Emit warning for broken object

changelog/5404.bugfix.rst

@@ -0,0 +1,2 @@
+Emit a warning when attempting to unwrap a broken object raises an exception,
+for easier debugging (`#5080 <https://github.com/pytest-dev/pytest/issues/5080>`__).

src/_pytest/doctest.py

@@ -3,6 +3,7 @@ import inspect
 import platform
 import sys
 import traceback
+import warnings
 from contextlib import contextmanager
 
 import pytest
@@ -12,6 +13,7 @@ from _pytest._code.code import TerminalRepr
 from _pytest.compat import safe_getattr
 from _pytest.fixtures import FixtureRequest
 from _pytest.outcomes import Skipped
+from _pytest.warning_types import PytestWarning
 
 DOCTEST_REPORT_CHOICE_NONE = "none"
 DOCTEST_REPORT_CHOICE_CDIFF = "cdiff"
@@ -368,10 +370,18 @@ def _patch_unwrap_mock_aware():
     else:
 
         def _mock_aware_unwrap(obj, stop=None):
-            if stop is None:
+            try:
+                if stop is None or stop is _is_mocked:
                     return real_unwrap(obj, stop=_is_mocked)
-            else:
                 return real_unwrap(obj, stop=lambda obj: _is_mocked(obj) or stop(obj))
+            except Exception as e:
+                warnings.warn(
+                    "Got %r when unwrapping %r.  This is usually caused "
+                    "by a violation of Python's object protocol; see e.g. "
+                    "https://github.com/pytest-dev/pytest/issues/5080" % (e, obj),
+                    PytestWarning,
+                )
+                raise
 
         inspect.unwrap = _mock_aware_unwrap
         try:

testing/test_doctest.py

@@ -1,7 +1,10 @@
+import inspect
 import textwrap
 
 import pytest
 from _pytest.compat import MODULE_NOT_FOUND_ERROR
+from _pytest.doctest import _is_mocked
+from _pytest.doctest import _patch_unwrap_mock_aware
 from _pytest.doctest import DoctestItem
 from _pytest.doctest import DoctestModule
 from _pytest.doctest import DoctestTextfile
@@ -1224,3 +1227,24 @@ def test_doctest_mock_objects_dont_recurse_missbehaved(mock_module, testdir):
     )
     result = testdir.runpytest("--doctest-modules")
     result.stdout.fnmatch_lines(["* 1 passed *"])
+
+
+class Broken:
+    def __getattr__(self, _):
+        raise KeyError("This should be an AttributeError")
+
+
+@pytest.mark.parametrize(  # pragma: no branch (lambdas are not called)
+    "stop", [None, _is_mocked, lambda f: None, lambda f: False, lambda f: True]
+)
+def test_warning_on_unwrap_of_broken_object(stop):
+    bad_instance = Broken()
+    assert inspect.unwrap.__module__ == "inspect"
+    with _patch_unwrap_mock_aware():
+        assert inspect.unwrap.__module__ != "inspect"
+        with pytest.warns(
+            pytest.PytestWarning, match="^Got KeyError.* when unwrapping"
+        ):
+            with pytest.raises(KeyError):
+                inspect.unwrap(bad_instance, stop=stop)
+    assert inspect.unwrap.__module__ == "inspect"