title: Python packages not installing in virtualenv using pip
body: <p><strong>I'm having trouble installing twisted</strong></p>
  
  <pre><code>pip --version
  </code></pre>
  
  <blockquote>
    <p>pip 1.1 from
    /home/chris/GL/GLBackend/glenv/lib/python2.7/site-packages/pip-1.1-py2.7.egg
    (python 2.7)</p>
  </blockquote>
  
  <p><strong>Create a virtual environment</strong></p>
  
  <pre><code>chris@chris-mint ~/GL/GLBackend $ sudo virtualenv -p python2.7 glenv
  </code></pre>
  
  <blockquote>
    <p>Running virtualenv with interpreter /usr/bin/python2.7 New python
    executable in glenv/bin/python2.7 Also creating executable in
    glenv/bin/python Installing
    distribute.............................................................................................................................................................................................done.
    Installing pip...............done.</p>
  </blockquote>
  
  <p><strong>Just in case, I'll enable all permissions</strong></p>
  
  <pre><code>chris@chris-mint ~/GL/GLBackend $ sudo chmod -R 777 glenv
  
  chris@chris-mint ~/GL/GLBackend $ source glenv/bin/activate
  
  
  (glenv)chris@chris-mint ~/GL/GLBackend $ pip freeze
  </code></pre>
  
  <blockquote>
    <p>argparse==1.2.1 distribute==0.6.24 wsgiref==0.1.2</p>
  </blockquote>
  
  <p><strong>twisted is not listed here as installed</strong></p>
  
  <pre><code>(glenv)chris@chris-mint ~/GL/GLBackend $ sudo pip install twisted
  </code></pre>
  
  <blockquote>
    <p>Requirement already satisfied (use --upgrade to upgrade): twisted in
    /usr/local/lib/python2.7/dist-packages Requirement already satisfied
    (use --upgrade to upgrade): zope.interface>=3.6.0 in
    /usr/local/lib/python2.7/dist-packages (from twisted) Requirement
    already satisfied (use --upgrade to upgrade): distribute in
    /usr/local/lib/python2.7/dist-packages (from
    zope.interface>=3.6.0->twisted) Cleaning up... (glenv)chris@chris-mint
    ~/GL/GLBackend $ pip uninstall twisted Cannot uninstall requirement
    twisted, not installed Storing complete log in
    /home/chris/.pip/pip.log</p>
  </blockquote>
  
  <p><strong>But when I install it it says that its already installed.
  Force the install:</strong></p>
  
  <pre><code>sudo pip install -I twisted
  </code></pre>
  
  <blockquote>
    <p>Downloading/unpacking twisted Downloading Twisted-12.3.0.tar.bz2
    (2.6Mb): 2.6Mb downloaded Running setup.py egg_info for package
    twisted
         . . .</p>
    
    <p>Successfully installed twisted zope.interface distribute Cleaning
    up...</p>
  </blockquote>
  
  <p><strong>And yet it still isn't installed</strong></p>
  
  <pre><code>(glenv)chris@chris-mint ~/GL/GLBackend $ pip freeze
  </code></pre>
  
  <blockquote>
    <p>argparse==1.2.1 distribute==0.6.24 wsgiref==0.1.2</p>
  </blockquote>
  
  <p>**When I try running Python scripts which use twisted, I get an error saying that twisted is not installed. That is:</p>
  
  <blockquote>
    <p>ImportError: No module named twisted.python**</p>
  </blockquote>