python: collect: ignore exceptions with isinstance

Fixes pytest-dev#4266.

doc/4266.bugfix.rst

@@ -0,0 +1 @@
+Handle (ignore) exceptions raised during collection, e.g. with Django's LazySettings proxy class.

src/_pytest/compat.py

@@ -334,6 +334,14 @@ def safe_getattr(object, name, default):
         return default
 
 
+def safe_isclass(obj):
+    """Ignore any exception via isinstance on Python 3."""
+    try:
+        return isclass(obj)
+    except Exception:
+        return False
+
+
 def _is_unittest_unexpected_success_a_failure():
     """Return if the test suite should fail if an @expectedFailure unittest test PASSES.
 

src/_pytest/python.py

@@ -33,6 +33,7 @@
 from _pytest.compat import NOTSET
 from _pytest.compat import REGEX_TYPE
 from _pytest.compat import safe_getattr
+from _pytest.compat import safe_isclass
 from _pytest.compat import safe_str
 from _pytest.compat import STRING_TYPES
 from _pytest.config import hookimpl
@@ -195,7 +196,7 @@ def pytest_pycollect_makeitem(collector, name, obj):
     if res is not None:
         return
     # nothing was collected elsewhere, let's do it here
-    if isclass(obj):
+    if safe_isclass(obj):
         if collector.istestclass(obj, name):
             Class = collector._getcustomclass("Class")
             outcome.force_result(Class(name, parent=collector))

testing/python/raises.py

@@ -1,5 +1,7 @@
 import sys
 
+import six
+
 import pytest
 from _pytest.outcomes import Failed
 
@@ -170,3 +172,25 @@ class ClassLooksIterableException(Exception):
             Failed, match="DID NOT RAISE <class 'raises.ClassLooksIterableException'>"
         ):
             pytest.raises(ClassLooksIterableException, lambda: None)
+
+    def test_raises_with_raising_dunder_class(self):
+        """Test current behavior with regard to exceptions via __class__ (#4284)."""
+
+        class CrappyClass(Exception):
+            @property
+            def __class__(self):
+                assert False, "via __class__"
+
+        if six.PY2:
+            with pytest.raises(pytest.fail.Exception) as excinfo:
+                with pytest.raises(CrappyClass()):
+                    pass
+            assert "DID NOT RAISE" in excinfo.value.args[0]
+
+            with pytest.raises(CrappyClass) as excinfo:
+                raise CrappyClass()
+        else:
+            with pytest.raises(AssertionError) as excinfo:
+                with pytest.raises(CrappyClass()):
+                    pass
+            assert "via __class__" in excinfo.value.args[0]

testing/test_collection.py

@@ -977,3 +977,30 @@ def fix():
     result.stdout.fnmatch_lines(
         ["Could not determine arguments of *.fix *: invalid method signature"]
     )
+
+
+def test_collect_handles_raising_on_dunder_class(testdir):
+    """Handle proxy classes like Django's LazySettings that might raise on
+    ``isinstance`` (#4266).
+    """
+    testdir.makepyfile(
+        """
+        class ImproperlyConfigured(Exception):
+            pass
+
+        class RaisesOnGetAttr(object):
+            def raises(self):
+                raise ImproperlyConfigured
+
+            __class__ = property(raises)
+
+        raises = RaisesOnGetAttr()
+
+
+        def test_1():
+            pass
+    """
+    )
+    result = testdir.runpytest()
+    assert result.ret == 0
+    result.stdout.fnmatch_lines(["*1 passed in*"])

testing/test_compat.py

@@ -12,6 +12,7 @@
 from _pytest.compat import get_real_func
 from _pytest.compat import is_generator
 from _pytest.compat import safe_getattr
+from _pytest.compat import safe_isclass
 from _pytest.outcomes import OutcomeException
 
 
@@ -140,3 +141,14 @@ def test_safe_getattr():
     helper = ErrorsHelper()
     assert safe_getattr(helper, "raise_exception", "default") == "default"
     assert safe_getattr(helper, "raise_fail", "default") == "default"
+
+
+def test_safe_isclass():
+    assert safe_isclass(type) is True
+
+    class CrappyClass(Exception):
+        @property
+        def __class__(self):
+            assert False, "Should be ignored"
+
+    assert safe_isclass(CrappyClass()) is False