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When you set a particular cache key and the item you are setting is larger than the size allotted for a cached item, it fails silently and your key gets set to None. (I know this because I have been bitten by it.)
Memcached uses pickle to cache objects, so at some point new_cache is getting pickled and it’s simply larger than the size allotted for cached items.
The memcached default size is 1MB, and you can increase it, but the bigger issue that seems a bit odd is that that you are using the same key over and over again and your single cached item just gets bigger and bigger.
Wouldn’t a better strategy be to set new items in the cache and to be sure that those items are small enough to be cached?
Anyway, if you want to see how large your item is growing, so you can test whether or not it’s going to go into the cache, you can do some of the following:
>>> import pickle
>>> some_object = [1, 2, 3]
>>> len(pickle.dumps(some_object, -1))
22
>>> new_object = list(range(1000000))
>>> len(pickle.dumps(new_object, -1))
4871352 # Wow, that got pretty big!
Note that this can grow a lot larger if you are pickling Django model instances, in which case it’s probably recommended just to pickle the values you want from the instance.
For more reading, see this other answer:
How to get the size of a python object in bytes on Google AppEngine?