From 9d3b7e24862ae2c480df12ef0e7141fb30a81b05 Mon Sep 17 00:00:00 2001
From: Vladimir Feinberg <vladimir.feinberg@gmail.com>
Date: Fri, 5 Feb 2021 21:22:58 -0800
Subject: [PATCH 01/16] bpo-43048: Catch RecursionError traceback
 RecursionError

When traceback is formatting a RecursionError with a long __context__ chain it runs the risk of itself raising a RecursionError. This patch catches the corresponding recursive call's RecursionError, truncates the traceback and emits a warning instead.
---
 Lib/test/test_traceback.py | 22 ++++++++++++++++++++++
 Lib/traceback.py           | 22 ++++++++++++++--------
 2 files changed, 36 insertions(+), 8 deletions(-)

diff --git a/Lib/test/test_traceback.py b/Lib/test/test_traceback.py
index 5bb3a58b2a103b..9d565fcaa4aa72 100644
--- a/Lib/test/test_traceback.py
+++ b/Lib/test/test_traceback.py
@@ -92,6 +92,28 @@ def test_base_exception(self):
         lst = traceback.format_exception_only(e.__class__, e)
         self.assertEqual(lst, ['KeyboardInterrupt\n'])
 
+    def test_traceback_recursionerror(self):
+        # Test that for long traceback chains traceback does not itself
+        # raise a recursion error while printing (Issue43048)
+
+        # Calling f() creates a stack-overflowing __context__ chain.
+        def f():
+            try:
+                raise ValueError('hello')
+            except ValueError:
+                f()
+
+        try:
+            f()
+        except RecursionError as e:
+            # Creating this exception should not fail if internally
+            # the traceback exception is constructed safely with respect
+            # to recursion context (since TracebackExceptions are used
+            # for printing and formatting exceptions we should allow their
+            # creation even for RecursionErrors with long chains).
+            exc_info = sys.exc_info()
+            traceback.TracebackException(exc_info[0], exc_info[1], exc_info[2])
+
     def test_format_exception_only_bad__str__(self):
         class X(Exception):
             def __str__(self):
diff --git a/Lib/traceback.py b/Lib/traceback.py
index d7fbdae680be62..b659f754879e7c 100644
--- a/Lib/traceback.py
+++ b/Lib/traceback.py
@@ -3,6 +3,7 @@
 import collections
 import itertools
 import linecache
+import warnings
 import sys
 
 __all__ = ['extract_stack', 'extract_tb', 'format_exception',
@@ -490,14 +491,19 @@ def __init__(self, exc_type, exc_value, exc_traceback, *, limit=None,
             cause = None
         if (exc_value and exc_value.__context__ is not None
             and id(exc_value.__context__) not in _seen):
-            context = TracebackException(
-                type(exc_value.__context__),
-                exc_value.__context__,
-                exc_value.__context__.__traceback__,
-                limit=limit,
-                lookup_lines=False,
-                capture_locals=capture_locals,
-                _seen=_seen)
+            try:
+                context = TracebackException(
+                    type(exc_value.__context__),
+                    exc_value.__context__,
+                    exc_value.__context__.__traceback__,
+                    limit=limit,
+                    lookup_lines=False,
+                    capture_locals=capture_locals,
+                    _seen=_seen)
+            except RecursionError:
+                warnings.warn(
+                    'A RecursionError occurred while processing an exception; truncating traceback.');
+                context = None
         else:
             context = None
         self.__cause__ = cause

From b4dcb16d19684f1d40b730bf2056eb59dc19464c Mon Sep 17 00:00:00 2001
From: "blurb-it[bot]" <43283697+blurb-it[bot]@users.noreply.github.com>
Date: Sat, 6 Feb 2021 05:34:02 +0000
Subject: [PATCH 02/16] =?UTF-8?q?=F0=9F=93=9C=F0=9F=A4=96=20Added=20by=20b?=
 =?UTF-8?q?lurb=5Fit.?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 .../NEWS.d/next/Library/2021-02-06-05-34-01.bpo-43048.yCPUmo.rst | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 Misc/NEWS.d/next/Library/2021-02-06-05-34-01.bpo-43048.yCPUmo.rst

