Make variables that have not been found global

This will allow tools to work that access variables from
the enclosing scope in a list/dict/tuple comprehension.

lib/galaxy/util/template.py

@@ -1,10 +1,19 @@
 """Entry point for the usage of Cheetah templating within Galaxy."""
+from __future__ import absolute_import
+
+import logging
+import sys
+import traceback
+
+from Cheetah.NameMapper import NotFound
 from Cheetah.Template import Template
 
 from . import unicodify
 
+log = logging.getLogger(__name__)
 
-def fill_template(template_text, context=None, **kwargs):
+
+def fill_template(template_text, context=None, retry=10, **kwargs):
     """Fill a cheetah template out for specified context.
 
     If template_text is None, an exception will be thrown, if context
@@ -15,4 +24,14 @@ def fill_template(template_text, context=None, **kwargs):
         raise TypeError("Template text specified as None to fill_template.")
     if not context:
         context = kwargs
-    return unicodify(Template(source=template_text, searchList=[context]))
+    try:
+        return unicodify(Template(source=template_text, searchList=[context]))
+    except NotFound as e:
+        if retry > 0 and sys.version_info.major > 2:
+            tb = e.__traceback__
+            if traceback.extract_tb(tb)[-1].name == '<listcomp>':
+                arg_to_set_global = e.args[0].split("'")[1]
+                log.warning("Replacing variable %s", arg_to_set_global)
+                new_template = "#set global $%s = None\n" % arg_to_set_global
+                return fill_template(template_text=new_template + template_text, context=context, retry=retry - 1, **kwargs)
+        raise