bpo-31299: make it possible to filter out frames from tracebacks (GH-28067)

Doc/library/traceback.rst

@@ -357,7 +357,8 @@ capture data for later printing in a lightweight fashion.
 
       Returns a string for printing one of the frames involved in the stack.
       This method gets called for each frame object to be printed in the
-      :class:`StackSummary`.
+      :class:`StackSummary`. If it returns ``None``, the frame is omitted
+      from the output.
 
       .. versionadded:: 3.11
 

Lib/test/test_traceback.py

@@ -1514,6 +1514,34 @@ def some_inner():
             s.format(),
             [f'{__file__}:{some_inner.__code__.co_firstlineno + 1}'])
 
+    def test_dropping_frames(self):
+         def f():
+             1/0
+
+         def g():
+             try:
+                 f()
+             except:
+                 return sys.exc_info()
+
+         exc_info = g()
+
+         class Skip_G(traceback.StackSummary):
+             def format_frame(self, frame):
+                 if frame.name == 'g':
+                     return None
+                 return super().format_frame(frame)
+
+         stack = Skip_G.extract(
+             traceback.walk_tb(exc_info[2])).format()
+
+         self.assertEqual(len(stack), 1)
+         lno = f.__code__.co_firstlineno + 1
+         self.assertEqual(
+             stack[0],
+             f'  File "{__file__}", line {lno}, in f\n    1/0\n'
+         )
+
 
 class TestTracebackException(unittest.TestCase):
 

Lib/traceback.py

@@ -515,6 +515,9 @@ def format(self):
         last_name = None
         count = 0
         for frame in self:
+            formatted_frame = self.format_frame(frame)
+            if formatted_frame is None:
+                continue
             if (last_file is None or last_file != frame.filename or
                 last_line is None or last_line != frame.lineno or
                 last_name is None or last_name != frame.name):

Misc/NEWS.d/next/Library/2021-08-30-13-55-09.bpo-31299.9QzjZs.rst

@@ -0,0 +1 @@
+Add option to completely drop frames from a traceback by returning ``None`` from a :meth:`~traceback.StackSummary.format_frame` override.