diff --git a/Misc/NEWS.d/next/Library/2021-02-06-05-34-01.bpo-43048.yCPUmo.rst b/Misc/NEWS.d/next/Library/2021-02-06-05-34-01.bpo-43048.yCPUmo.rst
new file mode 100644
index 00000000000000..476f0c15a9ed8b
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2021-02-06-05-34-01.bpo-43048.yCPUmo.rst
@@ -0,0 +1 @@
+When traceback is formatting a RecursionError with a long __context__ chain it runs the risk of itself raising a RecursionError. This patch catches the corresponding recursive call's RecursionError, truncates the traceback and emits a warning instead.
\ No newline at end of file

From cdfef13d8e6f553f72917f197b638fe7bbbfc449 Mon Sep 17 00:00:00 2001
From: Vladimir Feinberg <vladimir.feinberg@gmail.com>
Date: Sat, 6 Feb 2021 08:53:07 -0800
Subject: [PATCH 03/16] iritkatriel comments

---
 Lib/test/test_traceback.py |  2 +-
 Lib/traceback.py           | 11 ++++++++---
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/Lib/test/test_traceback.py b/Lib/test/test_traceback.py
index 9d565fcaa4aa72..56a9bc3f3e249d 100644
--- a/Lib/test/test_traceback.py
+++ b/Lib/test/test_traceback.py
@@ -92,7 +92,7 @@ def test_base_exception(self):
         lst = traceback.format_exception_only(e.__class__, e)
         self.assertEqual(lst, ['KeyboardInterrupt\n'])
 
-    def test_traceback_recursionerror(self):
+    def test_traceback_context_recursionerror(self):
         # Test that for long traceback chains traceback does not itself
         # raise a recursion error while printing (Issue43048)
 
diff --git a/Lib/traceback.py b/Lib/traceback.py
index b659f754879e7c..ec2bae39d84e55 100644
--- a/Lib/traceback.py
+++ b/Lib/traceback.py
@@ -3,7 +3,6 @@
 import collections
 import itertools
 import linecache
-import warnings
 import sys
 
 __all__ = ['extract_stack', 'extract_tb', 'format_exception',
@@ -489,6 +488,7 @@ def __init__(self, exc_type, exc_value, exc_traceback, *, limit=None,
                 _seen=_seen)
         else:
             cause = None
+        self._truncated_context = False
         if (exc_value and exc_value.__context__ is not None
             and id(exc_value.__context__) not in _seen):
             try:
@@ -501,8 +501,7 @@ def __init__(self, exc_type, exc_value, exc_traceback, *, limit=None,
                     capture_locals=capture_locals,
                     _seen=_seen)
             except RecursionError:
-                warnings.warn(
-                    'A RecursionError occurred while processing an exception; truncating traceback.');
+                self._truncated_context = True
                 context = None
         else:
             context = None
@@ -626,6 +625,12 @@ def format(self, *, chain=True):
                 not self.__suppress_context__):
                 yield from self.__context__.format(chain=chain)
                 yield _context_message
+            if self._truncated_context:
+                yield (
+                    'Warning: during handling of the above exception, '
+                    'more exceptions occurred, but were truncated to avoid '
+                    'stack overflow in the exception handler. '
+                    'See https://bugs.python.org/issue43048 for details.\n')
         if self.stack:
             yield 'Traceback (most recent call last):\n'
             yield from self.stack.format()

From c87b34bccc1fc2c35db34e1b7da26e7f1947485f Mon Sep 17 00:00:00 2001
From: Vladimir Feinberg <vladimir.feinberg@gmail.com>
Date: Sat, 6 Feb 2021 09:12:08 -0800
Subject: [PATCH 04/16] bonus bug: cause recursion

