Refactor aggregate method to directly handle expression conversion for group by and aggregation lists

Author: kosiew

python/datafusion/dataframe.py

@@ -546,8 +546,10 @@ def aggregate(
         group_by_list = group_by if isinstance(group_by, list) else [group_by]
         aggs_list = aggs if isinstance(aggs, list) else [aggs]
 
-        group_by_exprs = _to_expr_list(group_by_list)
-        aggs_exprs = _to_expr_list(aggs_list)
+        group_by_exprs = [
+            Expr.column(e).expr if isinstance(e, str) else e.expr for e in group_by_list
+        ]
+        aggs_exprs = [e.expr for e in aggs_list]
         return DataFrame(self.df.aggregate(group_by_exprs, aggs_exprs))
 
     def sort(self, *exprs: Expr | SortExpr | str) -> DataFrame: