How to check python coding style automatically via flake8 and isort in django project with unit test

test_coding_style.py

"""
Coding styles test.
You need to install flake8 and isort via pip.
pip install flake8
pip install isort
Add this test to any of your django apps tests.
"""

import subprocess

from django.test import TestCase


class TestCodingStyle(TestCase):
    """
    Test of the code for Flake8 and isort.
    """

    def test_flake8(self):
        """Testing the retval of flake8 command to be zero (PEP8 check)."""
        p = subprocess.Popen('flake8')
        retval = p.wait()
        self.assertEqual(retval, 0)

    def test_isort(self):
        """Testing the retval of `isort -rc -c .` command to be zero. (import sort order)"""
        p = subprocess.Popen('isort -rc -c .', shell=True)
        retval = p.wait()
        self.assertEqual(retval, 0)

strange, since this code is working fine for me. maybe some local file structure issue or python/isort version