title: why is the pip install process stuck on ''Installing collected packages" step?
body: <p>I'm trying to pip install some python libraries in a virtual environment created by conda create, but for some packages, the installation were stuck on the step "Installing collected packages: .
  Take pandas as an example:
  My command and output are as follows:</p>
  
  <pre><code>pip install pandas --no-cache-dir
  Collecting pandas
    Downloading https://files.pythonhosted.org/packages/99/12/bf4c58eea94cea4f91ff931f284146337814fb8546e6eb0b52584446fd52/pandas-0.24.1-cp36-cp36m-macosx_10_6_intel.macosx_10_9_intel.macosx_10_9_x86_64.macosx_10_10_intel.macosx_10_10_x86_64.whl (16.3MB)
  100% |████████████████████████████████| 16.3MB 11.4MB/s 
  Requirement already satisfied: numpy&gt;=1.12.0 in /anaconda/envs/testctds2/lib/python3.6/site-packages (from pandas) (1.16.1)
  Requirement already satisfied: pytz&gt;=2011k in /anaconda/envs/testctds2/lib/python3.6/site-packages (from pandas) (2018.9)
  Requirement already satisfied: python-dateutil&gt;=2.5.0 in /anaconda/envs/testctds2/lib/python3.6/site-packages (from pandas) (2.8.0)
  Requirement already satisfied: six&gt;=1.5 in /anaconda/envs/testctds2/lib/python3.6/site-packages (from python-dateutil&gt;=2.5.0-&gt;pandas) (1.12.0)
  Installing collected packages: pandas
  </code></pre>
  
  <p>The process just hang there (at least for 30 min) until I control+z to quit (control+c got no response).</p>
  
  <p>What I have tried:</p>
  
  <ol>
  <li><p>conda install pandas worked well, which is also the recommended way to install pandas. I just don't understand why pip install didn't work, as it's assumed to and this situation also happened to some other libraries such as numpy, scipy, and scikit-learn.</p></li>
  <li><p>I also tried without --no-cache-dir or -vvv to see more details, but in either case there were no more information or error code after the line "Installing collected packages: pandas" </p></li>
  <li><p>I tried the command in a new terminal window. Magically numpy can be installed very quickly by "pip install numpy", but it didn't work with pandas or scipy.</p></li>
  </ol>