/core/pipeline/pipeline.py

@@ -327,6 +327,7 @@ class Pipeline:
             conn_type=_type_name(connection.sender_socket.type),
             from_socket=connection.sender_socket,
             to_socket=connection.receiver_socket,
+            mandatory=connection.is_mandatory,
         )
 
         self._connections.append(connection)

releasenotes/notes/highlight-optional-conns-in-draw-cf107ee210b7b8e7.yaml

@@ -0,0 +1,3 @@
+---
+enhancements:
+  - Highlight optional connections in the `Pipeline.draw()` output.