title: str object doesn't support item assignment (when reading bytes)
body: <p>I have two binary data files and I want to replace the contents of part of the second binary data file 
  This is the sample code I have so far </p>
  
  <pre><code>Binary_file1 = open("File1.yuv","rb")
  Binary_file2 = open("File2.yuv","rb")
  
  data1 = Binary_file1.read()
  data2 = Binary_file2.read()
  
  bytes = iter(data1)
  
  for i in range(4, 10):
      data2[i] = next(bytes)  
  </code></pre>
  
  <p>It fails at the part where I equate the data2[i] with next(bytes) and gives me an error saying that “'str' object does not support item assignment” 
  The part I dont understand is that how is this a string object and how can I resolve this error ,Any help would be appreciated . 
  PLease note the Binary files here are huge and I would like to avoid creating duplicate files as I alwyas will run into Memory Issues</p>