title: How should I fork an existing python environment into a new environment using virtualenv?
body: <p>I have installed virtualenv for python. After I have installed some packages such as "nose" etc., I decided to have a try at installing some other packages without affect the former environment.
  I type the command,</p>
  
  <p>virtualenv --system-site-packages --always-copy some_new_env</p>
  
  <p>And it responded,</p>
  
  <p>New python executable in some_new_env\Scripts\python.exe
  Installing setuptools, pip...done.</p>
  
  <p>Then I looked into the folder some_new_env\lib\site-packages\, in it, still only with the following files and folders:</p>
  
  <pre><code>&lt;_markerlib&gt;
  &lt;pip&gt;
  &lt;pip-1.5.6.dist-info&gt;
  &lt;setuptools&gt;
  &lt;setuptools-3.6.dist-info&gt;
  easy_install.py
  easy_install.pyc
  pkg_resources.py
  pkg_resources.pyc
  </code></pre>
  
  <p>Nose etc. installed packages are not installed to this folder. Was the typed command incorrect? How should I type a right command to make those packages installed in the out environment copied to the new environment?</p>