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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 __check_field(self, key):
 """Raises a KeyError if the field doesn't exist."""

        if not self._props.get(key):
            raise KeyError(
                'The field "%s" does not exist on "%s"' % (
                    key, self.__class__.__name__,
                ),
            )