add ascii_only filter

IPython/nbconvert/exporters/templateexporter.py

@@ -62,6 +62,7 b' default_filters = {'
         'citation2latex': filters.citation2latex,
         'path2url': filters.path2url,
         'add_prompts': filters.add_prompts,
+        'ascii_only': filters.ascii_only,
 }
 
 #-----------------------------------------------------------------------------

IPython/nbconvert/filters/strings.py

@@ -43,7 +43,8 b' __all__ = ['
     'ipython2python',
     'posix_path',
     'path2url',
-    'add_prompts'
+    'add_prompts',
+    'ascii_only',
 ]
 
 
@@ -213,3 +214,8 b' def path2url(path):'
     """Turn a file path into a URL"""
     parts = path.split(os.path.sep)
     return '/'.join(quote(part) for part in parts)
+
+def ascii_only(s):
+    """ensure a string is ascii"""
+    s = py3compat.cast_unicode(s)
+    return s.encode('ascii', 'replace').decode('ascii')
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