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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 variable_names(self):
 """ Get all variable names required for this query """

        if self._variable_names is None:

            if self._operator is None:
                if self._operands is None:
                    self._variable_names = tuple()
                else:
                    self._variable_names = self._get_variable_names(self._operands)

            elif self._operator == 'NOT':
                self._variable_names = self._operands.variable_names

            else:
                v = list()
                for op in self._operands:
                    v.extend(op.variable_names)
                self._variable_names = tuple(set(v))

        return self._variable_names