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
<https://stackoverflow.com/questions/10524142/python-dictionary-return-requested-key-if-value-does-not-exist>
:
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
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