A key-aware default dictionary implementation.

gistfile1.py

class KeyAwareDefaultDict(dict):
    """
    Like a standard library defaultdict, but pass the key
    to the default factory.
    """
    def __init__(self, default_factory=None):
        self.default_factory = default_factory

    def __getitem__(self, key):
        if key not in self and self.default_factory is not None:
            self[key] = val = self.default_factory(key)
            return val
        else:
            raise KeyError(str(key))


def test_key_aware_default_dict():
    a = KeyAwareDefaultDict(str)
    assert a[5] == '5'
    assert a[4] == '4'
    assert a[(3, 2)] == '(3, 2)'
    try:
        b = KeyAwareDefaultDict()
        b[5]
    except KeyError:
        return
    assert False