title: How can one mark a flag as required with gflags?
body: <p>I use gflags like this:</p>
  
  <pre><code>flags.DEFINE_string('logdir', None,
                      'Directory where logs are stored.')
  </code></pre>
  
  <p>However, I would like gflags to show the help when somebody does not define <code>--logdir</code>. How can I make this flag required?</p>
  
  <p>(<a href="https://github.com/gflags/python-gflags/blob/master/tests/gflags_validators_test.py#L189" rel="noreferrer">This</a> looks a bit as if it should be possible, but I couldn't find any documentation about how to use gflags with Python.)</p>