Create cache of graph to avoid generating it more than once

src/wireviz/Harness.py

@@ -104,6 +104,7 @@ def connect(self, from_name: str, from_pin: (int, str), via_name: str, via_wire:
         if to_name in self.connectors:
             self.connectors[to_name].activate_pin(to_pin)
 
+
     def create_graph(self) -> Graph:
         dot = Graph()
         dot.body.append(f'// Graph generated by {APP_NAME} {__version__}')
@@ -457,10 +458,20 @@ def typecheck(name: str, value: Any, expect: type) -> None:
 
         return dot
 
+    # cache for the GraphViz Graph object
+    # do not access directly, use self.graph instead
+    _graph = None
+
+    @property
+    def graph(self):
+        if not self._graph:  # no cached graph exists, generate one
+            self._graph = self.create_graph()
+        return self._graph  # return cached graph
+
     @property
     def png(self):
         from io import BytesIO
-        graph = self.create_graph()
+        graph = self.graph
         data = BytesIO()
         data.write(graph.pipe(format='png'))
         data.seek(0)
@@ -469,15 +480,15 @@ def png(self):
     @property
     def svg(self):
         from io import BytesIO
-        graph = self.create_graph()
+        graph = self.graph
         data = BytesIO()
         data.write(graph.pipe(format='svg'))
         data.seek(0)
         return data.read()
 
     def output(self, filename: (str, Path), view: bool = False, cleanup: bool = True, fmt: tuple = ('html','png','svg','tsv')) -> None:
         # graphical output
-        graph = self.create_graph()
+        graph = self.graph
         svg_already_exists = Path(f'{filename}.svg').exists()  # if SVG already exists, do not delete later
         # graphical output
         for f in fmt: