fix: stop reusing shared PytestWarning instances whose tracebacks grow under -W error

changelog/14912.bugfix.rst

@@ -0,0 +1 @@
+:class:`~pytest.PytestWarning` instances defined in ``_pytest.deprecated`` are no longer reused across ``warnings.warn`` calls, so warnings raised as errors (e.g. with ``-W error``) no longer accumulate stale traceback frames from previous raises.

src/_pytest/config/__init__.py

@@ -280,7 +280,7 @@ def console_main() -> int:
 
     from _pytest.deprecated import CONSOLE_MAIN
 
-    warnings.warn(CONSOLE_MAIN, stacklevel=2)
+    warnings.warn(CONSOLE_MAIN.format(), stacklevel=2)
     return _console_main()
 
 
@@ -1191,7 +1191,7 @@ def inicfg(self) -> _DeprecatedInicfgProxy:
         @property
         def inicfg(self) -> _DeprecatedInicfgProxy:
             warnings.warn(
-                _pytest.deprecated.CONFIG_INICFG,
+                _pytest.deprecated.CONFIG_INICFG.format(),
                 stacklevel=2,
             )
             return _DeprecatedInicfgProxy(self)

src/_pytest/deprecated.py

@@ -4,9 +4,11 @@
 Keeping it in a central location makes it easy to track what is deprecated and should
 be removed when the time comes.
 
-All constants defined in this module should be either instances of
-:class:`PytestWarning`, or :class:`UnformattedWarning`
-in case of warnings which need to format their messages.
+All constants defined in this module should be instances of
+:class:`UnformattedWarning`, whose ``format`` method returns a fresh warning
+instance each time. Do not warn with shared warning instances: if a warning is
+raised as an error, CPython appends to the exception's existing ``__traceback__``,
+so a process-lifetime instance would accumulate stale frames from previous raises.
 """
 
 from __future__ import annotations
@@ -29,9 +31,10 @@
 
 
 # This could have been removed pytest 8, but it's harmless and common, so no rush to remove.
-YIELD_FIXTURE = PytestDeprecationWarning(
+YIELD_FIXTURE = UnformattedWarning(
+    PytestDeprecationWarning,
     "@pytest.yield_fixture is deprecated.\n"
-    "Use @pytest.fixture instead; they are the same."
+    "Use @pytest.fixture instead; they are the same.",
 )
 
 CLASS_FIXTURE_INSTANCE_METHOD = UnformattedWarning(
@@ -44,7 +47,10 @@
 )
 
 # This deprecation is never really meant to be removed.
-PRIVATE = PytestDeprecationWarning("A private pytest class or function was used.")
+PRIVATE = UnformattedWarning(
+    PytestDeprecationWarning,
+    "A private pytest class or function was used.",
+)
 
 
 HOOK_LEGACY_MARKING = UnformattedWarning(
@@ -56,11 +62,12 @@
     "#configuring-hook-specs-impls-using-markers",
 )
 
-MONKEYPATCH_LEGACY_NAMESPACE_PACKAGES = PytestRemovedIn10Warning(
+MONKEYPATCH_LEGACY_NAMESPACE_PACKAGES = UnformattedWarning(
+    PytestRemovedIn10Warning,
     "monkeypatch.syspath_prepend() called with pkg_resources legacy namespace packages detected.\n"
     "Legacy namespace packages (using pkg_resources.declare_namespace) are deprecated.\n"
     "Please use native namespace packages (PEP 420) instead.\n"
-    "See https://docs.pytest.org/en/stable/deprecations.html#monkeypatch-fixup-namespace-packages"
+    "See https://docs.pytest.org/en/stable/deprecations.html#monkeypatch-fixup-namespace-packages",
 )
 
 PARAMETRIZE_NON_COLLECTION_ITERABLE = UnformattedWarning(
@@ -71,15 +78,17 @@
     "See https://docs.pytest.org/en/stable/deprecations.html#parametrize-iterators",
 )
 
-CONSOLE_MAIN = PytestRemovedIn10Warning(
+CONSOLE_MAIN = UnformattedWarning(
+    PytestRemovedIn10Warning,
     "pytest.console_main() is deprecated and will be removed in pytest 10.\n"
     "It was never intended for programmatic use; use pytest.main() instead.\n"
-    "See https://docs.pytest.org/en/stable/deprecations.html#console-main"
+    "See https://docs.pytest.org/en/stable/deprecations.html#console-main",
 )
 
