def newOperator(self, key, operands):
    try:
        operator = self._operators[key]
        op = operator['imp'](operands)
        op.name = key
        op.category = operator['category']
        return op
    except:
        raise ExpressionError(('error with operator: ' + str(key)))
def newOperator(self, key, operands):
    try:
        operator = self._operators[key]
        op = operator['imp'](operands)
        op.name = key
        op.category = operator['category']
        return op
    except:
        raise ExpressionError(('error with operator: ' + str(key)))