Convert Python 3 ResourceWarnings into exception

Question

Is there a way to force a Python 3 unittest to fail, rather than simply print a warning to stderr, if it causes any ResourceWarning?

I've tried the following:

import warnings
warnings.simplefilter(action='error', category=ResourceWarning)

Which results in this output from unittest:

my_test (__main__.MyTest) ... Exception ignored in: <socket.socket fd=9, family=AddressFamily.AF_INET, type=SocketType.SOCK_STREAM, proto=0, laddr=('127.0.0.1', 54065), raddr=('127.0.0.1', 27017)>
ResourceWarning: unclosed <socket.socket fd=9, family=AddressFamily.AF_INET, type=SocketType.SOCK_STREAM, proto=0, laddr=('127.0.0.1', 54065), raddr=('127.0.0.1', 27017)>
ok

----------------------------------------------------------------------
Ran 1 test in 0.110s

Note the "Exception ignored" message. I'd rather the test failed, instead of requiring me to read its output looking for ResourceWarnings.

Answer 1

Here's a unit test that fails if ResourceWarning is generated by the code in with catch_warning() statement:

#!/usr/bin/env python3
import gc
import socket
import unittest
import warnings

class Test(unittest.TestCase):
    def test_resource_warning(self):
        s = socket.socket()
        ####s.close() #XXX uncomment to pass the test

        # generate resource warning when s is deleted
        with warnings.catch_warnings(record=True) as w:
            warnings.resetwarnings() # clear all filters
            warnings.simplefilter('ignore') # ignore all
            warnings.simplefilter('always', ResourceWarning) # add filter
            del s        # remove reference
            gc.collect() # run garbage collection (for pypy3)
            self.assertFalse(w and str(w[-1])) # test fails if there
                                               # are warnings

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

Answer 2

Unfortunately, it doesn't appear to be possible. That "Exception ignored in:" message is produced by the CPython function PyErr_WriteUnraisable in Python/errors.c. The comment before that function states:

/* Call when an exception has occurred but there is no way for Python
   to handle it.  Examples: exception in __del__ or during GC. */

The ResourceWarning is indeed being generated during garbage collection, and Python prints a message because it doesn't have a way to raise an exception at that point. This has to do with the core CPython implementation and there is nothing that unittest can do to override it.

Update: while the above is a correct technical analysis, there is another approach that will actually solve the OP's problem. See the answer by J.F. Sebastian for more details.