title: Install python packages with requirements.txt
body: <p>When I install packages with <code>pip install -r requirements.txt</code> after creating a virtual environment, they get installed but then when I run my python scripts, I still get the <code>ModuleNotFoundError</code></p>
  
  <p>I have also tried <code>pip install -r requirements.txt -t lib</code> but I still get the error. </p>
  
  <p>Is there anything more required to install those packages. I will appreciate your help. Thanks a lot in advance.</p>
  
  <p><strong>UPDATE</strong></p>
  
  <p><em>requirements.txt</em></p>
  
  <pre><code>pandas
  numpy
  scikit-learn
  flask
  flask_restful
  scipy
  sqlalchemy
  </code></pre>
  
  <blockquote>
    <p>ModuleNotFoundError: No module named 'flask'</p>
  </blockquote>
  
  <p>Basically, all modules give me an error. I have to download them separately one by one to get the script working.</p>