title: Unable to import mock in test using pytest
body: <p>I have a test file <code>tests.py</code>. Currently, to help isolate the problem, the file contains only the following line:</p>
  
  <pre><code>from mock import Mock
  </code></pre>
  
  <p>When I try to run this file with <code>pytest</code>:</p>
  
  <pre><code>$ py.test tests.py
  </code></pre>
  
  <p>I get the following error output:</p>
  
  <pre><code>tests.py:1: in &lt;module&gt;
      from mock import Mock
  E   ImportError: No module named mock
  =========================== 1 error in 0.00 seconds ============================
  </code></pre>
  
  <p>If I run the file with <code>python</code>:</p>
  
  <pre><code>$ python tests.py
  </code></pre>
  
  <p>I do not get this error. Similarly, if I open an interactive interpreter, I am able to import <code>mock</code> without issue. <strong>Why am I not able to access <code>mock</code> in this context?</strong> </p>