Two tracebacks on test failure in PyCharm 2018

Question

I have simple tests in site_tests.py file:

import unittest


class SiteTests(unittest.TestCase):

    def test(self):
        self.assertEqual('a', 'b')

if __name__ == '__main__':
     unittest.main()

When I run "Unittest in test_site.py" with default PyCharm configuration I'm getting:

Testing started at 23:45 ...
C:\Users\testSite\AppData\Local\Programs\Python\Python36-32\python.exe "C:\Program Files\JetBrains\PyCharm Community Edition 2018.1.3\helpers\pycharm\_jb_unittest_runner.py" --path C:/testSiteDemoTests/site_tests.py
Launching unittests with arguments python -m unittest C:/testSiteDemoTests/site_tests.py in C:\testSiteDemoTests


b != a

Expected :a
Actual   :b
 <Click to see difference>

Traceback (most recent call last):
  File "C:\Program Files\JetBrains\PyCharm Community Edition 2018.1.3\helpers\pycharm\teamcity\diff_tools.py", line 32, in _patched_equals
    old(self, first, second, msg)
  File "C:\Users\testSite\AppData\Local\Programs\Python\Python36-32\lib\unittest\case.py", line 829, in assertEqual
    assertion_func(first, second, msg=msg)
  File "C:\Users\testSite\AppData\Local\Programs\Python\Python36-32\lib\unittest\case.py", line 1202, in assertMultiLineEqual
    self.fail(self._formatMessage(msg, standardMsg))
  File "C:\Users\testSite\AppData\Local\Programs\Python\Python36-32\lib\unittest\case.py", line 670, in fail
    raise self.failureException(msg)
AssertionError: 'a' != 'b'
- a
+ b


During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\Users\testSite\AppData\Local\Programs\Python\Python36-32\lib\unittest\case.py", line 59, in testPartExecutor
    yield
  File "C:\Users\testSite\AppData\Local\Programs\Python\Python36-32\lib\unittest\case.py", line 605, in run
    testMethod()
  File "C:\testSiteDemoTests\site_tests.py", line 7, in test
    self.assertEqual('a', 'b')



Ran 1 test in 0.000s

FAILED (failures=1)

Process finished with exit code 1

The last part is very interesting since running this file without _jb_unittest_runner.py so C:\testSite>python lost_hat_tests.py the output is ok:

F
======================================================================
FAIL: test (__main__.SiteTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "lost_hat_tests.py", line 7, in test
    self.assertEqual('a', 'b')
AssertionError: 'a' != 'b'
- a
+ b


----------------------------------------------------------------------
Ran 1 test in 0.000s

FAILED (failures=1)

Is there simple answer why that second message appearing in PyCharm runner?

I also run the test in console with _jb_unittest_runner.py - hmm two testSuites - interesting

C:\testSiteDemoTests>python "C:\Program Files\JetBrains\PyCharm Community Edition 2018.1.3\helpers\pycharm\_jb_unittest_runner.py" --path C:/testSiteDemoTests/site_tests.py
##teamcity[enteredTheMatrix timestamp='2018-05-20T23:59:59.931']
Launching unittests with arguments python -m unittest C:/testSiteDemoTests/site_tests.py in C:\testSiteDemoTests

##teamcity[testCount timestamp='2018-05-20T23:59:59.946' count='1']
##teamcity[testSuiteStarted timestamp='2018-05-20T23:59:59.946' locationHint='python<C:\testSiteDemoTests>://site_tests' name='site_tests' nodeId='1' parentNodeId='0']
##teamcity[testSuiteStarted timestamp='2018-05-20T23:59:59.946' locationHint='python<C:\testSiteDemoTests>://site_tests.SiteTests' name='SiteTests' nodeId='2' parentNodeId='1']
##teamcity[testStarted timestamp='2018-05-20T23:59:59.962' captureStandardOutput='true' locationHint='python<C:\testSiteDemoTests>://site_tests.SiteTests.test' name='test' nodeId='3' parentNodeId='2']
##teamcity[testFailed timestamp='2018-05-20T23:59:59.977' actual='b' details='Traceback (most recent call last):|n  File "C:\Program Files\JetBrains\PyCharm Community Edition 2018.1.3\helpers\pycharm\teamcity\diff_tools.py", line 32, in _patched_equals|n    old(
self, first, second, msg)|n  File "C:\Users\testSite\AppData\Local\Programs\Python\Python36-32\lib\unittest\case.py", line 829, in assertEqual|n    assertion_func(first, second, msg=msg)|n  File "C:\Users\testSite\AppData\Local\Programs\Python\Python36-32\
lib\unittest\case.py", line 1202, in assertMultiLineEqual|n    self.fail(self._formatMessage(msg, standardMsg))|n  File "C:\Users\testSite\AppData\Local\Programs\Python\Python36-32\lib\unittest\case.py", line 670, in fail|n    raise self.failureException(msg)
|nAssertionError: |'a|' != |'b|'|n- a|n+ b|n|n|nDuring handling of the above exception, another exception occurred:|n|nTraceback (most recent call last):|n  File "C:\Users\testSite\AppData\Local\Programs\Python\Python36-32\lib\unittest\case.py", line 59, in t
estPartExecutor|n    yield|n  File "C:\Users\testSite\AppData\Local\Programs\Python\Python36-32\lib\unittest\case.py", line 605, in run|n    testMethod()|n  File "C:\testSiteDemoTests\site_tests.py", line 7, in test|n    self.assertEqual(|'a|', |'b|')|n' e
xpected='a' locationHint='python<C:\testSiteDemoTests>://site_tests.SiteTests.test' message='|nb != a|n' name='test' nodeId='3' parentNodeId='2' type='comparisonFailure']
##teamcity[testFinished timestamp='2018-05-20T23:59:59.977' duration='30' locationHint='python<C:\testSiteDemoTests>://site_tests.SiteTests.test' name='test' nodeId='3' parentNodeId='2']


Ran 1 test in 0.031s

FAILED (failures=1)
##teamcity[testSuiteFinished timestamp='2018-05-20T23:59:59.977' locationHint='python<C:\testSiteDemoTests>://site_tests.SiteTests' name='SiteTests' nodeId='2' parentNodeId='1']
##teamcity[testSuiteFinished timestamp='2018-05-20T23:59:59.977' locationHint='python<C:\testSiteDemoTests>://site_tests' name='site_tests' nodeId='1' parentNodeId='0']

Answer 1

This message shows up in exception's formatting in Python 3 if another exception was raised in an exception handler or finally clause for the first one:

A similar mechanism works implicitly if an exception is raised inside an exception handler or a finally clause: the previous exception is then attached as the new exception’s __context__ attribute

Setting a breakpoint on the test in PyCharm, then stepping further into the machinery shows where this second exception is thrown. _jb_unittest_runner patches the assert methods in unittest:

PyCharm Community Edition\helpers\pycharm\_jb_unittest_runner.py:

from teamcity import unittestpy

PyCharm Community Edition\helpers\pycharm\teamcity\unittestpy.py:

def run(self, test):
    <...>
    patch_unittest_diff(subtest_filter)
    <...>

PyCharm Community Edition\helpers\pycharm\teamcity\diff_tools.py:

def patch_unittest_diff(<...>):

    old = unittest.TestCase.assertEqual

    def _patched_equals(self, first, second, msg=None):
        try:
            old(self, first, second, msg)
            return
        except AssertionError as native_error:
            if not test_filter or test_filter(self):
                error = EqualsAssertionError(first, second, msg)
                if error.can_be_serialized():
                    raise error
            raise native_error

    unittest.TestCase.assertEqual = _patched_equals