The Gentoo Refleaks 3.7 buildbot is reporting that test_smtplib may be leaving with dangling threads:

https://buildbot.python.org/all/#builders/114/builds/179

FAILED (errors=7)
Warning -- threading._dangling was modified by test_smtplib
Before: <_weakrefset.WeakSet object at 0xb6918bcc>
After: <_weakrefset.WeakSet object at 0xb68e761c>
test test_smtplib failed
1 test failed again:
test_smtplib
== Tests result: FAILURE then FAILURE ==

New failure on AMD64 Windows8.1 Refleaks 3.6:

https://buildbot.python.org/all/#builders/64/builds/301

Ah, sorry. we just modernized test_smtplib's __main__ section, and I think we lost the thread cleanup code that used to be run around the test suite. We need to add the thread cleanup decorators.

I think it may be something extra as test_send_message_error_on_non_ascii_addrs_if_no_smtputf8 is failing when you run the test with -R:

./python -W error -m test test_smtplib -R 3:3

Warning -- threading._dangling was modified by test_smtplib
  Before: <_weakrefset.WeakSet object at 0x10ed7dd70>
  After:  <_weakrefset.WeakSet object at 0x10f619830>
test test_smtplib failed -- Traceback (most recent call last):
  File "/Users/pgalindo3/cpython/Lib/test/test_smtplib.py", line 1232, in test_send_message_error_on_non_ascii_addrs_if_no_smtputf8
    smtp.send_message(msg))
  File "/Users/pgalindo3/cpython/Lib/unittest/case.py", line 743, in assertRaises
    return context.handle('assertRaises', args, kwargs)
  File "/Users/pgalindo3/cpython/Lib/unittest/case.py", line 178, in handle
    callable_obj(*args, **kwargs)
TypeError: 'dict' object is not callable

test_smtplib failed

== Tests result: FAILURE ==

1 test failed:
test_smtplib

Total duration: 16 sec 566 ms
Tests result: FAILURE

The TypeError is because the test is misusing the assertRaises API:

         self.assertRaises(smtplib.SMTPNotSupportedError,
                          smtp.send_message(msg))

should be

         self.assertRaises(smtplib.SMTPNotSupportedError,
                          smtp.send_message, msg)

That's not the code I thought I merged. I'll have to take a look at what actually got merged.

That was from commit 48ed88a

Oh, sorry, my bad. That was precisely what commit 48ed88a changed. The problem with running with -R still happens:

======================================================================
ERROR: test_name_field_not_included_in_envelop_addresses (test.test_smtplib.SMTPSimTests)
----------------------------------------------------------------------

Traceback (most recent call last):
  File "/Users/pgalindo3/cpython/Lib/test/test_smtplib.py", line 1100, in test_name_field_not_included_in_envelop_addresses
    self.assertDictEqual(smtp.send_message(message), {})
  File "/Users/pgalindo3/cpython/Lib/smtplib.py", line 967, in send_message
    rcpt_options)
  File "/Users/pgalindo3/cpython/Lib/smtplib.py", line 861, in sendmail
    (code, resp) = self.mail(from_addr, esmtp_opts)
  File "/Users/pgalindo3/cpython/Lib/smtplib.py", line 532, in mail
    'SMTPUTF8 not supported by server')
smtplib.SMTPNotSupportedError: SMTPUTF8 not supported by server

Ran 60 tests in 1.365s

FAILED (errors=7)
Warning -- threading._dangling was modified by test_smtplib
Before: <_weakrefset.WeakSet object at 0x10f445e90>
After: <_weakrefset.WeakSet object at 0x10fe75ad0>
test test_smtplib failed
test_smtplib failed

== Tests result: FAILURE ==

1 test failed:
test_smtplib

Total duration: 2 sec 920 ms
Tests result: FAILURE

That's due to another bug, which will get fixed by bpo-32814, unless you'd like to generate a PR with the this fix in it (fixing the mutable default for smtp_options).

This is just to note that I'm investigating this issue as part of the CPython sprint in EuroPython.

See also bpo-32657.

New changeset d5fbe9b by Pablo Galindo (Pablo Aguiar) in branch 'master':
bpo-34246: Use no mutable default args in smtplib (GH-8554)
d5fbe9b

New changeset 9835696 by Miss Islington (bot) in branch '3.7':
bpo-34246: Use no mutable default args in smtplib (GH-8554)
9835696

New changeset 5b7a2cb by Pablo Galindo in branch 'master':
bpo-34246: Make sure test_smtplib always cleans resources when finished (GH-9108)
5b7a2cb

New changeset eb6ab73 by Pablo Galindo in branch '3.6':
[3.6] bpo-34246: Use no mutable default args in smtplib (GH-8554) (bpo-9112)
eb6ab73