In which case pip install building wheel? : <p>I found that in different folders, sometimes "pip install" will build wheel which take a lot of time, while sometimes it won't. I'm not sure why's that and how to control that. Anyone can help on this.</p>

<p>The command I use: "bin/python -m pip install -r ../requirements.txt" (due to shebang line length limitation, so don't use pip directly)</p>

<p>The output without building wheel (just take a few seconds)</p>

<pre><code>Collecting numpy==1.10.4 (from -r ../requirements.txt (line 1))
Installing collected packages: numpy
Successfully installed numpy-1.10.4
</code></pre>

<p>The output with building wheel (take at least 2 minutes)</p>

<pre><code>Collecting numpy==1.10.4 (from -r ../requirements.txt (line 1))
  Downloading numpy-1.10.4.tar.gz (4.1MB)
    100% |████████████████████████████████| 4.1MB 92kB/s
Building wheels for collected packages: numpy
  Running setup.py bdist_wheel for numpy ... done
  Stored in directory: /root/.cache/pip/wheels/66/f5/d7/f6ddd78b61037fcb51a3e32c9cd276e292343cdd62d5384efd
Successfully built numpy
Installing collected packages: numpy
Successfully installed numpy-1.10.4
</code></pre>

<p>The contents of requirements.tt</p>

<pre><code>numpy==1.10.4
</code></pre>
