How do I convert a symbolic math expression (containing variables) into prefix notation?

Question

I am trying to convert math expressions (which contain trigonometric func, log func, etc and variable x ) into prefix form.

1. I have generated the expressions in the form of a pandas dataframe which contains expressions and their derivatives (in 2 separate columns)
2. I need to convert them into prefix notation
3. After running the code, the values are changing to <pythonds.trees.binaryTree.BinaryTree object at 0x00000137A622CC40>

Below is my code:

import sympy as sp
import pandas as pd
import numpy as np
from pythonds.basic import Stack
from pythonds.trees import BinaryTree

def buildParseTree(fpexp):
    fplist = fpexp.split()

    pStack = Stack()
    eTree = BinaryTree('')
    pStack.push(eTree)
    currentTree = eTree

    for i in fplist:
        if i == '(':
            currentTree.insertLeft('')
            pStack.push(currentTree)
            currentTree = currentTree.getLeftChild()

        elif i in ['+', '-', '*', '/','sin','cos','tan','cot','sec','csc','log','sqrt']:
            currentTree.setRootVal(i)
            currentTree.insertRight('')
            pStack.push(currentTree)
            currentTree = currentTree.getRightChild()

        elif i == ')':
            currentTree = pStack.pop()

        elif i.isnumeric() or i=='x':
            try:
                currentTree.setRootVal(i)
                parent = pStack.pop()
                currentTree = parent

            except ValueError:
                raise ValueError("token '{}' is not a valid integer".format(i))

    return eTree


def preorder(self):
    print(self.key)
    if self.leftChild:
        self.leftChild.preorder()
    if self.rightChild:
        self.rightChild.preorder()

def separator(element):

# element=dataset['Answers'][0]
    a=list(element)
    temp_list = []
#     print(a)
    i=0
    while(i<len(a)):
        if(a[i].isalpha() and (a[i]!="x")):
            joinedstring = (''.join(a[i:i+3:]))
            temp_list.append(joinedstring)
            i+=3
        else:
            temp_list.append(a[i])
            i+=1
    return temp_list

ques = []
ans = []
count = 0
x = sp.symbols("x")
i = sp.symbols("i")

for i in range (1,101):
    ques.append("sin"+"("+str(i)+"*"+ "x"+")")
    ques.append("cos"+"("+str(i)+"*"+ "x"+")")
    ques.append("tan"+"("+str(i)+"*"+ "x"+")")
    ques.append("cot"+"("+str(i)+"*"+ "x"+")")
    ques.append("sec"+"("+str(i)+"*"+ "x"+")")
    ques.append("csc"+"("+str(i)+"*"+ "x"+")")
    ques.append("sqrt"+"("+str(i)+"*"+ "x"+")")
    ques.append("log"+"("+str(i)+"*"+ "x"+")")

    
    count+=8


    
for q in ques:
    k=(sp.diff(q))
    ans.append(str(k))

dataset = pd.DataFrame(list(zip(ques,ans)), columns=['Questions','Answers'])
print(dataset)
print(count)


    

## Exporting Data into Excel file
# # saving the excel
# sol.to_excel(file_name) 
## Converting the generated math expressions into lists and separating operators and operands
for ques in dataset['Questions']:
    ques = separator(ques)
    print(ques)
for ans in dataset['Answers']:
    ans = separator(ans)
    print(ans)

## Converting the differentiated expressions into prefix notation

for i in range(count):
    pt = buildParseTree(str(dataset.loc[i].at['Questions']))
    pt.preorder()
    dataset.loc[i].at['Questions'] = pt
    
    pt = buildParseTree(str(dataset.loc[i].at['Answers']))
    pt.preorder()
    dataset.loc[i].at['Answers'] = pt