ignore absent pygraphviz - wrap render call (to_agraph) in try/except ImportError in agent_chains

flatland/envs/agent_chains.py

@@ -182,14 +182,19 @@ class MotionCheck(object):
             
 
 def render(omc:MotionCheck, horizontal=True):
-    oAG = nx.drawing.nx_agraph.to_agraph(omc.G)
-    oAG.layout("dot")
-    sDot = oAG.to_string()
-    if horizontal:
-        sDot = sDot.replace('{', '{ rankdir="LR" ')
-    #return oAG.draw(format="png")
-    # This returns a graphviz object which implements __repr_svg
-    return gv.Source(sDot)
+    try:
+        oAG = nx.drawing.nx_agraph.to_agraph(omc.G)
+        oAG.layout("dot")
+        sDot = oAG.to_string()
+        if horizontal:
+            sDot = sDot.replace('{', '{ rankdir="LR" ')
+        #return oAG.draw(format="png")
+        # This returns a graphviz object which implements __repr_svg
+        return gv.Source(sDot)
+    except ImportError as oError:
+        print("Flatland agent_chains ignoring ImportError - install pygraphviz to render graphs")
+        return None
+
 
 class ChainTestEnv(object):
     """ Just for testing agent chains