In /Lib/test/test_urllib2_localnet.py there is an incorrect usage of unittest.TestCase.fail (an extra comma makes it use two arguments instead of one):

class BasicAuthTests(unittest.TestCase):
    ...

    def test_basic_auth_success(self):
        ...
        try:
            self.assertTrue(urllib.request.urlopen(self.server_url))
        except urllib.error.HTTPError:
            self.fail("Basic auth failed for the url: %s", self.server_url)

here self.fail takes two argument, but it only admits one. This produces this traceback if there are proxy issues when running the tests:

Traceback (most recent call last):
  File "/home7/pablogsal/Python-3.7.0/Lib/test/test_urllib2_localnet.py", line 307, in test_basic_auth_success
    self.assertTrue(urllib.request.urlopen(self.server_url))
  File "/home/pablogsal/Python-3.7.0/Lib/urllib/request.py", line 222, in urlopen
    return opener.open(url, data, timeout)
  File "/home/pablogsal/Python-3.7.0/Lib/urllib/request.py", line 531, in open
    response = meth(req, response)
  File "/home/pablogsal/Python-3.7.0/Lib/urllib/request.py", line 641, in http_response
    'http', request, response, code, msg, hdrs)
  File "/home/pablogsal/Python-3.7.0/Lib/urllib/request.py", line 569, in error
    return self._call_chain(*args)
  File "/home/pablogsal/Python-3.7.0/Lib/urllib/request.py", line 503, in _call_chain
    result = func(*args)
  File "/home/pablogsal/Python-3.7.0/Lib/urllib/request.py", line 649, in http_error_default
    raise HTTPError(req.full_url, code, msg, hdrs, fp)
urllib.error.HTTPError: HTTP Error 403: Forbidden

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/pablogsal/Python-3.7.0/Lib/test/test_urllib2_localnet.py", line 309, in test_basic_auth_success
    self.fail("Basic auth failed for the url: %s", self.server_url)
TypeError: fail() takes from 1 to 2 positional arguments but 3 were given

----------------------------------------------------------------------

Hey again @pablogsal. Thanks for reviewing the email issue again. I'll try to fix and submit a PR for this if that's ok.

This might be slightly unrelated but I grepped for similar cases and came across self.fail, "Broken file" in Lib/test/test_file.py at https://github.com/python/cpython/blob/a390cb6b6fb6638e91d699513167e8f35b47f846/Lib/test/test_file.py#L245. I think this is not triggered for broken file which I tried in the test by removing the if condition. Kindly correct me if I am wrong on the syntax or this should be raised as a separate issue.

Thanks

New changeset 6b490b5db40fc29588e8e6cc23bb89c4fed74ad5 by Tal Einat (Bradley Laney) in branch 'master':
bpo-34031: fix incorrect usage of self.fail in two tests (GH-8091)
https://github.com/python/cpython/commit/6b490b5db40fc29588e8e6cc23bb89c4fed74ad5

New changeset 125371d2c617d886e1a15b8f940ce29124cdc79c by Miss Islington (bot) in branch '3.7':
bpo-34031: fix incorrect usage of self.fail in two tests (GH-8091)
https://github.com/python/cpython/commit/125371d2c617d886e1a15b8f940ce29124cdc79c

New changeset cb14eb7eedf36cd21aaf41c4d3eb1e6521fa7b11 by Miss Islington (bot) in branch '3.6':
bpo-34031: fix incorrect usage of self.fail in two tests (GH-8091)
https://github.com/python/cpython/commit/cb14eb7eedf36cd21aaf41c4d3eb1e6521fa7b11

New changeset 6f036bb67d5a20c86c29ea5aeace563e3751baab by Tal Einat in branch '2.7':
[2.7] bpo-34031: fix incorrect usage of self.fail in two tests (GH-8091) (#8225)
https://github.com/python/cpython/commit/6f036bb67d5a20c86c29ea5aeace563e3751baab

Thanks for the PR, stoksc!