title: What is the use of opener argument in built-in open() function?
body: <p>I am playing with open function in python 3.5. I don't understand the usages of opener argument (Last argument) in open function. According to python documentation: A custom opener can be used by passing a callable as opener. </p>
  
  <pre><code>open(file, mode='r', buffering=-1, encoding=None, errors=None, newline=None, closefd=True, opener=None)
  </code></pre>
  
  <p>Can someone explain me why I need opener argument with help of an example.</p>