-CONFIG_INICFG = PytestRemovedIn10Warning(
+CONFIG_INICFG = UnformattedWarning(
+    PytestRemovedIn10Warning,
     "config.inicfg is deprecated, use config.getini() to access configuration values instead.\n"
-    "See https://docs.pytest.org/en/stable/deprecations.html#config-inicfg"
+    "See https://docs.pytest.org/en/stable/deprecations.html#config-inicfg",
 )
 
 FIXTURE_GETFIXTUREVALUE_DURING_TEARDOWN = UnformattedWarning(
@@ -90,10 +99,11 @@
     "See https://docs.pytest.org/en/stable/deprecations.html#dynamic-fixture-request-during-teardown",
 )
 
-PASTEBIN = PytestRemovedIn10Warning(
+PASTEBIN = UnformattedWarning(
+    PytestRemovedIn10Warning,
     "The --pastebin option is deprecated. "
     "The functionality is now available in an external plugin package, pytest-pastebin.\n"
-    "See https://docs.pytest.org/en/stable/deprecations.html#the-pastebin-option"
+    "See https://docs.pytest.org/en/stable/deprecations.html#the-pastebin-option",
 )
 
 # You want to make some `__init__` or function "private".
@@ -115,33 +125,38 @@
 # the warning (possibly error in the future).
 
 
-FIXTURE_BASEID_DEPRECATED = PytestRemovedIn10Warning(
-    "Passing baseid to FixtureDef is deprecated. Pass node instead for fixture scoping."
+FIXTURE_BASEID_DEPRECATED = UnformattedWarning(
+    PytestRemovedIn10Warning,
+    "Passing baseid to FixtureDef is deprecated. Pass node instead for fixture scoping.",
 )
 
-FIXTURE_NODEID_DEPRECATED = PytestRemovedIn10Warning(
+FIXTURE_NODEID_DEPRECATED = UnformattedWarning(
+    PytestRemovedIn10Warning,
     "Passing nodeid to _register_fixture is deprecated. "
-    "Pass node instead for fixture scoping."
+    "Pass node instead for fixture scoping.",
 )
 
-FIXTUREDEF_HAS_LOCATION_DEPRECATED = PytestRemovedIn10Warning(
+FIXTUREDEF_HAS_LOCATION_DEPRECATED = UnformattedWarning(
+    PytestRemovedIn10Warning,
     "FixtureDef.has_location is deprecated and will be removed in pytest 10. "
-    "See https://docs.pytest.org/en/stable/deprecations.html#fixturedef-has-location-deprecated"
+    "See https://docs.pytest.org/en/stable/deprecations.html#fixturedef-has-location-deprecated",
 )
 
-PARSEFACTORIES_NODEID_DEPRECATED = PytestRemovedIn10Warning(
+PARSEFACTORIES_NODEID_DEPRECATED = UnformattedWarning(
+    PytestRemovedIn10Warning,
     "Passing nodeid string to parsefactories is deprecated. "
-    "Use parsefactories(holder=obj, node=node) instead."
+    "Use parsefactories(holder=obj, node=node) instead.",
 )
 
-CALLSPEC2_RENAMED = PytestRemovedIn10Warning(
+CALLSPEC2_RENAMED = UnformattedWarning(
+    PytestRemovedIn10Warning,
     "_pytest.python.CallSpec2 has been renamed to CallSpec.\n"
     "The CallSpec2 alias will be removed in pytest 10.\n"
     "Update imports to use CallSpec instead.\n"
-    "See https://docs.pytest.org/en/stable/deprecations.html#callspec2-renamed"
+    "See https://docs.pytest.org/en/stable/deprecations.html#callspec2-renamed",
 )
 
 
 def check_ispytest(ispytest: bool) -> None:
     if not ispytest:
-        warn(PRIVATE, stacklevel=3)
+        warn(PRIVATE.format(), stacklevel=3)

src/_pytest/fixtures.py

@@ -1133,7 +1133,7 @@ def __init__(
         check_ispytest(_ispytest)
         # Emit deprecation warning if deprecated baseid string is used.
         if node is NOTSET:
-            warnings.warn(FIXTURE_BASEID_DEPRECATED, stacklevel=2)
+            warnings.warn(FIXTURE_BASEID_DEPRECATED.format(), stacklevel=2)
         if baseid is NOTSET:
             baseid = None
         # The node where this fixture was defined, if available.
@@ -1198,7 +1198,7 @@ def scope(self) -> ScopeName:
 
