gh-87822: Make traceback module robust to exceptions from repr() of local values (GH-94691)

Doc/library/traceback.rst

@@ -341,6 +341,10 @@ capture data for later printing in a lightweight fashion.
       local variables in each :class:`FrameSummary` are captured as object
       representations.
 
+      .. versionchanged:: 3.12
+         Exceptions raised from :func:`repr` on a local variable (when
+         *capture_locals* is ``True``) are no longer propagated to the caller.
+
    .. classmethod:: from_list(a_list)
 
       Construct a :class:`StackSummary` object from a supplied list of

Lib/test/test_traceback.py

@@ -2279,6 +2279,9 @@ class TestStack(unittest.TestCase):
             f'  File "{__file__}", line {lno}, in f\n    1/0\n'
         )
 
+class Unrepresentable:
+    def __repr__(self) -> str:
+        raise Exception("Unrepresentable")
 
 class TestTracebackException(unittest.TestCase):
 
@@ -2546,12 +2549,13 @@ class TestTracebackException(unittest.TestCase):
         linecache.updatecache('/foo.py', globals())
         e = Exception("uh oh")
         c = test_code('/foo.py', 'method')
-        f = test_frame(c, globals(), {'something': 1, 'other': 'string'})
+        f = test_frame(c, globals(), {'something': 1, 'other': 'string', 'unrepresentable': Unrepresentable()})
         tb = test_tb(f, 6, None, 0)
         exc = traceback.TracebackException(
             Exception, e, tb, capture_locals=True)
         self.assertEqual(
-            exc.stack[0].locals, {'something': '1', 'other': "'string'"})
+            exc.stack[0].locals,
+            {'something': '1', 'other': "'string'", 'unrepresentable': '<local repr() failed>'})
 
     def test_no_locals(self):
         linecache.updatecache('/foo.py', globals())

Lib/traceback.py

@@ -279,7 +279,8 @@ class FrameSummary:
         self._line = line
         if lookup_line:
             self.line
-        self.locals = {k: repr(v) for k, v in locals.items()} if locals else None
+        self.locals = {k: _safe_string(v, 'local', func=repr)
+            for k, v in locals.items()} if locals else None
         self.end_lineno = end_lineno
         self.colno = colno
         self.end_colno = end_colno

Misc/ACKS

@@ -1590,6 +1590,7 @@ Ed Schouten
 Scott Schram
 Robin Schreiber
 Chad J. Schroeder
+Simon-Martin Schroeder
 Christian Schubert
 Sam Schulenburg
 Andreas Schwab

Misc/NEWS.d/next/Library/2022-07-08-17-49-12.gh-issue-87822.F9dzkf.rst

@@ -0,0 +1 @@
+When called with ``capture_locals=True``, the :mod:`traceback` module functions swallow exceptions raised from calls to ``repr()`` on local variables of frames. This is in order to prioritize the original exception over rendering errors.  An indication of the failure is printed in place of the missing value.  (Patch by Simon-Martin Schroeder).