title: What is the Python test file type and identifiers?
body: <p>I am reading this article: <a href="https://docs.python.org/3.8/library/unittest.html#test-discovery" rel="nofollow noreferrer">https://docs.python.org/3.8/library/unittest.html#test-discovery</a></p>
  
  <p>I know that a python file (.py) is a module. A folder with an <code>__init__.py</code> file is a package. A folder without an <code>__init__.py</code> file is a namespace package.</p>
  
  <p>I am confused with below statement from the link:</p>
  
  <blockquote>
    <p>In order to be compatible with test discovery, all of the test files
    must be modules or packages (including namespace packages) importable
    from the top-level directory of the project (this means that their
    filenames must be valid identifiers).</p>
  </blockquote>
  
  <p>What does "all of the test files must be modules or packages" and "importable from the top-level directory of the project" mean?</p>