bpo-31299: Make it possible to filter out frames from tracebacks

Doc/library/traceback.rst

@@ -212,7 +212,7 @@ The module also defines the following classes:
 :class:`TracebackException` objects are created from actual exceptions to
 capture data for later printing in a lightweight fashion.
 
-.. class:: TracebackException(exc_type, exc_value, exc_traceback, *, limit=None, lookup_lines=True, capture_locals=False, compact=False)
+.. class:: TracebackException(exc_type, exc_value, exc_traceback, *, limit=None, lookup_lines=True, capture_locals=False, compact=False, stack_summary_cls=None)
 
    Capture an exception for later rendering. *limit*, *lookup_lines* and
    *capture_locals* are as for the :class:`StackSummary` class.
@@ -222,6 +222,10 @@ capture data for later printing in a lightweight fashion.
    ``__context__`` field is calculated only if ``__cause__`` is ``None`` and
    ``__suppress_context__`` is false.
 
+   If *stack_summary_cls* is not ``None``, it is a class to be used instead of
+   the default :class:`~traceback.StackSummary` to format the stack (typically
+   a subclass that overrides :meth:`~traceback.StackSummary.format_frame`).
+
    Note that when locals are captured, they are also shown in the traceback.
 
    .. attribute:: __cause__
@@ -309,6 +313,8 @@ capture data for later printing in a lightweight fashion.
    .. versionchanged:: 3.10
       Added the *compact* parameter.
 
+   .. versionchanged:: 3.11
+      Added the *stack_summary_cls* paramter.
 
 :class:`StackSummary` Objects
 -----------------------------
@@ -357,7 +363,8 @@ 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

@@ -1443,6 +1443,40 @@ 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)
+
+        def get_output(stack_summary_cls=None):
+            output = StringIO()
+            traceback.TracebackException(
+                *exc_info, stack_summary_cls=stack_summary_cls,
+            ).print(file=output)
+            return output.getvalue().split('\n')
+
+        default = get_output()
+        skip_g = get_output(Skip_G)
+
+        for l in skip_g:
+            default.remove(l)
+        # Only the lines for g's frame should remain:
+        self.assertEqual(len(default), 3)
+        self.assertRegex(default[0], ', line [0-9]*, in g')
+        self.assertEqual(default[1], '    f()')
+        self.assertEqual(default[2], '    ^^^')
+
 
 class TestTracebackException(unittest.TestCase):
 

Lib/traceback.py

@@ -453,7 +453,8 @@ def format_frame(self, frame):
         """Format the lines for a single frame.
 
         Returns a string representing one frame involved in the stack. This
-        gets called for every frame to be printed in the stack summary.
+        gets called for every frame to be printed in the stack summary. If
+        it returns ``None``, the frame is omitted from the output.
         """
         row = []
         row.append('  File "{}", line {}, in {}\n'.format(
@@ -507,25 +508,32 @@ def format(self):
         last_file = None
         last_line = None
         last_name = None
+        last_line_displayed = False
         count = 0
         for frame in self:
             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):
                 if count > _RECURSIVE_CUTOFF:
                     count -= _RECURSIVE_CUTOFF
-                    result.append(
-                        f'  [Previous line repeated {count} more '
-                        f'time{"s" if count > 1 else ""}]\n'
-                    )
+                    if last_line_displayed:
+                        result.append(
+                            f'  [Previous line repeated {count} more '
+                            f'time{"s" if count > 1 else ""}]\n'
+                        )
                 last_file = frame.filename
                 last_line = frame.lineno
                 last_name = frame.name
                 count = 0
             count += 1
             if count > _RECURSIVE_CUTOFF:
                 continue
-            result.append(self.format_frame(frame))
+            formatted_frame = self.format_frame(frame)
+            if formatted_frame is not None:
+                last_line_displayed = True
+                result.append(formatted_frame)
+            else:
+                last_line_displayed = False
 
         if count > _RECURSIVE_CUTOFF:
             count -= _RECURSIVE_CUTOFF
@@ -618,7 +626,7 @@ class TracebackException:
 
     def __init__(self, exc_type, exc_value, exc_traceback, *, limit=None,
             lookup_lines=True, capture_locals=False, compact=False,
-            _seen=None):
+            stack_summary_cls=None, _seen=None):
         # NB: we need to accept exc_traceback, exc_value, exc_traceback to
         # permit backwards compat with the existing API, otherwise we
         # need stub thunk objects just to glue it together.
@@ -628,8 +636,9 @@ def __init__(self, exc_type, exc_value, exc_traceback, *, limit=None,
             _seen = set()
         _seen.add(id(exc_value))
 
-        # TODO: locals.
-        self.stack = StackSummary._extract_from_extended_frame_gen(
+        if stack_summary_cls is None:
+            stack_summary_cls = StackSummary
+        self.stack = stack_summary_cls._extract_from_extended_frame_gen(
             _walk_tb_with_full_positions(exc_traceback),
             limit=limit, lookup_lines=lookup_lines,
             capture_locals=capture_locals)

Misc/NEWS.d/next/Library/2021-06-17-16-16-33.bpo-31299.d4WDz7.rst

@@ -0,0 +1 @@
+Added the ``stack_summary_cls`` parameter to :class:`TracebackException`, to allow fine-grained control over the content of a formatted traceback.  Added option to completely drop frames from the output by returning ``None`` from a :meth:`~StackSummary.format_frame` override.