title: Pass South random unique default values when migrating
body: <p>I am trying to forward migrate a model with existing data. The model has a new field with constraints unique=True and null=False. 
  When I do</p>
  
  <pre><code>./manage.py schemamigration myapp --auto
  </code></pre>
  
  <p>South lets me specify a default value for the new field by asking:</p>
  
  <pre><code>Specify a one-off value to use for existing columns now
  </code></pre>
  
  <p>Usually I set this to None but since this field needs to be unique I was wondering if it is possible to pass South a unique value via:</p>
  
  <pre><code> &gt;&gt;&gt; import uuid; uuid.uuid1().hex[0:35]
  </code></pre>
  
  <p>This gives me an error message </p>
  
  <pre><code>! Invalid input: invalid syntax 
  </code></pre>
  
  <p>Any ideas if it is possible to pass South random unique default values when migrating via the commandline?</p>
  
  <p>Thanks.</p>