how to make a default dict that returns Id of key?

[Chris Smith]

Am I missing some obvious way to have a default dict that returns the key if it is not present? If not, I found this by googling:

from collections import defaultdict

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

>>> lookup = KeyAwareDefaultDict(Id)
>>> lookup[x]
x
>>> lookup[x] = y; lookup[x]
y

[Aaron Meurer]

That's a generalized version of defaultdict. For just returning the
key, you can just use

class default_to_key(dict):
def __missing__(self, key):
return key

Worth noting is a potentially important difference between subclassing
dict and defaultdict: when you lookup a missing entry in a
defaultdict, it gets saved for future lookups

>>> lookup = KeyAwareDefaultDict(Id)
>>> lookup[x]
x
>>> lookup

defaultdict(Lambda(_x, _x), {x: x})

This is wasteful when your lookup function is fast and stateless
(always returns the same output for a given input). If it's not fast
or not stateless, defaultdict is better, but for Id, I think the dict
solution might be preferred.

Aaron Meurer

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[Chris Smith]

Nice! I thought there had to be a better way. Hopefully we are using this in SparseMatrix.