---
 Lib/test/test_traceback.py | 37 ++++++++++++++++++++++++++++++++++++-
 Lib/traceback.py           | 30 ++++++++++++++++++++++--------
 2 files changed, 58 insertions(+), 9 deletions(-)

diff --git a/Lib/test/test_traceback.py b/Lib/test/test_traceback.py
index 56a9bc3f3e249d..8bfbc45593a730 100644
--- a/Lib/test/test_traceback.py
+++ b/Lib/test/test_traceback.py
@@ -105,7 +105,7 @@ def f():
 
         try:
             f()
-        except RecursionError as e:
+        except RecursionError:
             # Creating this exception should not fail if internally
             # the traceback exception is constructed safely with respect
             # to recursion context (since TracebackExceptions are used
@@ -114,6 +114,41 @@ def f():
             exc_info = sys.exc_info()
             traceback.TracebackException(exc_info[0], exc_info[1], exc_info[2])
 
+    def test_traceback_cause_recursionerror(self):
+        # Same as test_traceback_context_recursionerror, but with
+        # a __cause__ chain.
+
+        def f():
+            e = None
+            try:
+                f()
+            except Exception as exc:
+                e = exc
+            raise Exception from e
+
+        try:
+            f()
+        except Exception:
+            exc_info = sys.exc_info()
+            traceback.TracebackException(exc_info[0], exc_info[1], exc_info[2])
+
+    def test_traceback_cause_context_recursionerror(self):
+        # Same as test_traceback_context_recursionerror, but with
+        # both a __cause__ and __context__ chain.
+
+        def f():
+            e = None
+            try:
+                f()
+            except Exception as exc:
+                raise RuntimeException() from exc
+
+        try:
+            f()
+        except Exception:
+            exc_info = sys.exc_info()
+            traceback.TracebackException(exc_info[0], exc_info[1], exc_info[2])
+
     def test_format_exception_only_bad__str__(self):
         class X(Exception):
             def __str__(self):
diff --git a/Lib/traceback.py b/Lib/traceback.py
index ec2bae39d84e55..f767d5d85bfc5c 100644
--- a/Lib/traceback.py
+++ b/Lib/traceback.py
@@ -476,16 +476,24 @@ def __init__(self, exc_type, exc_value, exc_traceback, *, limit=None,
         _seen.add(id(exc_value))
         # Gracefully handle (the way Python 2.4 and earlier did) the case of
         # being called with no type or value (None, None, None).
+        self._truncated_cause = False
         if (exc_value and exc_value.__cause__ is not None
             and id(exc_value.__cause__) not in _seen):
-            cause = TracebackException(
-                type(exc_value.__cause__),
-                exc_value.__cause__,
-                exc_value.__cause__.__traceback__,
-                limit=limit,
-                lookup_lines=False,
-                capture_locals=capture_locals,
-                _seen=_seen)
+            try:
+                cause = TracebackException(
+                    type(exc_value.__cause__),
+                    exc_value.__cause__,
+                    exc_value.__cause__.__traceback__,
+                    limit=limit,
+                    lookup_lines=False,
+                    capture_locals=capture_locals,
+                    _seen=_seen)
+            except RecursionError:
+                # The recursive call to the constructor in the try above
+                # may result in a stack overflow for long exception chains,
+                # so we must truncate.
+                self._truncated_cause = True
+                cause = None
         else:
             cause = None
         self._truncated_context = False
@@ -625,6 +633,12 @@ def format(self, *, chain=True):
                 not self.__suppress_context__):
                 yield from self.__context__.format(chain=chain)
                 yield _context_message
+            if self._truncated_cause:
+                yield (
+                    'Warning: the above exception was the cause of other '
+                    'exceptions, but those were truncated to avoid '
+                    'stack overflow in the exception handler. '
+                    'See https://bugs.python.org/issue43048 for details.\n')
             if self._truncated_context:
                 yield (
                     'Warning: during handling of the above exception, '

From e1243dde14b8e25046e479a01eef6abcf550ed23 Mon Sep 17 00:00:00 2001
From: Vladimir Feinberg <vladimir.feinberg@gmail.com>
Date: Sat, 6 Feb 2021 09:36:03 -0800
Subject: [PATCH 05/16] simplify exception

---
 Lib/test/test_traceback.py | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/Lib/test/test_traceback.py b/Lib/test/test_traceback.py
index 8bfbc45593a730..772c667c845fe8 100644
--- a/Lib/test/test_traceback.py
+++ b/Lib/test/test_traceback.py
@@ -137,11 +137,10 @@ def test_traceback_cause_context_recursionerror(self):
         # both a __cause__ and __context__ chain.
 
