Issue #25651: Allow falsy values to be used for msg parameter of subTest()

Lib/unittest/case.py

@@ -17,6 +17,7 @@ from .util import (strclass, safe_repr, _count_diff_all_purpose,
 
 __unittest = True
 
+_subtest_msg_sentinel = object()
 
 DIFF_OMITTED = ('\nDiff is %s characters long. '
                  'Set self.maxDiff to None to see it.')
@@ -497,7 +498,7 @@ class TestCase(object):
             result.addSuccess(test_case)
 
     @contextlib.contextmanager
-    def subTest(self, msg=None, **params):
+    def subTest(self, msg=_subtest_msg_sentinel, **params):
         """Return a context manager that will return the enclosed block
         of code in a subtest identified by the optional message and
         keyword parameters.  A failure in the subtest marks the test
@@ -1397,7 +1398,7 @@ class _SubTest(TestCase):
 
     def _subDescription(self):
         parts = []
-        if self._message:
+        if self._message is not _subtest_msg_sentinel:
             parts.append("[{}]".format(self._message))
         if self.params:
             params_desc = ', '.join(

Lib/unittest/test/test_result.py

@@ -323,6 +323,16 @@ class Test_TestResult(unittest.TestCase):
                     'testGetSubTestDescriptionWithoutDocstringAndParams '
                     '(' + __name__ + '.Test_TestResult) (<subtest>)')
 
+    def testGetSubTestDescriptionForFalsyValues(self):
+        expected = 'testGetSubTestDescriptionForFalsyValues (%s.Test_TestResult) [%s]'
+        result = unittest.TextTestResult(None, True, 1)
+        for arg in [0, None, []]:
+            with self.subTest(arg):
+                self.assertEqual(
+                    result.getDescription(self._subtest),
+                    expected % (__name__, arg)
+                )
+
     def testGetNestedSubTestDescriptionWithoutDocstring(self):
         with self.subTest(foo=1):
             with self.subTest(bar=2):

Misc/NEWS

@@ -77,6 +77,8 @@ Core and Builtins
 Library
 -------
 
+- Issue #25651: Allow falsy values to be used for msg parameter of subTest().
+
 - Issue #27932: Prevent memory leak in win32_ver().
 
 - Fix UnboundLocalError in socket._sendfile_use_sendfile.