This can be accomplished with defaultdict, too. It is instantiated with a callable which is then called when a nonexisting element is accessed.
from collections import defaultdict
d = defaultdict(noisy_default)
d[1] # noise
d[1] # no noise
The caveat with defaultdict is that the callable gets no arguments, so you can not derive the default value from the key as you could with dict.setdefault. This can be mitigated by overriding __missing__ in a subclass:
from collections import defaultdict
class defaultdict2(defaultdict):
def __missing__(self, key):
value = self.default_factory(key)
self[key] = value
return value
def noisy_default_with_key(key):
print key
return key + 1
d = defaultdict2(noisy_default_with_key)
d[1] # prints 1, sets 2, returns 2
d[1] # does not print anything, does not set anything, returns 2
For more information, see the collections module.