code: def do_xmlattr(_eval_ctx, d, autospace=True):
      
      rv = u' '.join(
          u'%s="%s"' % (escape(key), escape(value))
          for key, value in d.iteritems()
          if value is not None and not isinstance(value, Undefined)
      )
      if autospace and rv:
          rv = u' ' + rv
      if _eval_ctx.autoescape:
          rv = Markup(rv)
      return rv
docs: Create an SGML/XML attribute string based on the items in a dict.
      All values that are neither `none` nor `undefined` are automatically
      escaped:
  
      .. sourcecode:: html+jinja
  
          <ul{{ {'class': 'my_list', 'missing': none,
                  'id': 'list-%d'|format(variable)}|xmlattr }}>
          ...
          </ul>
  
      Results in something like this:
  
      .. sourcecode:: html
  
          <ul class="my_list" id="list-42">
          ...
          </ul>
  
      As you can see it automatically prepends a space in front of the item
      if the filter returned something unless the second parameter is false.