         def f():
-            e = None
             try:
                 f()
             except Exception as exc:
-                raise RuntimeException() from exc
+                raise Exception() from exc
 
         try:
             f()

From e13a98b3c2a33cf3f9db2603fe7b72bb083f02a7 Mon Sep 17 00:00:00 2001
From: Vladimir Feinberg <vladimir.feinberg@gmail.com>
Date: Sat, 6 Feb 2021 12:52:23 -0800
Subject: [PATCH 06/16] test format_exception over TracebackException to
 validate recursion in format()

---
 Lib/test/test_traceback.py | 10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

diff --git a/Lib/test/test_traceback.py b/Lib/test/test_traceback.py
index 772c667c845fe8..d25249500193d3 100644
--- a/Lib/test/test_traceback.py
+++ b/Lib/test/test_traceback.py
@@ -106,13 +106,8 @@ def f():
         try:
             f()
         except RecursionError:
-            # Creating this exception should not fail if internally
-            # the traceback exception is constructed safely with respect
-            # to recursion context (since TracebackExceptions are used
-            # for printing and formatting exceptions we should allow their
-            # creation even for RecursionErrors with long chains).
             exc_info = sys.exc_info()
-            traceback.TracebackException(exc_info[0], exc_info[1], exc_info[2])
+            traceback.format_exception(exc_info[0], exc_info[1], exc_info[2])
 
     def test_traceback_cause_recursionerror(self):
         # Same as test_traceback_context_recursionerror, but with
@@ -130,7 +125,7 @@ def f():
             f()
         except Exception:
             exc_info = sys.exc_info()
-            traceback.TracebackException(exc_info[0], exc_info[1], exc_info[2])
+            traceback.format_exception(exc_info[0], exc_info[1], exc_info[2])
 
     def test_traceback_cause_context_recursionerror(self):
         # Same as test_traceback_context_recursionerror, but with
@@ -140,6 +135,7 @@ def f():
             try:
                 f()
             except Exception as exc:
+                # exc is both the cause and context of this new fresh exception
                 raise Exception() from exc
 
         try:

From 8550dbdd53b87dde1f32288f250507bc43b68130 Mon Sep 17 00:00:00 2001
From: Vladimir Feinberg <vladimir.feinberg@gmail.com>
Date: Sat, 6 Feb 2021 14:14:48 -0800
Subject: [PATCH 07/16] add stub

---
 Lib/test/test_traceback.py | 2 +-
 Lib/traceback.py           | 5 +++++
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/Lib/test/test_traceback.py b/Lib/test/test_traceback.py
index d25249500193d3..1f4da7576107ba 100644
--- a/Lib/test/test_traceback.py
+++ b/Lib/test/test_traceback.py
@@ -142,7 +142,7 @@ def f():
             f()
         except Exception:
             exc_info = sys.exc_info()
-            traceback.TracebackException(exc_info[0], exc_info[1], exc_info[2])
+            traceback.format_exception(exc_info[0], exc_info[1], exc_info[2])
 
     def test_format_exception_only_bad__str__(self):
         class X(Exception):
diff --git a/Lib/traceback.py b/Lib/traceback.py
index f767d5d85bfc5c..2db761f7799c73 100644
--- a/Lib/traceback.py
+++ b/Lib/traceback.py
@@ -465,6 +465,9 @@ class TracebackException:
     - :attr:`msg` For syntax errors - the compiler error message.
     """
 
+    def hello(self, exc_value):
+        return id(exc_value) and exc_value
+
     def __init__(self, exc_type, exc_value, exc_traceback, *, limit=None,
             lookup_lines=True, capture_locals=False, _seen=None):
         # NB: we need to accept exc_traceback, exc_value, exc_traceback to
@@ -476,6 +479,8 @@ def __init__(self, exc_type, exc_value, exc_traceback, *, limit=None,
         _seen.add(id(exc_value))
         # Gracefully handle (the way Python 2.4 and earlier did) the case of
         # being called with no type or value (None, None, None).
+
+        self.hello(exc_value)
         self._truncated_cause = False
         if (exc_value and exc_value.__cause__ is not None
             and id(exc_value.__cause__) not in _seen):

