BUG : fix svg corner case

tacaswell · tacaswell · commit 66e9fa21039c · 2015-04-28T00:23:19.000-04:00
Under certain conditions the ``id`` of the `clipPath` node emitted by the svg writer will be the same for multiple files. If both svg fragments are embedded in the same document, then one of them is removed, the clipping of the remaining fragment can become inconsistent. See ipython/ipython#8133 Closes matplotlib#4349
diff --git a/doc/api/api_changes/2015-04-27_svgclip_url.rst b/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.
diff --git a/lib/matplotlib/backends/backend_svg.py b/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']:
