Unable to efficiently retrieve keys by tag #341

I have a number of keys organized by tags and would like to get a list of keys with a certain tag. However, I cannot find a straightforward way to do this.

Current workaround:

for key in cache.iterkeys():
    if cache.get(key, tag=True)[-1] == 'counter':
        # ...

This approach is awkward and inefficient. It is of course possible to do

select = "SELECT key, raw, value FROM Cache WHERE tag = ?"
for key, raw, value in cache._sql(select, ('counter',))}:
        # get key, value from key and raw
        

but I would prefer not messing with the internal of diskcache

Proposed enhancement: It would be beneficial to add a tag parameter to the iterkeys method to allow:

    def iterkeys(self, reverse=False, tag=False):

This enhancement would simplify the code and improve efficiency when working with tagged keys.