code: def __str__(self) -> str:
      expr_str = ""
      if self.etype:
          expr_str = f"{str(self.get_symbol())}()"
      return expr_str
  
file_path: evadb/evadb/expression/aggregation_expression.py
imports: from evadb.expression.abstract_expression import AbstractExpression, ExpressionReturnType, ExpressionType
  from evadb.models.storage.batch import Batch
label_desc: Use image node_id 5 for calling the AggregationExpression obj's underlying member method code with example usage: obj.__str__() and returns: expr_str