title: Python initialise Struct
body: <p>I have a block of memory with binary data. 
  The block was created with <code>ctypes.create_string_buffer</code>, so the data is mutable, and accessible as an array.</p>
  
  <p>Each 32 bits are made up of a pair, a 20 bit unsigned integer, and a 12 bit unsigned integer.</p>
  
  <p>I want to access the nth element pair, and change the values in the memory block.</p>
  
  <p>I have a structure:</p>
  
  <pre><code>from ctypes import *
  class Int(Structure):
      _fields_ = [("first",  c_uint, 20),
                  ("second",  c_uint, 12)]
  </code></pre>
  
  <p>How do I populate the Structure from my data?
  Is there a C like pointer I can set to point the structure at my data?</p>