Better pydotprint error message

theano/printing.py

@@ -23,19 +23,26 @@ from theano.compile import Function, debugmode, SharedVariable
 from theano.compile.profilemode import ProfileMode
 
 pydot_imported = False
+pydot_imported_msg = ""
 try:
     # pydot-ng is a fork of pydot that is better maintained
     import pydot_ng as pd
     if pd.find_graphviz():
         pydot_imported = True
+    else:
+        pydot_imported_msg = "pydot-ng can't find graphviz"
 except ImportError:
     try:
         # fall back on pydot if necessary
         import pydot as pd
         if pd.find_graphviz():
             pydot_imported = True
+        else:
+            pydot_imported_msg = "pydot can't find graphviz"
     except ImportError:
-        pass  # tests should not fail on optional dependency
+        # tests should not fail on optional dependency
+        pydot_imported_msg = "Install the python package pydot or pydot-ng."
+
 
 _logger = logging.getLogger("theano.printing")
 VALID_ASSOC = set(['left', 'right', 'either'])
@@ -728,7 +735,8 @@ def pydotprint(fct, outfile=None,
         topo = fct.toposort()
     if not pydot_imported:
         raise RuntimeError("Failed to import pydot. You must install pydot"
-                           " and graphviz for `pydotprint` to work.")
+                           " and graphviz for `pydotprint` to work.",
+                           pydot_imported_msg)
 
     g = pd.Dot()
 
@@ -1063,7 +1071,8 @@ def pydotprint_variables(vars,
                                config.device + '.' + format)
     if not pydot_imported:
         raise RuntimeError("Failed to import pydot. You must install pydot"
-                           " and graphviz for `pydotprint_variables` to work.")
+                           " and graphviz for `pydotprint_variables` to work.",
+                           pydot_imported_msg)
     if pd.__name__ == "pydot_ng":
         raise RuntimeError("pydotprint_variables do not support pydot_ng."
                            "pydotprint_variables is also deprecated, "