title: python: UnboundLocalError: local variable 'open' referenced before assignment
body: <pre><code>def read_lines():
      readFileName = "readfile.txt"
      f = open(readFileName, 'r+')
      contents = f.read()
          ... # and so on 
  
  read_lines()
  </code></pre>
  
  <p>When I run this, I get an error:</p>
  
  <pre><code>f = open(readFileName, 'r+')
  UnboundLocalError: local variable 'open' referenced before assignment
  </code></pre>