     @property
     def has_location(self) -> bool:
-        warnings.warn(FIXTUREDEF_HAS_LOCATION_DEPRECATED, stacklevel=2)
+        warnings.warn(FIXTUREDEF_HAS_LOCATION_DEPRECATED.format(), stacklevel=2)
         return self._has_location
 
     def addfinalizer(self, finalizer: Callable[[], object]) -> None:
@@ -1621,7 +1621,7 @@ def yield_fixture(
     .. deprecated:: 3.0
         Use :py:func:`pytest.fixture` directly instead.
     """
-    warnings.warn(YIELD_FIXTURE, stacklevel=2)
+    warnings.warn(YIELD_FIXTURE.format(), stacklevel=2)
     return fixture(
         fixture_function,
         *args,
@@ -2081,7 +2081,7 @@ def _register_fixture(
         """
         # Emit deprecation warning if nodeid string.
         if nodeid is not NOTSET or node is NOTSET:
-            warnings.warn(FIXTURE_NODEID_DEPRECATED, stacklevel=2)
+            warnings.warn(FIXTURE_NODEID_DEPRECATED.format(), stacklevel=2)
         fixture_def = FixtureDef(
             config=self.config,
             baseid=nodeid,
@@ -2278,7 +2278,7 @@ def parsefactories(
             raise TypeError("parsefactories() requires holder or node_or_obj")
         elif nodeid is not NOTSET:
             # Legacy: parsefactories(obj, nodeid) - string-based scoping only.
-            warnings.warn(PARSEFACTORIES_NODEID_DEPRECATED, stacklevel=2)
+            warnings.warn(PARSEFACTORIES_NODEID_DEPRECATED.format(), stacklevel=2)
             holderobj = node_or_obj
             effective_nodeid = nodeid
         else:

src/_pytest/monkeypatch.py

@@ -359,7 +359,7 @@ def syspath_prepend(self, path) -> None:
                     ns_pkg_path = path_obj / ns_pkg.replace(".", os.sep)
                     if ns_pkg_path.is_dir():
                         warnings.warn(
-                            MONKEYPATCH_LEGACY_NAMESPACE_PACKAGES, stacklevel=2
+                            MONKEYPATCH_LEGACY_NAMESPACE_PACKAGES.format(), stacklevel=2
                         )
                         break
 

src/_pytest/pastebin.py

@@ -35,7 +35,7 @@ def pytest_addoption(parser: Parser) -> None:
 @pytest.hookimpl(trylast=True)
 def pytest_configure(config: Config) -> None:
     if config.option.pastebin:
-        config.issue_config_time_warning(PASTEBIN, 2)
+        config.issue_config_time_warning(PASTEBIN.format(), 2)
 
     if config.option.pastebin == "all":
         tr = config.pluginmanager.getplugin("terminalreporter")

src/_pytest/python.py

@@ -1837,6 +1837,6 @@ def runtest(self) -> None:
 
 def __getattr__(name: str) -> object:
     if name == "CallSpec2":
-        warnings.warn(CALLSPEC2_RENAMED, stacklevel=2)
+        warnings.warn(CALLSPEC2_RENAMED.format(), stacklevel=2)
         return CallSpec
     raise AttributeError(f"module {__name__!r} has no attribute {name!r}")

testing/deprecated_test.py

@@ -364,3 +364,37 @@ def test_callspec2_renamed() -> None:
 
     with pytest.warns(pytest.PytestRemovedIn10Warning, match="CallSpec2"):
         assert python_mod.CallSpec2 is CallSpec
+
+
+def test_deprecation_warning_instance_not_shared_across_raises() -> None:
+    """Each raise must use a fresh warning instance (#14912).
+
+    When a warning is raised as an error, CPython appends to the exception's
+    existing ``__traceback__`` instead of replacing it, so reusing a single
+    module-level instance accumulates stale frames from previous raises.
+    """
+    import warnings
+
+    class PrivateClass:
+        def __init__(self) -> None:
+            deprecated.check_ispytest(False)
+
+    def count_frames(tb: object) -> int:
+        n = 0
+        while tb is not None:
+            n += 1
+            tb = tb.tb_next
+        return n
+
+    def raise_and_count() -> int:
+        with warnings.catch_warnings():
+            warnings.simplefilter("error", PytestDeprecationWarning)
+            with pytest.raises(PytestDeprecationWarning) as excinfo:
+                PrivateClass()
+        return count_frames(excinfo.value.__traceback__)
+
+    first = raise_and_count()
+    second = raise_and_count()
+
+    # The traceback of a shared instance would grow with every raise.
+    assert second <= first