django.template.VariableDoesNotExist.__init__ prototype forces work to be done up front that is typically thrown away

[(removed)]

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.

[(removed)]

VariableDoesNotExist.init refactoring.

[Jacob]

Fixed in [4461].