[patch] Make template system scoped to the parser

[rjwittams]

So currently, template tags and filters are loaded into module level dictionaries.
This means that tags and filters are available in places that they haven't been loaded.

What this leads to is namespace pollution by previous requests - so your code might work one minute, as a previous request loaded the lib, and break the next if you are the first request.

So this patch moves filters and tags onto parsers. As a bonus, filters are now parsed at compile time rather than runtime, so they don't get parsed multiple times in loops.

The changes are:
At the top of a tag/filter library,
do

import django.core.template 

register = template.Library()

then to register tags or filters, use

register.tag('name', func)
register.filter('name', func) # No has_arg, see below

In tags, resolve_variable_with_filters is gone.
The functionality is split in two.
In your compile function, do

filter_expr =  parser.compile(filter_string)

pass this into your node class as usual.

in render, do

filter_result = filter_expr.resolve(context) 

instead of resolve_variable_with_filters.

the tag decorators are also now members of the library class, so do

@register.simple_tag

and

@register.inclusion_tag('cow_detail')

for those.

Filters can now have default arguments, and whether they have an argument is inferred from the signature. So any

def filter_func(obj, _):

should be changed to

def filter_func(obj):

and the has_arg function is no longer accepted or useful.
Filter arguments can be also now be variables which are resolved in the context as well as constant strings.

[Adrian Holovaty]

(Fixed formatting in description.)