Add utility cache_function

This allows caching the results of an arbitrary function and its arguments
in the Django-managed cache, e.g. memcached in production.

Signed-off-by: Dan McGee <dan@archlinux.org>

main/utils.py

@@ -1,2 +1,37 @@
+try:
+    import cPickle as pickle
+except ImportError:
+    import pickle
+from django.utils.hashcompat import md5_constructor
+
+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):
+            from django.core.cache import cache
+
+            raw = [func.__name__, func.__module__, args, kwargs]
+            pickled = pickle.dumps(raw, protocol=pickle.HIGHEST_PROTOCOL)
+            key = md5_constructor(pickled).hexdigest()
+            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
+
 #utility to make a pair of django choices
 make_choice = lambda l: [(str(m), str(m)) for m in l]