Short version:
Callable context variables do not get called during resolution. If
invoked as a simple variable they return something like <function foo
at 0x8c454fc>. If used in a for loop, they blow up the template. I'm
running trunk:7374
Longer version:
In order to delay a *very* expensive DB operation, I decided to put it
in a function and pass the function as a context variable. The
template uses fragment caching with vary_on, so only if the template
fragment is stale would the function be called and invoke the heavy
lifting.
Sounded great to me ... but it blows up with a 500 error.
Simple example:
=========================
def some_view(request):
def foo():
return [1, 2, 3]
return render_to_response('some_template.html', {'foo': foo, })
some_template.html:
{{foo}} ==>> yields <function foo at 0x8c454fc>
{% for x in foo %}{{x}}{% endif %} ==>> 500 error
=========================
The problem seems to be in (or is related to)
template.__init__.Variable._resolve_lookup.
[...]
current = context
for bit in self.lookups:
try: # dictionary lookup
current = current[bit]
## the following fixed the problem (but needs error handling?)
if callable(current):
current = current()
##
except (TypeError, AttributeError, KeyError):
try: # attribute lookup
current = getattr(current, bit)
if callable(current):
if getattr(current, 'alters_data', False):
current =
settings.TEMPLATE_STRING_IF_INVALID
else:
try: # method call (assuming no args
required)
current = current()
[...]
Note there is a check for callable, but only if it's a method. Is
there some reason why callability is OK in some context variables/
expressions and not others?
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