BUG : don't use mutable objects as dictionary keys

Base key on the id of the transform.  This maintains
the old behavior and does not break the python hashable
object model.

changed the variable id -> pid so as to not shadow the
method `id`

Closes matplotlib#2828

CHANGELOG

@@ -1,3 +1,5 @@
+2014-06-07 Fixed bug so radial plots can be saved as ps in py3k.
+
 2014-06-01 Changed the fmt kwarg of errorbar to support the
            the mpl convention that "none" means "don't draw it",
            and to default to the empty string, so that plotting

lib/matplotlib/backends/backend_ps.py

@@ -554,15 +554,16 @@ def _convert_path(self, path, transform, clip=False, simplify=None):
         return ps
 
     def _get_clip_path(self, clippath, clippath_transform):
-        id = self._clip_paths.get((clippath, clippath_transform))
-        if id is None:
-            id = 'c%x' % len(self._clip_paths)
-            ps_cmd = ['/%s {' % id]
+        key = (clippath, id(clippath_transform))
+        pid = self._clip_paths.get(key)
+        if pid is None:
+            pid = 'c%x' % len(self._clip_paths)
+            ps_cmd = ['/%s {' % pid]
             ps_cmd.append(self._convert_path(clippath, clippath_transform,
                                              simplify=False))
             ps_cmd.extend(['clip', 'newpath', '} bind def\n'])
             self._pswriter.write('\n'.join(ps_cmd))
-            self._clip_paths[(clippath, clippath_transform)] = id
+            self._clip_paths[key] = pid
         return id
 
     def draw_path(self, gc, path, transform, rgbFace=None):