diff --git a/pisi/graph.py b/pisi/graph.py
index 65386e19..ffc15396 100644
--- a/pisi/graph.py
+++ b/pisi/graph.py
@@ -13,6 +13,8 @@
 
 
 import sys
+import string
+
 import pisi
 
 import gettext
@@ -143,8 +145,12 @@ class Digraph(object):
         return list
 
     def id_str(self, u):
-        s = str(u)
-        return s.replace('-', '_')
+        def repl(char):
+            if char in string.punctuation:
+                return '_'
+            else:
+                return char
+        return pisi.util.concat(map(repl, str(u)))
 
     def write_graphviz(self, f):
         f.write('digraph G {\n')
diff --git a/pisi/pgraph.py b/pisi/pgraph.py
index dee7a9e6..d36b43e9 100644
--- a/pisi/pgraph.py
+++ b/pisi/pgraph.py
@@ -10,7 +10,9 @@
 # Please read the COPYING file.
 #
 
-# PISI package relation graph that represents the state of packagedb
+"""PISI package relation graph that represents the state of packagedb"""
+
+import string
 
 from graph import *