Flask "Error: The file/path provided does not appear to exist" although the file does exist : <p>I use <code>export FLASK_APP=flask_app</code> and then do <code>flask run</code> but I get the error:</p>

<blockquote>
  <p>Error: The file/path provided (flask_app) does not appear to exist.  Please verify the path is correct.  If app is not on PYTHONPATH, ensure the extension is .py</p>
</blockquote>

<p>However, the file <em>does</em> exist and is even in the present working directory. Using the complete path to the file does not work either.</p>