From 1ac601dfd38a3e55e895936e9f47681ac6b3ee66 Mon Sep 17 00:00:00 2001
From: Vladimir Feinberg <vladimir.feinberg@gmail.com>
Date: Sun, 7 Feb 2021 10:48:03 -0800
Subject: [PATCH 08/16] iritkatriel comments: remove no-op, fix indent, concise
 truncation messages

There seems to be another bug that's out-of-scope for this PR, which is handling the tstate->recursion_headroom clearing that's needed for the cause_context test case to pass. I fixed this temporarily by calling a no-op function which I think allows cpython to notice that the stack has been unwound, but it's a convoluted hack and is better addressed head-on by a fix in the ceval code.
---
 Lib/test/test_traceback.py | 21 +++------------------
 Lib/traceback.py           | 12 ++++--------
 2 files changed, 7 insertions(+), 26 deletions(-)

diff --git a/Lib/test/test_traceback.py b/Lib/test/test_traceback.py
index 1f4da7576107ba..987fcb955087e4 100644
--- a/Lib/test/test_traceback.py
+++ b/Lib/test/test_traceback.py
@@ -107,7 +107,8 @@ def f():
             f()
         except RecursionError:
             exc_info = sys.exc_info()
-            traceback.format_exception(exc_info[0], exc_info[1], exc_info[2])
+
+        traceback.format_exception(exc_info[0], exc_info[1], exc_info[2])
 
     def test_traceback_cause_recursionerror(self):
         # Same as test_traceback_context_recursionerror, but with
@@ -125,24 +126,8 @@ def f():
             f()
         except Exception:
             exc_info = sys.exc_info()
-            traceback.format_exception(exc_info[0], exc_info[1], exc_info[2])
-
-    def test_traceback_cause_context_recursionerror(self):
-        # Same as test_traceback_context_recursionerror, but with
-        # both a __cause__ and __context__ chain.
-
-        def f():
-            try:
-                f()
-            except Exception as exc:
-                # exc is both the cause and context of this new fresh exception
-                raise Exception() from exc
 
-        try:
-            f()
-        except Exception:
-            exc_info = sys.exc_info()
-            traceback.format_exception(exc_info[0], exc_info[1], exc_info[2])
+        traceback.format_exception(exc_info[0], exc_info[1], exc_info[2])
 
     def test_format_exception_only_bad__str__(self):
         class X(Exception):
diff --git a/Lib/traceback.py b/Lib/traceback.py
index 2db761f7799c73..fb05306d3589f4 100644
--- a/Lib/traceback.py
+++ b/Lib/traceback.py
@@ -640,16 +640,12 @@ def format(self, *, chain=True):
                 yield _context_message
             if self._truncated_cause:
                 yield (
-                    'Warning: the above exception was the cause of other '
-                    'exceptions, but those were truncated to avoid '
-                    'stack overflow in the exception handler. '
-                    'See https://bugs.python.org/issue43048 for details.\n')
+                    'Explicitly chained exceptions have been truncated to avoid '
+                    'stack overflow in traceback formatting:\n')
             if self._truncated_context:
                 yield (
-                    'Warning: during handling of the above exception, '
-                    'more exceptions occurred, but were truncated to avoid '
-                    'stack overflow in the exception handler. '
-                    'See https://bugs.python.org/issue43048 for details.\n')
+                    'Implicitly chained exceptions have been truncated to avoid '
+                    'stack overflow in traceback formatting:\n')
         if self.stack:
             yield 'Traceback (most recent call last):\n'
             yield from self.stack.format()

From 4d028b3e8f612b9ddcf86abb981d621dec153e20 Mon Sep 17 00:00:00 2001
From: Vladimir Feinberg <vladimir.feinberg@gmail.com>
Date: Sun, 7 Feb 2021 10:49:12 -0800
Subject: [PATCH 09/16] goodbye

---
 Lib/traceback.py | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/Lib/traceback.py b/Lib/traceback.py
index fb05306d3589f4..e47d5d76f01a12 100644
--- a/Lib/traceback.py
+++ b/Lib/traceback.py
@@ -465,9 +465,6 @@ class TracebackException:
     - :attr:`msg` For syntax errors - the compiler error message.
     """
 
