title: requirements.txt in python package
body: <p>I'm studying packages, and I have a question on creating a requirements.txt file.</p>
  
  <p>If a module is imported from python, does it need to be mentioned in the requirements.txt file or can be left out?</p>
  
  <p>I know that for separate modules, they need to be pip installed. For those modules that need to be pip installed, should they be written in the requirements.txt file as 'pip install modulename'?</p>