#3441: django.template.VariableDoesNotExist.__init__ prototype forces work to be
done up front that is typically thrown away
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Reporter: Brian Harring <[EMAIL PROTECTED]> | Owner: adrian
Status: new | Component: Template system
Version: SVN | Keywords:
Stage: Unreviewed | Has_patch: 1
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Kind of a cornercase, but VariableDoesNotExist derives from Exception,
overriding nothing- meaning the str/repr for it must be done up front.
Thus django.template.resolve_variable raises it via
{{{
raise VariableDoesNotExist, "Failed lookup for key [%s] in %r" % (bits[0],
current)
}}}
So... it's effectively raising VariableDoesNotExist(msg). Problem here is
that a rather large amount of code just catches the exception, and
substitutes a different value, TEMPLATE_STRING_IF_INVALID for example- in
other words, they don't even *care* about the msg, the exception is just a
passing mechanism. Thus the msg creation is pure overhead to most
codeblocks I've found in django.
Said overhead can add up; a context level lookup ( {{ foo }} for example),
raising VariableDoesNotExist results in repr(context), repr'ing all
mappings in context- if say your base template is just checking for a var,
if so, outputing a queryset, else not outputting it, the current code
forces a repr on context, in other words forcing the query to be executed.
Which is fairly daft, if pretty much all code just throws out the
exception, substituting a different value in.
Attached is a patch changing VariableDoesNotExist's __init__ to (self,
msg, params=()); via that, can delay the actual msg creation until it's
required, making catching the exception just to substitute a different
value far cheaper.
Within the code, only resolve_variable throws the exception- google code
search, nobody else is throwing it, thus it looks pretty safe to change
the __init__ to delay the cost.
Alternative, just derive NewStyleVariableDoesNotExist from
VariableDoesNotExist, and have resolve_variable throw that; either way,
it's work being done that isn't needed so should be tweaked.
--
Ticket URL: <http://code.djangoproject.com/ticket/3441>
Django Code <http://code.djangoproject.com/>
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