title: What does sys.stdin read?
body: <p>I get how to open files, and then use Python's pre built in functions with them. But how does sys.stdin work? </p>
  
  <pre><code>for something in sys.stdin:
      some stuff here
  
  lines = sys.stdin.readlines()
  </code></pre>
  
  <p>What's the difference between the above two different uses on sys.stdin? Where is it reading the information from? Is it via keyboard, or do we still have to provide a file?</p>