"Lazy load" of data from a context processor
If you pass a callable object into the template context, Django will evaluate it when it is used in the template. This provides one simple way to do laziness — just pass in callables:
def my_context_processor(request):
def complicated_query():
do_stuff()
return result
return {'my_info': complicated_query}
The problem with this is it does not memoize the call — if you use it multiple times in a template, complicated_query
gets called multiple times.
The fix is to use something like SimpleLazyObject
as in the other answer, or to use something like functools.lru_cache
:
from functools import lru_cache:
def my_context_processor(request):
@lru_cache()
def complicated_query():
result = do_stuff()
return result
return {'my_info': complicated_query}
You can now use my_info
in your template, and it will be evaluated lazily, just once.
Or, if the function already exists, you would do it like this:
from somewhere import complicated_query
def my_context_processor(request):
return {'my_info': lru_cache()(complicated_query)}
I would prefer this method over SimpleLazyObject
because the latter can produce some strange bugs sometimes.
(I was the one who originally implemented LazyObject
and SimpleLazyObject
, and discovered for myself that there is curse on any code artefact labelled simple.)
Django has a SimpleLazyObject
. In Django 1.3, this is used by the auth context processor (source code). This makes user
available in the template context for every query, but the user is only accessed when the template contains {{ user }}
.
You should be able to do something similar in your context processor.
from django.utils.functional import SimpleLazyObject
def my_context_processor(request):
def complicated_query():
do_stuff()
return result
return {
'result': SimpleLazyObject(complicated_query)