Merge pull request #4388 from tacaswell/fix_svg_id

BUG : fix svg corner case

doc/api/api_changes/2015-04-27_svgclip_url.rst

@@ -0,0 +1,13 @@
+Add salt to cilpPath id
+```````````````````````
+
+Add salt to the hash used to determine the id of the ``clipPath``
+nodes.  This is to avoid conflicts in two svg documents with the same
+clip path are included in the same document (see
+https://github.com/ipython/ipython/issues/8133 and
+https://github.com/matplotlib/matplotlib/issues/4349 ), however this
+means that the svg output is no longer deterministic if the same
+figure is saved twice.  It is not expected that this will affect any
+users as the current ids are generated from an md5 hash of properties
+of the clip path and any user would have a very difficult time
+anticipating the value of the id.

lib/matplotlib/backends/backend_svg.py

@@ -10,6 +10,7 @@
 import numpy as np
 
 from hashlib import md5
+import uuid
 
 from matplotlib import verbose, __version__, rcParams
 from matplotlib.backend_bases import RendererBase, GraphicsContextBase,\
@@ -255,7 +256,7 @@ def __init__(self, width, height, svgwriter, basename=None, image_dpi=72):
         self.width = width
         self.height = height
         self.writer = XMLWriter(svgwriter)
-        self.image_dpi = image_dpi # the actual dpi we want to rasterize stuff with
+        self.image_dpi = image_dpi  # the actual dpi we want to rasterize stuff with
 
         self._groupd = {}
         if not rcParams['svg.image_inline']:
@@ -309,9 +310,14 @@ def _write_default_style(self):
 
     def _make_id(self, type, content):
         content = str(content)
+        salt = str(uuid.uuid4())
         if six.PY3:
             content = content.encode('utf8')
-        return '%s%s' % (type, md5(content).hexdigest()[:10])
+            salt = salt.encode('utf8')
+        m = md5()
+        m.update(salt)
+        m.update(content)
+        return '%s%s' % (type, m.hexdigest()[:10])
 
     def _make_flip_transform(self, transform):
         return (transform +
@@ -532,7 +538,7 @@ def close_group(self, s):
 
     def option_image_nocomposite(self):
         """
-        return whether to generate a composite image from multiple images on 
+        return whether to generate a composite image from multiple images on
         a set of axes
         """
         if rcParams['svg.image_noscale']: