bpo-29564: warnings suggests to enable tracemalloc (GH-10486)

The warnings module now suggests to enable tracemalloc if the source
is specified, tracemalloc module is available, but tracemalloc is not
tracing memory allocations.

Lib/test/test_warnings/__init__.py

@@ -897,12 +897,27 @@ class PyWarningsDisplayTests(WarningsDisplayTests, unittest.TestCase):
                 func()
             """))
 
-        res = assert_python_ok('-Wd', '-X', 'tracemalloc=2', support.TESTFN)
-
+        def run(*args):
+            res = assert_python_ok(*args)
             stderr = res.err.decode('ascii', 'replace')
-        # normalize newlines
             stderr = '\n'.join(stderr.splitlines())
+
+            # normalize newlines
             stderr = re.sub('<.*>', '<...>', stderr)
+            return stderr
+
+        # tracemalloc disabled
+        stderr = run('-Wd', support.TESTFN)
+        expected = textwrap.dedent('''
+            {fname}:5: ResourceWarning: unclosed file <...>
+              f = None
+            ResourceWarning: Enable tracemalloc to get the object allocation traceback
+        ''')
+        expected = expected.format(fname=support.TESTFN).strip()
+        self.assertEqual(stderr, expected)
+
+        # tracemalloc enabled
+        stderr = run('-Wd', '-X', 'tracemalloc=2', support.TESTFN)
         expected = textwrap.dedent('''
             {fname}:5: ResourceWarning: unclosed file <...>
               f = None

Lib/warnings.py

@@ -33,9 +33,8 @@ def _showwarnmsg_impl(msg):
         pass
 
 def _formatwarnmsg_impl(msg):
-    s =  ("%s:%s: %s: %s\n"
-          % (msg.filename, msg.lineno, msg.category.__name__,
-             msg.message))
+    category = msg.category.__name__
+    s =  f"{msg.filename}:{msg.lineno}: {category}: {msg.message}\n"
 
     if msg.line is None:
         try:
@@ -55,6 +54,15 @@ def _formatwarnmsg_impl(msg):
     if msg.source is not None:
         try:
             import tracemalloc
+        # Logging a warning should not raise a new exception:
+        # catch Exception, not only ImportError and RecursionError.
+        except Exception:
+            # don't suggest to enable tracemalloc if it's not available
+            tracing = True
+            tb = None
+        else:
+            tracing = tracemalloc.is_tracing()
+            try:
                 tb = tracemalloc.get_object_traceback(msg.source)
             except Exception:
                 # When a warning is logged during Python shutdown, tracemalloc
@@ -77,6 +85,9 @@ def _formatwarnmsg_impl(msg):
                 if line:
                     line = line.strip()
                     s += '    %s\n' % line
+        elif not tracing:
+            s += (f'{category}: Enable tracemalloc to get the object '
+                  f'allocation traceback\n')
     return s
 
 # Keep a reference to check if the function was replaced

Misc/NEWS.d/next/Library/2018-11-12-17-40-04.bpo-29564.SFNBT5.rst

+The warnings module now suggests to enable tracemalloc if the source is
+specified, the tracemalloc module is available, but tracemalloc is not
+tracing memory allocations.