如何在Graphviz Python中让箭头从左侧进入注释节点？

需求说明 #

我用Graphviz的Python库绘图，有个特定需求：标记在annotations列表里的半矩形样式注释节点，需要让箭头从左侧进入，而非默认的底部。

代码背景 #

• model对象包含edge对象，每个edge有source和target节点（本质是字符串）
• 维护annotations列表，用来标记需要设置为半矩形样式的节点
• 核心目标：让这类节点的入箭头改为从左侧接入

原代码 #

g = Digraph('G', filename='./output/model_' + str(mi), format='png', strict=True)
g.graph_attr.update(rankdir='BT')
g.attr('node', shape='note')
sources = list()
targets = list()
annotations = list()
PossibleModel.MAX_DEPTH = 3

"""Let us first see what the possible inputs are"""
"""  
for edge in model.get_edges():
    sources.append(str(edge.source))
    targets.append(str(edge.target))
"""  
for edge in model.get_edges():
    sources.append(edge.source.small_string2())
    targets.append(edge.target.small_string2())
  
difference = list(set(sources) - set(targets))
print('difference', difference)

for edge in model.get_edges():
    edge_contained = ''
    for rule in br_rules:
        if str(edge.source) in rule or str(edge.target) in rule:
            edge_contained += rule + '\n'
    if edge_contained == '':
        g.edge(edge.source.small_string2(), edge.target.small_string2(), label=str(edge.weight), color='red')
        g.edge(edge.source.small_string2(), edge.source.small_string3(), color='black', arrowhead='none', style='dashed')
        g.edge(edge.target.small_string2(), edge.target.small_string3(), color='black', arrowhead='none', style='dashed')
        annotations.append(edge.source.small_string3())
        annotations.append(edge.target.small_string3())
    else:
        g.attr('edge', color='blue')
        g.edge(str(edge.source), str(edge.target), label=edge_contained)

for node in difference:
    g.node(node, style='rounded', shape='ellipse')

for node in targets:
    g.node(node, shape='box')
    
for node in annotations:
    g.node(node, color='transparent', image='test.png', fontsize='11')

解决方案 #

要实现箭头从左侧接入注释节点，核心是利用Graphviz的**端口（port）**特性，指定边连接到节点的左侧端口（west）。具体修改如下：

1. 修改指向注释节点的边：在创建指向annotations列表中节点的边时，把目标节点格式改为{节点名}:west，强制箭头连接到节点左侧。
2. （可选）调整节点锚点：如果使用自定义图片作为节点样式，可以设置imagepos="ml"（middle left），让图片的左侧作为端口锚点，确保箭头位置准确。

修改后的关键代码片段 #

# 原代码中创建指向注释节点的边部分，替换为：
g.edge(edge.source.small_string2(), f"{edge.source.small_string3()}:west", color='black', arrowhead='none', style='dashed')
g.edge(edge.target.small_string2(), f"{edge.target.small_string3()}:west", color='black', arrowhead='none', style='dashed')

# 注释节点的设置可添加imagepos属性（如果需要）：
for node in annotations:
    g.node(node, color='transparent', image='test.png', fontsize='11', imagepos="ml")

完整修改后的代码 #

g = Digraph('G', filename='./output/model_' + str(mi), format='png', strict=True)
g.graph_attr.update(rankdir='BT')
g.attr('node', shape='note')
sources = list()
targets = list()
annotations = list()
PossibleModel.MAX_DEPTH = 3

"""Let us first see what the possible inputs are"""
"""  
for edge in model.get_edges():
    sources.append(str(edge.source))
    targets.append(str(edge.target))
"""  
for edge in model.get_edges():
    sources.append(edge.source.small_string2())
    targets.append(edge.target.small_string2())
  
difference = list(set(sources) - set(targets))
print('difference', difference)

for edge in model.get_edges():
    edge_contained = ''
    for rule in br_rules:
        if str(edge.source) in rule or str(edge.target) in rule:
            edge_contained += rule + '\n'
    if edge_contained == '':
        g.edge(edge.source.small_string2(), edge.target.small_string2(), label=str(edge.weight), color='red')
        # 修改这里，指定目标节点的west端口
        g.edge(edge.source.small_string2(), f"{edge.source.small_string3()}:west", color='black', arrowhead='none', style='dashed')
        g.edge(edge.target.small_string2(), f"{edge.target.small_string3()}:west", color='black', arrowhead='none', style='dashed')
        annotations.append(edge.source.small_string3())
        annotations.append(edge.target.small_string3())
    else:
        g.attr('edge', color='blue')
        g.edge(str(edge.source), str(edge.target), label=edge_contained)

for node in difference:
    g.node(node, style='rounded', shape='ellipse')

for node in targets:
    g.node(node, shape='box')
    
# 可选：添加imagepos确保图片锚点在左侧
for node in annotations:
    g.node(node, color='transparent', image='test.png', fontsize='11', imagepos="ml")

原理说明 #

Graphviz的节点支持通过:方向格式指定连接端口，常用的方向包括：

• west：左侧
• east：右侧
• north：顶部
• south：底部

因为你的图设置了rankdir='BT'（从下到上布局），默认箭头会沿着布局方向从底部接入节点，指定:west后，强制边连接到节点的左侧，满足需求。

内容的提问来源于stack exchange，提问作者Alexandre Goossens