Is there a clever way to pass the key to defaultdict's default_factory?

python dictionary defaultdict
Benjamin Nitlehoo picture Benjamin Nitlehoo · May 26, 2010 · Viewed 15.8k times · Source

A class has a constructor which takes one parameter: 

class C(object):
    def __init__(self, v):
        self.v = v
        ...

Somewhere in the code, it is useful for values in a dict to know their keys.
I want to use a defaultdict with the key passed to newborn default values: 

d = defaultdict(lambda : C(here_i_wish_the_key_to_be))

Any suggestions?

Answer

Jochen Ritzel picture Jochen Ritzel · May 26, 2010

It hardly qualifies as clever - but subclassing is your friend:

class keydefaultdict(defaultdict):
    def __missing__(self, key):
        if self.default_factory is None:
            raise KeyError( key )
        else:
            ret = self[key] = self.default_factory(key)
            return ret

d = keydefaultdict(C)
d[x] # returns C(x)

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