title: Identifying the dependency relationship for python packages installed with pip
body: <p>When I do a pip freeze I see large number of Python packages that I didn't explicitly install, e.g.</p>
  
  <pre><code>$ pip freeze
  Cheetah==2.4.3
  GnuPGInterface==0.3.2
  Landscape-Client==11.01
  M2Crypto==0.20.1
  PAM==0.4.2
  PIL==1.1.7
  PyYAML==3.09
  Twisted-Core==10.2.0
  Twisted-Web==10.2.0
  (etc.)
  </code></pre>
  
  <p>Is there a way for me to determine why pip installed these particular dependent packages?  In other words, how do I determine the parent package that had these packages as dependencies?  </p>
  
  <p>For example, I might want to use Twisted and I don't want to depend on a package until I know more about not accidentally uninstalling it or upgrading it.</p>