Python search and replace in an xml file

I have an xml called sample.xml. I use minidom to read and parse that xml file. Here is the content of the xml file which I am mostly concerned about:-

<sub>-s ${student.name} -e '${student.subject}' -c junior</sub>

The python script student.py requires two command line arguments to run.

I want to replace whats between -s ${student.name} -e by the first argument which I provide and I want to replace what is between -e '${student.subject}' -c by the second argument I provide to run the script.

Here is my code snippet:-

from xml.dom import minidom
import sys
import re
import commands
from optparse import OptionParser
parser.add_option("-e", "--student",
                    action="store",
                    dest="student",
                    type="str",
                    help="student name")
parser.add_option("-s", "--subject",
                    action="store",
                    dest="subject",
                    type="str",
                    help="Subject name") 
StudentXML = minidom.parse("sample.xml")
class = StudenXML.getElementsByTagName("class")
for stu in class:
        name = stu.getElementsByTagName("sub")[0]
        getName = stu.childNodes[0]
        details = Name.data
        if "-s" in details:
                #replace ${student.name} by sam
        if "-e"  in details:
                # replace ${student.subject} by chemistry

This is the format to run the script:-

python student.py -e sam -s chemistry