Markdown in Python: Footnote IDs can become tangled when multiple documents are on the same page.

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If you emit multiple footnote-containing documents onto one HTML page, and their authors both used, e.g., a footnote labeled "[^1]", they will be assigned identical anchor tags, and the footnote links for whichever document is second on the page will not work.

I have a patch that adds a 'UNIQUE_IDS' option to the footnotes extension.  When that option is enabled, each separate document (as determined by a call to 'reset()') processed by the same 'Markdown' object will have its own namespace for footnote IDs and all links will be usable.

diff --git a/markdown/extensions/footnotes.py b/markdown/extensions/footnotes.py
	index 6dacab7..751779d 100644
	--- a/markdown/extensions/footnotes.py
	+++ b/markdown/extensions/footnotes.py
	@@ -38,11 +38,18 @@ class FootnoteExtension(markdown.Extension):
	         """ Setup configs. """
	         self.config = {'PLACE_MARKER':
	                        ["///Footnotes Go Here///",
	-                        "The text string that marks where the footnotes go"]}
	+                        "The text string that marks where the footnotes go"],
	+                       'UNIQUE_IDS':
	+                       [False,
	+                        "Whether to prevent footnote name collisions in "
	+                        "one Markdown object across calls to reset()."]}
	 
	         for key, value in configs:
	             self.config[key][0] = value
	-            
	+
	+        # In multiple invocations, emit links that don't get tangled.
	+        self.unique_prefix = 0
	+
	         self.reset()
	 
	     def extendMarkdown(self, md, md_globals):
	@@ -66,8 +73,9 @@ class FootnoteExtension(markdown.Extension):
	                                   ">amp_substitute")
	 
	     def reset(self):
	-        """ Clear the footnotes on reset. """
	+        """ Clear the footnotes on reset, and prepare for a distinct document. """
	         self.footnotes = markdown.odict.OrderedDict()
	+        self.unique_prefix += 1
	 
	     def findFootnotesPlaceholder(self, root):
	         """ Return ElementTree Element that contains Footnote placeholder. """
	@@ -91,11 +99,17 @@ class FootnoteExtension(markdown.Extension):
	 
	     def makeFootnoteId(self, id):
	         """ Return footnote link id. """
	-        return 'fn:%s' % id
	+        if self.getConfig("UNIQUE_IDS"):
	+            return 'fn:%d-%s' % (self.unique_prefix, id)
	+        else:
	+            return 'fn:%s' % id
	 
	     def makeFootnoteRefId(self, id):
	         """ Return footnote back-link id. """
	-        return 'fnref:%s' % id
	+        if self.getConfig("UNIQUE_IDS"):
	+            return 'fnref:%d-%s' % (self.unique_prefix, id)
	+        else:
	+            return 'fnref:%s' % id
	 
	     def makeFootnotesDiv(self, root):
	         """ Return div of footnotes as et Element. """

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