How to edit a text file at a specific line or location

I have a text file of the format below, and I am trying to edit/update text in the file.

VAR_GROUP
Var1 : DATATYPE1;(Description Var1)
Var2 : DATATYPE2;(Text to be added here)
Var3 : DATATYPE3;(Description Var3)
Var4 : DATATYPE4;(Text to be added here)
END_GROUP

Using Python I am trying to add certain description of for eg., Var3 and Var4. With code I wrote the logic is working fine but the text is added to the end of the file and not at the required position.

def search_write_in_file(file_name, string_to_search, description):
with open(file_name, 'r+') as file_obj:
    # Read all lines in the file
    for line in file_obj:
        # For each line, check if line contains the string
        line_number += 1
        if (string_to_search in line) and flag_found == 0:
            line = line[:-1]+description+'\n'
            file_obj.write(line)
            flag_found =1

read_obj.close()

Current Output
VAR_GROUP
Var1 : DATATYPE1;(Description Var)
Var2 : DATATYPE2;
Var3 : DATATYPE3;(Description Var3)
Var4 : DATATYPE4;
END_GROUP
Var1 : DATATYPE1;(Description Var1)
Var2 : DATATYPE2;(Description Var2)
Var3 : DATATYPE3;(Description Var3)
Var4 : DATATYPE4;(Description Var4)

What could be the possible reason that the the mentioned specific location is not edited, rather added at the end. Thanks in advance.

I'd use regular expressions to match and replace text inside your file

import re

def search_write_in_file(file_name, string_to_search, description):
    with open(file_name, 'r+') as file_obj:
        text = file_obj.read()
    new_text = re.sub(string_to_search,r"\1 {0}\n".format(description),text)
    with open(file_name, 'w') as file_obj:
        file_obj.write(new_text)
    print(new_text)

if __name__ == '__main__':
    search_write_in_file('text_to_edit.txt',r'(DATATYPE2;\n)',"test2")
    search_write_in_file('text_to_edit.txt',r'(DATATYPE4;\n)',"test4")

This will update the existing file to be

VAR_GROUP
Var1 : DATATYPE1;(Description Var)
Var2 : DATATYPE2; test2
Var3 : DATATYPE3;(Description Var3)
Var4 : DATATYPE4; test4
END_GROUP