How to fix Python unittest __init__() takes 1 positional argument but 2 were given

Problem:

You are trying to run your Python unit tests using the unittest package, but you see this unspecific stack trace:

Traceback (most recent call last):
  File "/usr/lib/python3.7/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/lib/python3.7/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/usr/lib/python3.7/unittest/__main__.py", line 18, in <module>
    main(module=None)
  File "/usr/lib/python3.7/unittest/main.py", line 100, in __init__
    self.parseArgs(argv)
  File "/usr/lib/python3.7/unittest/main.py", line 124, in parseArgs
    self._do_discovery(argv[2:])
  File "/usr/lib/python3.7/unittest/main.py", line 244, in _do_discovery
    self.createTests(from_discovery=True, Loader=Loader)
  File "/usr/lib/python3.7/unittest/main.py", line 154, in createTests
    self.test = loader.discover(self.start, self.pattern, self.top)
  File "/usr/lib/python3.7/unittest/loader.py", line 349, in discover
    tests = list(self._find_tests(start_dir, pattern))
  File "/usr/lib/python3.7/unittest/loader.py", line 414, in _find_tests
    yield from self._find_tests(full_path, pattern, namespace)
  File "/usr/lib/python3.7/unittest/loader.py", line 406, in _find_tests
    full_path, pattern, namespace)
  File "/usr/lib/python3.7/unittest/loader.py", line 460, in _find_test_path
    return self.loadTestsFromModule(module, pattern=pattern), False
  File "/usr/lib/python3.7/unittest/loader.py", line 124, in loadTestsFromModule
    tests.append(self.loadTestsFromTestCase(obj))
  File "/usr/lib/python3.7/unittest/loader.py", line 93, in loadTestsFromTestCase
    loaded_suite = self.suiteClass(map(testCaseClass, testCaseNames))
  File "/usr/lib/python3.7/unittest/suite.py", line 24, in __init__
    self.addTests(tests)
  File "/usr/lib/python3.7/unittest/suite.py", line 57, in addTests
    for test in tests:
TypeError: __init__() takes 1 positional argument but 2 were given

Solution

You have at least one test case like this one:

class MyTest(unittest.TestCase):
    def __init__(self):
        self.x = 1.0

    def test_stuff(self):
        assert(self.x == 1.0)

Overriding __init__(...) is not possible in this way when using unittest. You need to use setUp() instead.

Usually, just replacing def __init__(self): by def setUp(self): will do the trick. unittests will call setUp() automatically.

Our example will look like this:

class MyTest(unittest.TestCase):
    def setUp(self):
        self.x = 1.0

    def test_stuff(self):
        assert(self.x == 1.0)

If the error still persists, check if you have more testcases overriding the __init__() method.