title: python String replace function argument
body: <pre><code>str1 = ["is" , "first"]       
  str2 = ["was" , "second"]          
  str3 = "he is doing his first year graduation"       
  for i in RANGE(len(str1)):      
  str4 = str3.replace(str1[i],str2[i])    
  </code></pre>
  
  <p>Due to the immutable property only the second change ("first" to "second"). How can i make both changes together. Actually I have a huge size of string. I'm new to python, please help me</p>