fix(utilities): stop truncating every traceback for a loud BaseError

docs/source/pcapkit/utilities/exceptions.rst

@@ -13,9 +13,12 @@ Loud and Quiet Errors
 Raising a :class:`~pcapkit.utilities.exceptions.BaseError` is, by default, a
 **loud** act: the error is logged once at :data:`logging.CRITICAL` on the
 :data:`~pcapkit.utilities.logging.logger` logger, and outside development mode
-:data:`sys.tracebacklimit` is set to ``0``, which suppresses the traceback frames
-entirely so the user sees the exception line rather than a walk through
-:mod:`pcapkit`'s internals.
+an exception hook is installed on :data:`sys.excepthook` and
+:data:`threading.excepthook`, the first time a loud error needs it, so the user
+sees the exception line rather than a walk through :mod:`pcapkit`'s internals.
+That hook only shortens the printing of a
+:class:`~pcapkit.utilities.exceptions.BaseError` itself; every other exception
+is handed on to whichever hook was previously installed, unchanged.
 
 ``quiet=True`` marks an error that :mod:`pcapkit` raises as **internal control
 flow** and expects to catch itself -- the
@@ -42,8 +45,8 @@ It is still an ordinary exception carrying its message, so ``except`` clauses an
    :show-inheritance:
 
    :param quiet: If :data:`True`, the error is neither logged nor allowed to
-      alter :data:`sys.tracebacklimit`; it is raised silently, as internal
-      control flow.
+      install the exception hook; it is raised silently, as internal control
+      flow.
    :param \*args: Arbitrary positional arguments.
    :param \*\*kwargs: Arbitrary keyword arguments.
 

tests/utilities/test_decorators.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import io
+import sys
 import unittest
 
 from tests._support import (bootstrap_core_modules, install_fake_payload_protocols,
@@ -13,10 +14,41 @@ def setUp(self) -> None:
         # binds partially-initialised real modules under ``pcapkit.*`` names, and
         # ``_payload_stand_ins`` binds outright stand-ins. See #660.
         isolate_modules(self)
+        self._protect_global_exception_state()
         modules = bootstrap_core_modules()
         self.decorators = modules['decorators']
         self.exceptions = modules['exceptions']
 
+    def _protect_global_exception_state(self) -> None:
+        """Restore ``sys.excepthook`` (and, defensively, ``sys.tracebacklimit``).
+
+        Several tests below construct a loud ``BaseError`` directly --
+        :meth:`test_beholder_wraps_struct_eof_with_no_payload`'s
+        ``StructError('unexpected eof', eof=True)`` does not pass ``quiet=True``,
+        for one -- and a loud error outside development mode installs
+        :mod:`pcapkit.utilities.exceptions`'s own exception hook on first use
+        (GitHub issue #719). This file had no ``tearDown`` before that issue and
+        never needed one: the equivalent side effect at the time,
+        ``sys.tracebacklimit = 0``, was already left unrestored, corrupting
+        every test that ran afterwards in the same process -- the #981 shape,
+        just not previously visible as a failure here. Addressed the way
+        ``isolate_modules`` addresses its own restore: via ``addCleanup``, so it
+        still runs if the rest of ``setUp`` raises partway through.
+
+        """
+        saved_hook = sys.excepthook
+        had_limit = hasattr(sys, 'tracebacklimit')
+        saved_limit = getattr(sys, 'tracebacklimit', None)
+
+        def _restore() -> None:
+            sys.excepthook = saved_hook
+            if had_limit:
+                sys.tracebacklimit = saved_limit
+            elif hasattr(sys, 'tracebacklimit'):
+                del sys.tracebacklimit
+
+        self.addCleanup(_restore)
+
     def test_seekset_restores_original_offset(self) -> None:
         class DemoProtocol:
             def __init__(self) -> None: