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try:
import cPickle as pickle
except ImportError:
import pickle
from datetime import datetime
from django.core.cache import cache
from django.utils.hashcompat import md5_constructor
CACHE_TIMEOUT = 1800
INVALIDATE_TIMEOUT = 10
CACHE_LATEST_PREFIX = 'cache_latest_'
def cache_function_key(func, args, kwargs):
raw = [func.__name__, func.__module__, args, kwargs]
pickled = pickle.dumps(raw, protocol=pickle.HIGHEST_PROTOCOL)
key = md5_constructor(pickled).hexdigest()
return 'cache_function.' + func.__name__ + '.' + key
def cache_function(length):
"""
A variant of the snippet posted by Jeff Wheeler at
http://www.djangosnippets.org/snippets/109/
Caches a function, using the function and its arguments as the key, and the
return value as the value saved. It passes all arguments on to the
function, as it should.
The decorator itself takes a length argument, which is the number of
seconds the cache will keep the result around.
"""
def decorator(func):
def inner_func(*args, **kwargs):
key = cache_function_key(func, args, kwargs)
value = cache.get(key)
if value is not None:
return value
else:
result = func(*args, **kwargs)
cache.set(key, result, length)
return result
return inner_func
return decorator
def clear_cache_function(func, args, kwargs):
key = cache_function_key(func, args, kwargs)
cache.delete(key)
# utility to make a pair of django choices
make_choice = lambda l: [(str(m), str(m)) for m in l]
# These are in here because we would be jumping around in some import circles
# and hoops otherwise. The only thing currently using these keys is the feed
# caching stuff.
def refresh_latest(**kwargs):
'''A post_save signal handler to clear out the cached latest value for a
given model.'''
cache_key = CACHE_LATEST_PREFIX + kwargs['sender'].__name__
# We could delete the value, but that could open a race condition
# where the new data wouldn't have been committed yet by the calling
# thread. Instead, explicitly set it to None for a short amount of time.
# Hopefully by the time it expires we will have committed, and the cache
# will be valid again. See "Scaling Django" by Mike Malone, slide 30.
cache.set(cache_key, None, INVALIDATE_TIMEOUT)
def retrieve_latest(sender):
# we could break this down based on the request url, but it would probably
# cost us more in query time to do so.
cache_key = CACHE_LATEST_PREFIX + sender.__name__
latest = cache.get(cache_key)
if latest:
return latest
try:
latest_by = sender._meta.get_latest_by
latest = sender.objects.values(latest_by).latest()[latest_by]
# Using add means "don't overwrite anything in there". What could be in
# there is an explicit None value that our refresh signal set, which
# means we want to avoid race condition possibilities for a bit.
cache.add(cache_key, latest, CACHE_TIMEOUT)
return latest
except sender.DoesNotExist:
pass
return None
def set_created_field(sender, **kwargs):
'''This will set the 'created' field on any object to datetime.utcnow() if
it is unset. For use as a pre_save signal handler.'''
obj = kwargs['instance']
if hasattr(obj, 'created') and not obj.created:
obj.created = datetime.utcnow()
# vim: set ts=4 sw=4 et:
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