title: what's the use of lambda in this case
body: <p>I used lambda several times. When reading this tutorial <a href="https://pypi.python.org/pypi/django-autoslug" rel="nofollow">django-autoslug</a></p>
  
  <p>there are two examples:</p>
  
  <pre><code>slug = AutoSlugField(populate_from='title')
  </code></pre>
  
  <p>and more complex:</p>
  
  <pre><code>slug = AutoSlugField(populate_from=lambda instance: instance.title,
                           unique_with=['author__name', 'pub_date__month'],
                           slugify=lambda value: value.replace(' ','-'))
  </code></pre>
  
  <p>I'm interested in that row: <code>populate_from='title'</code> and <code>populate_from=lambda instance: instance.title</code>
  <strong>What's the difference?</strong>
  And can you give some tutorial(not documentation) with examples of <code>lambda</code>?</p>