title: Why are python generator frames' (gi_frame) f_back attribute always none?
body: <p>The title is pretty self-explanatory. I'm doing something like:</p>
  
  <pre><code>gen = obj #some generator instance running
  frame = obj.gi_frame
  prevframe = frame.f_back
  </code></pre>
  
  <p>But I always get <code>None</code> for <code>prevframe</code>. Why is this the case. Also, is there some workaround for this?</p>
  
  <p>CONTEXT: I'm trying to write a simple call stack method to determine what called a particular function.  I'm using twisted manhole and telnetting into a running process, where I then execute these commands but I can't seem to access the previous frames.</p>