Dictionary with .default value

DefaultDict.py

class DefaultDict(dict):
    """Returns DefaultDict.default value for missing key, or raises KeyError if default has not been set."""
    def __missing__(self, key):
        if getattr(self, 'default', None):
            return self.default
        raise KeyError(
            f"Key '{key}' does not exist and default has not been set!"
        )