Currently, we're writing templates like the following to escape
literal substitutions in script blocks:

  $.validated.decorateError($(${"[name=%s]" % script(key)|script,n},
form), ${unicode(error)|script,n});

  <a href="javascript:deleteEmail(${script(email.id)},
${script(reply.id)});void 0;">Delete email</a>

In the above, script is just:

  def script(string): return re.sub('</', r'<\/', simplejson.dumps(string))

The intent is basically the following unsafe versions of the above:

  $.validated.decorateError($("[name='${key}']", form), "${error}");

  <a href="javascript:deleteEmail('${email.id}', '${reply.id}'); void
0;">Delete email</a>

Is this the simplest way to escape literals in scripts?

Are there any clever tools that might understand the context from
which substitutions are being invoked, and select the right filters
accordingly?

Thanks.
--
Yang Zhang
http://yz.mit.edu/

Have you looked at Mako's filtering syntax? It is perhaps just a little
simpler than how you're doing it.  You can even define your own filters.

   http://www.makotemplates.org/docs/filtering.html

In fact, you're mostly there. Just make sure your script() function
is defined in global scope (or import it from a python module),

<%!
    import re
    def script(string):
        return re.sub('</', r'<\/', simplejson.dumps(string))
%>

And then use it, like:

   ${email.id | script}

As far as escaping </ into <\/ inside Javascript, which I suppose
is to protect against it matching an HTML/XML end tag, I'm sure
there are probably other better methods.  If you are outputting
XHTML for example you could/should put all your Javascript
inside CDATA blocks.

However, when you're mixing in Javascript inside of HTML
attribute values (opposed to <script> elements), then more
correct would be to use the built-in |h filter rather than your regular
expression.  You can then define a filter which only converts
values to javascript, but without the re part.  Also note I'm using
the \-end of line syntax to keep things more readable.

<%!
    import simplejson
    def jsliteral( value ):
        return simplejson.dumps( value )
%>

<a href="javascript:deleteEmail(\
${ email.id | jsliteral, h }\
,\
${ reply.id | jsliteral, h }\
);\
void 0;\
">Delete email</a>

I'm not quite sure what you're attempting to do with the
validated.decorateError, so I can't comment on that.

Not that I know of. Filters are generally idempotent and unaware
of their calling context.

--
Deron Meranda
http://deron.meranda.us/