Improve error message for module-level __getattr__ issues

changelog/8265.improvement.rst

@@ -0,0 +1,4 @@
+Emit a ``PytestCollectionWarning`` when a module-level ``__getattr__`` returns ``None`` for ``pytestmark`` instead of raising ``AttributeError``.
+
+Previously this caused a cryptic ``TypeError: got None instead of Mark`` error.
+Now pytest issues a helpful warning and continues collecting the module normally.

src/_pytest/mark/structures.py

@@ -30,6 +30,7 @@
 from _pytest.outcomes import fail
 from _pytest.raises import AbstractRaises
 from _pytest.scope import _ScopeName
+from _pytest.warning_types import PytestCollectionWarning
 from _pytest.warning_types import PytestUnknownMarkWarning
 
 
@@ -443,7 +444,18 @@ def get_unpacked_marks(
                 mark_list.append(item)
     else:
         mark_attribute = getattr(obj, "pytestmark", [])
-        if isinstance(mark_attribute, list):
+        if mark_attribute is None:
+            warnings.warn(
+                "Module defines a `__getattr__` which returns None for "
+                "'pytestmark' instead of raising AttributeError. "
+                "Make sure `__getattr__` raises AttributeError for "
+                "attributes it does not provide. "
+                "See https://github.com/pytest-dev/pytest/issues/8265",
+                PytestCollectionWarning,
+                stacklevel=2,
+            )
+            mark_list = []
+        elif isinstance(mark_attribute, list):
             mark_list = mark_attribute
         else:
             mark_list = [mark_attribute]

testing/test_mark.py

@@ -1344,3 +1344,31 @@ def test_fixture_disallowed_between_marks() -> None:
         @pytest.mark.usefixtures("tmp_path")
         def foo():
             raise NotImplementedError()
+
+
+def test_module_getattr_without_attributeerror(pytester: Pytester) -> None:
+    """
+    Test that a helpful warning is emitted when a module-level
+    __getattr__ returns None instead of raising AttributeError.
+
+    Regression test for https://github.com/pytest-dev/pytest/issues/8265
+    """
+    pytester.makepyfile(
+        """
+        def __getattr__(key):
+            # Bug: should raise AttributeError, but returns None
+            return None
+
+        def test_something():
+            assert True
+        """
+    )
+    result = pytester.runpytest("-W", "always::pytest.PytestCollectionWarning")
+    result.stdout.fnmatch_lines(
+        [
+            "*PytestCollectionWarning*__getattr__*returns None*AttributeError*",
+        ]
+    )
+    # The module is buggy (__getattr__ returns None for all attributes),
+    # so no tests are collected, but pytest should NOT crash with a TypeError.
+    assert result.ret != ExitCode.INTERNAL_ERROR