-    def hello(self, exc_value):
-        return id(exc_value) and exc_value
-
     def __init__(self, exc_type, exc_value, exc_traceback, *, limit=None,
             lookup_lines=True, capture_locals=False, _seen=None):
         # NB: we need to accept exc_traceback, exc_value, exc_traceback to
@@ -480,7 +477,6 @@ def __init__(self, exc_type, exc_value, exc_traceback, *, limit=None,
         # Gracefully handle (the way Python 2.4 and earlier did) the case of
         # being called with no type or value (None, None, None).
 
-        self.hello(exc_value)
         self._truncated_cause = False
         if (exc_value and exc_value.__cause__ is not None
             and id(exc_value.__cause__) not in _seen):

From 567b478e2310c3447479811a7b6530f29b20f836 Mon Sep 17 00:00:00 2001
From: Vladimir Feinberg <vladimir.feinberg@gmail.com>
Date: Sun, 7 Feb 2021 12:15:35 -0800
Subject: [PATCH 10/16] retrigger checks: macos test failure is an unrelated
 connection flake


From 31782e56345b053ad8a321899f1c34ad72cc11df Mon Sep 17 00:00:00 2001
From: Vladimir Feinberg <vladimir.feinberg@gmail.com>
Date: Sun, 7 Feb 2021 14:20:18 -0800
Subject: [PATCH 11/16] single type of truncation across cause/context

---
 Lib/traceback.py | 15 +++++----------
 1 file changed, 5 insertions(+), 10 deletions(-)

diff --git a/Lib/traceback.py b/Lib/traceback.py
index e47d5d76f01a12..f46314e1661977 100644
--- a/Lib/traceback.py
+++ b/Lib/traceback.py
@@ -477,7 +477,7 @@ def __init__(self, exc_type, exc_value, exc_traceback, *, limit=None,
         # Gracefully handle (the way Python 2.4 and earlier did) the case of
         # being called with no type or value (None, None, None).
 
-        self._truncated_cause = False
+        self._truncated = False
         if (exc_value and exc_value.__cause__ is not None
             and id(exc_value.__cause__) not in _seen):
             try:
@@ -493,11 +493,10 @@ def __init__(self, exc_type, exc_value, exc_traceback, *, limit=None,
                 # The recursive call to the constructor in the try above
                 # may result in a stack overflow for long exception chains,
                 # so we must truncate.
-                self._truncated_cause = True
+                self._truncated = True
                 cause = None
         else:
             cause = None
-        self._truncated_context = False
         if (exc_value and exc_value.__context__ is not None
             and id(exc_value.__context__) not in _seen):
             try:
@@ -510,7 +509,7 @@ def __init__(self, exc_type, exc_value, exc_traceback, *, limit=None,
                     capture_locals=capture_locals,
                     _seen=_seen)
             except RecursionError:
-                self._truncated_context = True
+                self._truncated = True
                 context = None
         else:
             context = None
@@ -634,13 +633,9 @@ def format(self, *, chain=True):
                 not self.__suppress_context__):
                 yield from self.__context__.format(chain=chain)
                 yield _context_message
-            if self._truncated_cause:
+            if self._truncated:
                 yield (
-                    'Explicitly chained exceptions have been truncated to avoid '
-                    'stack overflow in traceback formatting:\n')
-            if self._truncated_context:
-                yield (
-                    'Implicitly chained exceptions have been truncated to avoid '
+                    'Chained exceptions have been truncated to avoid '
                     'stack overflow in traceback formatting:\n')
         if self.stack:
             yield 'Traceback (most recent call last):\n'

From 041fcfbe3a72bc4f0893f4c6bc2836cbe8b5e9a0 Mon Sep 17 00:00:00 2001
From: Vladimir Feinberg <vladimir.feinberg@gmail.com>
Date: Mon, 8 Feb 2021 08:23:25 -0800
Subject: [PATCH 12/16] don't print context if you have (possibly truncated)
 cause

