python: Allow ovs-flowviz to run without graphviz.

The graphviz library has a large set of dependencies, and as such
it may be desireable to not install it by default.

Before this patch execution of ovs-flowviz would end in
ImportError when graphviz is unavailable.

Exit with a friendly message instead, allowing the user to consume
the parts of the program that do not depend on graphviz.

Signed-off-by: Frode Nordahl <fnordahl@ubuntu.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

python/ovs/flowviz/odp/graph.py

@@ -14,16 +14,26 @@
 
 """ Defines a Datapath Graph using graphviz. """
 import colorsys
-import graphviz
 import random
+import sys
 
 from ovs.flowviz.odp.html import HTMLTree, HTMLFormatter
 from ovs.flowviz.odp.tree import FlowTree
 from ovs.flowviz.process import FileProcessor
 
+try:
+    import graphviz
+except ImportError:
+    graphviz = None
+
 
 class GraphProcessor(FileProcessor):
     def __init__(self, opts):
+        if graphviz is None:
+            print("ERROR: The graph sub-command depends on the graphviz "
+                  "Python library, which does not appear to be installed.",
+                  file=sys.stderr)
+            sys.exit(1)
         super().__init__(opts, "odp")
 
     def start_file(self, name, filename):