core: Add concept of conditional edge to graph rendering (#20480)

- implement for mermaid, graphviz and ascii
- this is to be used in langgraph

libs/core/langchain_core/runnables/graph.py

@@ -19,7 +19,6 @@ from typing import (
 from uuid import UUID, uuid4
 
 from langchain_core.pydantic_v1 import BaseModel
-from langchain_core.runnables.graph_ascii import draw_ascii
 
 if TYPE_CHECKING:
     from langchain_core.runnables.base import Runnable as RunnableType
@@ -44,6 +43,7 @@ class Edge(NamedTuple):
     source: str
     target: str
     data: Optional[str] = None
+    conditional: bool = False
 
 
 class Node(NamedTuple):
@@ -219,13 +219,21 @@ class Graph:
             if edge.source != node.id and edge.target != node.id
         ]
 
-    def add_edge(self, source: Node, target: Node, data: Optional[str] = None) -> Edge:
+    def add_edge(
+        self,
+        source: Node,
+        target: Node,
+        data: Optional[str] = None,
+        conditional: bool = False,
+    ) -> Edge:
         """Add an edge to the graph and return it."""
         if source.id not in self.nodes:
             raise ValueError(f"Source node {source.id} not in graph")
         if target.id not in self.nodes:
             raise ValueError(f"Target node {target.id} not in graph")
-        edge = Edge(source=source.id, target=target.id, data=data)
+        edge = Edge(
+            source=source.id, target=target.id, data=data, conditional=conditional
+        )
         self.edges.append(edge)
         return edge
 
@@ -283,9 +291,11 @@ class Graph:
                 self.remove_node(last_node)
 
     def draw_ascii(self) -> str:
+        from langchain_core.runnables.graph_ascii import draw_ascii
+
         return draw_ascii(
             {node.id: node_data_str(node) for node in self.nodes.values()},
-            [(edge.source, edge.target) for edge in self.edges],
+            self.edges,
         )
 
     def print_ascii(self) -> None:

libs/core/langchain_core/runnables/graph_ascii.py

@@ -3,7 +3,9 @@ Adapted from https://github.com/iterative/dvc/blob/main/dvc/dagascii.py"""
 
 import math
 import os
-from typing import Any, Mapping, Sequence, Tuple
+from typing import Any, Mapping, Sequence
+
+from langchain_core.runnables.graph import Edge as LangEdge
 
 
 class VertexViewer:
@@ -156,7 +158,7 @@ class AsciiCanvas:
 
 
 def _build_sugiyama_layout(
-    vertices: Mapping[str, str], edges: Sequence[Tuple[str, str]]
+    vertices: Mapping[str, str], edges: Sequence[LangEdge]
 ) -> Any:
     try:
         from grandalf.graphs import Edge, Graph, Vertex  # type: ignore[import]
@@ -181,7 +183,7 @@ def _build_sugiyama_layout(
     #
 
     vertices_ = {id: Vertex(f" {data} ") for id, data in vertices.items()}
-    edges_ = [Edge(vertices_[s], vertices_[e]) for s, e in edges]
+    edges_ = [Edge(vertices_[s], vertices_[e], data=cond) for s, e, _, cond in edges]
     vertices_list = vertices_.values()
     graph = Graph(vertices_list, edges_)
 
@@ -209,7 +211,7 @@ def _build_sugiyama_layout(
     return sug
 
 
-def draw_ascii(vertices: Mapping[str, str], edges: Sequence[Tuple[str, str]]) -> str:
+def draw_ascii(vertices: Mapping[str, str], edges: Sequence[LangEdge]) -> str:
     """Build a DAG and draw it in ASCII.
 
     Args:
@@ -220,7 +222,6 @@ def draw_ascii(vertices: Mapping[str, str], edges: Sequence[Tuple[str, str]]) ->
         str: ASCII representation
 
     Example:
-        >>> from dvc.dagascii import draw
         >>> vertices = [1, 2, 3, 4]
         >>> edges = [(1, 2), (2, 3), (2, 4), (1, 4)]
         >>> print(draw(vertices, edges))
@@ -287,7 +288,7 @@ def draw_ascii(vertices: Mapping[str, str], edges: Sequence[Tuple[str, str]]) ->
             assert end_x >= 0
             assert end_y >= 0
 
-            canvas.line(start_x, start_y, end_x, end_y, "*")
+            canvas.line(start_x, start_y, end_x, end_y, "." if edge.data else "*")
 
     for vertex in sug.g.sV:
         # NOTE: moving boxes w/2 to the left

libs/core/langchain_core/runnables/graph_mermaid.py

@@ -76,7 +76,13 @@ def draw_mermaid(
                         for i in range(0, len(words), wrap_label_n_words)
                     ]
                 )
+            if edge.conditional:
+                edge_label = f" -. {edge_data} .-> "
+            else:
                 edge_label = f" -- {edge_data} --> "
+        else:
+            if edge.conditional:
+                edge_label = " -.-> "
             else:
                 edge_label = " --> "
         mermaid_graph += (

libs/core/langchain_core/runnables/graph_png.py

@@ -52,7 +52,12 @@ class PngDrawer:
         )
 
     def add_edge(
-        self, viz: Any, source: str, target: str, label: Optional[str] = None
+        self,
+        viz: Any,
+        source: str,
+        target: str,
+        label: Optional[str] = None,
+        conditional: bool = False,
     ) -> None:
         viz.add_edge(
             source,
@@ -60,6 +65,7 @@ class PngDrawer:
             label=self.get_edge_label(label) if label else "",
             fontsize=12,
             fontname=self.fontname,
+            style="dotted" if conditional else "solid",
         )
 
     def draw(self, graph: Graph, output_path: Optional[str] = None) -> Optional[bytes]:
@@ -98,8 +104,8 @@ class PngDrawer:
             self.add_node(viz, node)
 
     def add_edges(self, viz: Any, graph: Graph) -> None:
-        for start, end, label in graph.edges:
+        for start, end, label, cond in graph.edges:
-            self.add_edge(viz, start, end, label)
+            self.add_edge(viz, start, end, label, cond)
 
     def update_styles(self, viz: Any, graph: Graph) -> None:
         if first := graph.first_node():