---
 Lib/traceback.py | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/Lib/traceback.py b/Lib/traceback.py
index f46314e1661977..083bde19e2e6bd 100644
--- a/Lib/traceback.py
+++ b/Lib/traceback.py
@@ -476,7 +476,7 @@ def __init__(self, exc_type, exc_value, exc_traceback, *, limit=None,
         _seen.add(id(exc_value))
         # Gracefully handle (the way Python 2.4 and earlier did) the case of
         # being called with no type or value (None, None, None).
-
+        self.__suppress_context__ = False
         self._truncated = False
         if (exc_value and exc_value.__cause__ is not None
             and id(exc_value.__cause__) not in _seen):
@@ -495,6 +495,9 @@ def __init__(self, exc_type, exc_value, exc_traceback, *, limit=None,
                 # so we must truncate.
                 self._truncated = True
                 cause = None
+                # Suppress the context during printing, so that even when
+                # a cause is truncated we don't print the context.
+                self.__suppress_context__ = True
         else:
             cause = None
         if (exc_value and exc_value.__context__ is not None
@@ -515,8 +518,8 @@ def __init__(self, exc_type, exc_value, exc_traceback, *, limit=None,
             context = None
         self.__cause__ = cause
         self.__context__ = context
-        self.__suppress_context__ = \
-            exc_value.__suppress_context__ if exc_value else False
+        self.__suppress_context__ = self.__suppress_context__ or (
+            exc_value.__suppress_context__ if exc_value else False)
         # TODO: locals.
         self.stack = StackSummary.extract(
             walk_tb(exc_traceback), limit=limit, lookup_lines=lookup_lines,

From f93d09f9bdea3f3f46cb978aadf4da863ecc926d Mon Sep 17 00:00:00 2001
From: Vladimir Feinberg <vladimir.feinberg@gmail.com>
Date: Mon, 8 Feb 2021 08:57:22 -0800
Subject: [PATCH 13/16] just have one try/except

---
 Lib/traceback.py | 35 ++++++++++++++---------------------
 1 file changed, 14 insertions(+), 21 deletions(-)

diff --git a/Lib/traceback.py b/Lib/traceback.py
index 083bde19e2e6bd..ffe1012f2dd264 100644
--- a/Lib/traceback.py
+++ b/Lib/traceback.py
@@ -476,11 +476,10 @@ def __init__(self, exc_type, exc_value, exc_traceback, *, limit=None,
         _seen.add(id(exc_value))
         # Gracefully handle (the way Python 2.4 and earlier did) the case of
         # being called with no type or value (None, None, None).
-        self.__suppress_context__ = False
         self._truncated = False
-        if (exc_value and exc_value.__cause__ is not None
-            and id(exc_value.__cause__) not in _seen):
-            try:
+        try:
+            if (exc_value and exc_value.__cause__ is not None
+                and id(exc_value.__cause__) not in _seen):
                 cause = TracebackException(
                     type(exc_value.__cause__),
                     exc_value.__cause__,
@@ -489,20 +488,10 @@ def __init__(self, exc_type, exc_value, exc_traceback, *, limit=None,
                     lookup_lines=False,
                     capture_locals=capture_locals,
                     _seen=_seen)
-            except RecursionError:
-                # The recursive call to the constructor in the try above
-                # may result in a stack overflow for long exception chains,
-                # so we must truncate.
-                self._truncated = True
+            else:
                 cause = None
-                # Suppress the context during printing, so that even when
-                # a cause is truncated we don't print the context.
-                self.__suppress_context__ = True
-        else:
-            cause = None
-        if (exc_value and exc_value.__context__ is not None
-            and id(exc_value.__context__) not in _seen):
-            try:
+            if (exc_value and exc_value.__context__ is not None
+                and id(exc_value.__context__) not in _seen):
                 context = TracebackException(
                     type(exc_value.__context__),
                     exc_value.__context__,
@@ -511,14 +500,18 @@ def __init__(self, exc_type, exc_value, exc_traceback, *, limit=None,
                     lookup_lines=False,
                     capture_locals=capture_locals,
                     _seen=_seen)
-            except RecursionError:
-                self._truncated = True
+            else:
                 context = None
-        else:
+        except RecursionError:
+            # The recursive call to the constructors above
+            # may result in a stack overflow for long exception chains,
+            # so we must truncate.
+            self._truncated = True
+            cause = None
             context = None
         self.__cause__ = cause
         self.__context__ = context
-        self.__suppress_context__ = self.__suppress_context__ or (
+        self.__suppress_context__ = (
             exc_value.__suppress_context__ if exc_value else False)
         # TODO: locals.
         self.stack = StackSummary.extract(

