Tests should not fail on optional dependency.

theano/d3viz/formatting.py

@@ -18,11 +18,16 @@ pydot_imported = False
 try:
     # pydot-ng is a fork of pydot that is better maintained
     import pydot_ng as pd
+    if pd.find_graphviz():
+        pydot_imported = True
 except ImportError:
+    try:
         # fall back on pydot if necessary
         import pydot as pd
-if pd.find_graphviz():
+        if pd.find_graphviz():
             pydot_imported = True
+    except ImportError:
+        pass  # tests should not fail on optional dependency
 
 
 class PyDotFormatter(object):

theano/printing.py

@@ -26,11 +26,16 @@ pydot_imported = False
 try:
     # pydot-ng is a fork of pydot that is better maintained
     import pydot_ng as pd
+    if pd.find_graphviz():
+        pydot_imported = True
 except ImportError:
+    try:
         # fall back on pydot if necessary
         import pydot as pd
-if pd.find_graphviz():
+        if pd.find_graphviz():
             pydot_imported = True
+    except ImportError:
+        pass  # tests should not fail on optional dependency
 
 _logger = logging.getLogger("theano.printing")
 VALID_ASSOC = set(['left', 'right', 'either'])