title: Python defaultdict with string as type of value
body: <p>Need to create a defaultdict, value type is normal Python string, it seems my below method does not work? Post compile error message. Using Python 2.7 and wondering any good ideas how to fix? Thanks.</p>
  
  <p><strong>Code</strong></p>
  
  <pre><code>import collections
  
  a = collections.defaultdict("")
  
  a[1]="Hello"
  a[2]="World"
  
  print a
  </code></pre>
  
  <p><strong>Error Message</strong></p>
  
  <pre><code>  a = collections.defaultdict("")
  TypeError: first argument must be callable
  </code></pre>