title: When does "pip install" build a wheel?
body: <p>I found that in different folders, sometimes <code>pip install</code> will build wheel which takes a lot of time, while sometimes it doesn't. I'm not sure why and how to control that.</p>
  <p>My command: <code>bin/python -m pip install -r ../requirements.txt</code> (due to the <code>!#</code> shebang line-length limitation, so I don't use pip directly)</p>
  <p>The output without building a wheel (just takes 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 <code>requirements.txt</code>:</p>
  <pre><code>numpy==1.10.4
  </code></pre>