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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def is_default(self):
 """ ``True`` if the item's value is its default value or if no value and no default value are set. If the item is backed by an environment variable, this will be ``True`` only if the environment variable is set and is different to the default value of the item. """

        envvar_value = self._get_envvar_value()
        if envvar_value is not not_set:
            return envvar_value == self.default
        else:
            return self._value is not_set or self._value == self.default