From c0ceb243073fb2664b4448ebb7ac2ce77906a5d9 Mon Sep 17 00:00:00 2001
From: Vladimir Feinberg <vladimir.feinberg@gmail.com>
Date: Mon, 8 Feb 2021 08:59:01 -0800
Subject: [PATCH 14/16] no edit

---
 Lib/traceback.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/Lib/traceback.py b/Lib/traceback.py
index ffe1012f2dd264..d65a6098cc621f 100644
--- a/Lib/traceback.py
+++ b/Lib/traceback.py
@@ -511,8 +511,8 @@ def __init__(self, exc_type, exc_value, exc_traceback, *, limit=None,
             context = None
         self.__cause__ = cause
         self.__context__ = context
-        self.__suppress_context__ = (
-            exc_value.__suppress_context__ if exc_value else False)
+        self.__suppress_context__ = \
+            exc_value.__suppress_context__ if exc_value else False
         # TODO: locals.
         self.stack = StackSummary.extract(
             walk_tb(exc_traceback), limit=limit, lookup_lines=lookup_lines,

From 0b29f89df97388a690bed49b912e4125ee307d6d Mon Sep 17 00:00:00 2001
From: Vladimir Feinberg <vladimir.feinberg@gmail.com>
Date: Mon, 8 Feb 2021 09:16:46 -0800
Subject: [PATCH 15/16] update news

---
 .../next/Library/2021-02-06-05-34-01.bpo-43048.yCPUmo.rst       | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Misc/NEWS.d/next/Library/2021-02-06-05-34-01.bpo-43048.yCPUmo.rst b/Misc/NEWS.d/next/Library/2021-02-06-05-34-01.bpo-43048.yCPUmo.rst
index 476f0c15a9ed8b..90958cb344fec8 100644
--- a/Misc/NEWS.d/next/Library/2021-02-06-05-34-01.bpo-43048.yCPUmo.rst
+++ b/Misc/NEWS.d/next/Library/2021-02-06-05-34-01.bpo-43048.yCPUmo.rst
@@ -1 +1 @@
-When traceback is formatting a RecursionError with a long __context__ chain it runs the risk of itself raising a RecursionError. This patch catches the corresponding recursive call's RecursionError, truncates the traceback and emits a warning instead.
\ No newline at end of file
+Handle `RecursionError`s in :class:`~traceback.TracebackException`'s constructor, so that long exceptions chains are truncated instead of causing traceback formatting to fail.

From 1b585aa6c7e0f11e6799941204536f47595fe0d6 Mon Sep 17 00:00:00 2001
From: Vladimir Feinberg <vladimir.feinberg@gmail.com>
Date: Mon, 8 Feb 2021 17:06:31 -0800
Subject: [PATCH 16/16] singular recursionerror

---
 .../next/Library/2021-02-06-05-34-01.bpo-43048.yCPUmo.rst       | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Misc/NEWS.d/next/Library/2021-02-06-05-34-01.bpo-43048.yCPUmo.rst b/Misc/NEWS.d/next/Library/2021-02-06-05-34-01.bpo-43048.yCPUmo.rst
index 90958cb344fec8..99f6b2bbe9e30b 100644
--- a/Misc/NEWS.d/next/Library/2021-02-06-05-34-01.bpo-43048.yCPUmo.rst
+++ b/Misc/NEWS.d/next/Library/2021-02-06-05-34-01.bpo-43048.yCPUmo.rst
@@ -1 +1 @@
-Handle `RecursionError`s in :class:`~traceback.TracebackException`'s constructor, so that long exceptions chains are truncated instead of causing traceback formatting to fail.
+Handle `RecursionError` in :class:`~traceback.TracebackException`'s constructor, so that long exceptions chains are truncated instead of causing traceback formatting to fail.