config: deprecate non-string values for string-typed ini options in ini mode

changelog/14808.deprecation.rst

@@ -0,0 +1,4 @@
+In ini mode (``.ini`` files and ``[tool.pytest.ini_options]`` in ``pyproject.toml``),
+reading a non-string value for a :confval:`string`-typed ini option (including
+options registered without an explicit type, which default to ``"string"``) is now
+deprecated and will raise a ``TypeError`` in pytest 10.

src/_pytest/config/__init__.py

@@ -79,6 +79,7 @@
 from _pytest.pathlib import safe_exists
 from _pytest.stash import Stash
 from _pytest.warning_types import PytestConfigWarning
+from _pytest.warning_types import PytestDeprecationWarning
 from _pytest.warning_types import warn_explicit_for
 
 
@@ -1949,6 +1950,15 @@ def _getini_ini(
         elif type == "bool":
             return _strtobool(str(value).strip())
         elif type == "string":
+            if not isinstance(value, str):
+                warnings.warn(
+                    PytestDeprecationWarning(
+                        f"{self.inipath}: config option '{name}' expects a string value, "
+                        f"got {builtins.type(value).__name__}: {value!r}. This will raise "
+                        "a TypeError in pytest 10."
+                    ),
+                    stacklevel=2,
+                )
             return value
         elif type == "int":
             if not isinstance(value, str):

testing/test_config.py

@@ -949,6 +949,33 @@ def pytest_addoption(parser):
         with pytest.raises(ValueError):
             config.getini("other")
 
+    def test_addini_string_type_toml_list_deprecated(self, pytester: Pytester) -> None:
+        # https://github.com/pytest-dev/pytest/issues/14808
+        pytester.makeconftest(
+            """
+            def pytest_addoption(parser):
+                parser.addini("mystr", "a string option", type="string")
+                parser.addini("myuntyped", "an untyped option")
+        """
+        )
+        pytester.makepyprojecttoml(
+            """
+            [tool.pytest.ini_options]
+            mystr = ["a", "b"]
+            myuntyped = ["c", "d"]
+        """
+        )
+        config = pytester.parseconfig()
+        with pytest.warns(
+            pytest.PytestDeprecationWarning, match="expects a string value"
+        ):
+            assert config.getini("mystr") == ["a", "b"]
+        # Options registered without an explicit type default to "string".
+        with pytest.warns(
+            pytest.PytestDeprecationWarning, match="expects a string value"
+        ):
+            assert config.getini("myuntyped") == ["c", "d"]
+
     @pytest.mark.parametrize("config_type", ["ini", "pyproject"])
     def test_addini_paths(self, pytester: Pytester, config_type: str) -> None:
         pytester.makeconftest(