Call a python unittest from another script and export all the error messages

You need to use a test runner

test runner A test runner is a component which orchestrates the execution of tests and provides the outcome to the user. The runner may use a graphical interface, a textual interface, or return a special value to indicate the results of executing the tests.

from unittest.case import TestCase
import unittest
from StringIO import StringIO
class MyTestCase(TestCase):
    def testTrue(self):
        '''
        Always true
        '''
        assert True

    def testFail(self):
        '''
        Always fails
        '''
        assert False

from pprint import pprint
stream = StringIO()
runner = unittest.TextTestRunner(stream=stream)
result = runner.run(unittest.makeSuite(MyTestCase))
print 'Tests run ', result.testsRun
print 'Errors ', result.errors
pprint(result.failures)
stream.seek(0)
print 'Test output\n', stream.read()

---

>>> Output:  
>>> Tests run  2
>>> Errors  []
>>> [(<__main__.MyTestCase testMethod=testFail>,
>>> 'Traceback (most recent call last):\n  File "leanwx/test.py", line 15, in testFail\n                assert False\nAssertionError\n')]
>>> Test output
>>> F.
>>> ======================================================================
>>> FAIL: testFail (__main__.MyTestCase)
>>> ----------------------------------------------------------------------
>>> Traceback (most recent call last):
>>>   File "leanwx/test.py", line 15, in testFail
>>>     assert False
>>> AssertionError
>>>
>>>----------------------------------------------------------------------
>>>Ran 2 tests in 0.001s
>>>
>>>FAILED (failures=1)

Assuming you have your unit tests located in the "test" directory:

# Tested in Python 3.8
# https://docs.python.org/3/library/unittest.html#module-unittest
from unittest import TestLoader, TestResult
from pathlib import Path


def run_tests():
    test_loader = TestLoader()
    test_result = TestResult()

    # Use resolve() to get an absolute path
    # https://docs.python.org/3/library/pathlib.html#pathlib.Path.resolve
    test_directory = str(Path(__file__).resolve().parent / 'test')

    test_suite = test_loader.discover(test_directory, pattern='test_*.py')
    test_suite.run(result=test_result)

    # See the docs for details on the TestResult object
    # https://docs.python.org/3/library/unittest.html#unittest.TestResult

    if test_result.wasSuccessful():
        exit(0)
    else:
        # Here you can either print or log your test errors and failures
        # test_result.errors or test_result.failures
        exit(-1)

if __name__ == '__main__':
    run_tests()