title: strange exceptions.SystemExit in Python 2.7
body: <p>Here is my code and error message, anyone have any ideas why there are such exception? Thanks.</p>
  
  <p><strong>Source Code</strong>,</p>
  
  <pre><code>import sys
  import tensorflow as tf
  
  def main(argv):
    print 'in main'
  def f():
      # this method will call def main(argv)
      try:
          tf.app.run()
      except:
          print "tf.app.run error ", sys.exc_info()
  
  if __name__ == "__main__":
      f()
  </code></pre>
  
  <p><strong>Error Code</strong>,</p>
  
  <pre><code>in main
  tf.app.run error  (&lt;type 'exceptions.SystemExit'&gt;, SystemExit(), &lt;traceback object at 0x10fa33f38&gt;)
  </code></pre>