title: How to make 'pip install' not uninstall other versions?
body: <p>I am managing several modules on an HPC, and want to install some requirements for a tool using pip.</p>
  
  <p>I won't use virtualenv because they don't work well with our module system.  I want to install module-local versions of packages and will set <code>PYTHONPATH</code> correctly when the module is loaded, and this has worked just fine when the packages I am installing are not also installed in the default python environment.</p>
  
  <p>What I do not want to do is uninstall the default python's versions of packages while I am installing module-local versions.</p>
  
  <p>For example, one package requires <code>numpy==1.6</code>, and the default version installed with the python I am using is <code>1.8.0</code>.  When I </p>
  
  
  
  <pre class="lang-sh prettyprint-override"><code>pip install --install-option="--prefix=$RE_PYTHON" numpy==1.6
  </code></pre>
  
  <p>where <code>RE_PYTHON</code> points to the top of the module-local site-packages directory, <code>numpy==1.6</code> installs fine, then pip goes ahead and starts uninstalling <code>1.8.0</code> from the tree of the python I am using (why it wants to uninstall a newer version is beyond me but I want to avoid this even when I am doing a local install of e.g. <code>numpy==1.10.1</code>).</p>
  
  <p>How can I prevent pip from doing that?  It is really annoying and I have not been able to find a solution that doesn't involve virtualenv.</p>