title: Replace line containing string with Python
body: <p>I'm trying to search through a file for a line that contain certain text and then replace that entire line with a new line.</p>
  
  <p>I'm trying to use:</p>
  
  <pre><code>pattern = "Hello"
  file = open('C:/rtemp/output.txt','w')
  
  for line in file:
      if pattern in line:
          line = "Hi\n"
          file.write(line)
  </code></pre>
  
  <p>I get an error saying:</p>
  
  <pre><code>io.UnsupportedOperation: not readable
  </code></pre>
  
  <p>I'm not sure what I'm doing wrong, please can someone assist.</p>