interface versioned dictionary

i have an versioned request store i wish opening by an dict interface.
common use opening latest supplement (get, set, del), nonetheless should means opening specific revisions too (keys always str/unicode int).

from userdict import dictmixin
class vdict(dictmixin):   
    def __getitem__(self, key):
        isinstance(key, tuple):
            docid, rev = key
        else:
            docid = key
            rev = zero  # set tip rev

        imitation docid, rev
        # relapse ...

in [1]: d = vdict()

in [2]: d[2]
2 none

in [3]: d[2, 1]
2 1

this fortitude small bit machiavellian i'm certain clean, graphic interface. should i yield duty

def getrev(self, docid, rev):
   ...

instead?