Fix for warning test arguments with default values

src/_pytest/compat.py

@@ -174,6 +174,15 @@ def get_default_arg_names(function: Callable[..., Any]) -> tuple[str, ...]:
     )
 
 
+def get_default_name_val(function: Callable[..., Any]) -> dict[str, Any]:
+    sig = signature(function)
+    return {
+        param.name: param.default
+        for param in sig.parameters.values()
+        if param.default is not Parameter.empty
+    }
+
+
 _non_printable_ascii_translate_table = {
     i: f"\\x{i:02x}" for i in range(128) if i not in range(32, 127)
 }

src/_pytest/fixtures.py

@@ -41,6 +41,7 @@
 from _pytest._code.code import TerminalRepr
 from _pytest._io import TerminalWriter
 from _pytest.compat import _PytestWrapper
+from _pytest.compat import get_default_name_val
 from _pytest.compat import assert_never
 from _pytest.compat import get_real_func
 from _pytest.compat import get_real_method
@@ -70,7 +71,7 @@
 from _pytest.scope import _ScopeName
 from _pytest.scope import HIGH_SCOPES
 from _pytest.scope import Scope
-from _pytest.warning_types import PytestRemovedIn9Warning
+from _pytest.warning_types import PytestDefaultArgumentWarning, PytestRemovedIn9Warning
 from _pytest.warning_types import PytestWarning
 
 
@@ -1448,6 +1449,20 @@ def deduplicate_names(*seqs: Iterable[str]) -> tuple[str, ...]:
     return tuple(dict.fromkeys(name for seq in seqs for name in seq))
 
 
+def default_arg_warn(nodeid: str, function_name, param_name, param_val) -> None:
+    msg = (
+        f"Test function '{function_name}' has a default argument '{param_name}={param_val}'.\n"
+    )
+    warnings.simplefilter("always", PytestDefaultArgumentWarning)
+    warnings.warn(PytestDefaultArgumentWarning(msg))
+
+
+def check_default_arguments(func_name, default_args, nodeid):
+    """Check for default arguments in the function and issue warnings."""
+    for arg_name, default_val in default_args.items():
+        default_arg_warn(nodeid, func_name, arg_name, default_val)
+
+
 class FixtureManager:
     """pytest fixture definitions and information is stored and managed
     from this class.
@@ -1528,6 +1543,11 @@ def getfixtureinfo(
             ignore_args=direct_parametrize_args,
         )
 
+        if func is not None:
+            function_name = func.__name__
+            default_args = get_default_name_val(func)
+            check_default_arguments(function_name, default_args, node.nodeid)
+
         return FuncFixtureInfo(argnames, initialnames, names_closure, arg2fixturedefs)
 
     def pytest_plugin_registered(self, plugin: _PluggyPlugin, plugin_name: str) -> None:

src/_pytest/python.py

@@ -69,7 +69,7 @@
 from _pytest.scope import _ScopeName
 from _pytest.scope import Scope
 from _pytest.stash import StashKey
-from _pytest.warning_types import PytestCollectionWarning
+from _pytest.warning_types import PytestCollectionWarning, PytestDefaultArgumentWarning, warn_explicit_for
 
 
 if TYPE_CHECKING:
@@ -146,6 +146,7 @@ def async_fail(nodeid: str) -> None:
 @hookimpl(trylast=True)
 def pytest_pyfunc_call(pyfuncitem: Function) -> object | None:
     testfunction = pyfuncitem.obj
+
     if is_async_function(testfunction):
         async_fail(pyfuncitem.nodeid)
     funcargs = pyfuncitem.funcargs

src/_pytest/warning_types.py

@@ -44,6 +44,13 @@ class PytestCollectionWarning(PytestWarning):
     __module__ = "pytest"
 
 
+@final
+class PytestDefaultArgumentWarning(PytestWarning):
+    """Warning emitted when a test function has default arguments."""
+
+    __module__ = "pytest"
+
+
 class PytestDeprecationWarning(PytestWarning, DeprecationWarning):
     """Warning class for features that will be removed in a future version."""
 

testing/test_default_params.py

@@ -0,0 +1,52 @@
+from _pytest.pytester import Pytester
+
+def test_no_default_argument(pytester: Pytester) -> None:
+    pytester.makepyfile(
+        """
+        def test_with_default_param(param):
+            assert param == 42
+        """
+    )
+    result = pytester.runpytest()
+    result.stdout.fnmatch_lines([
+        "*fixture 'param' not found*"
+    ])
+
+
+def test_default_argument_warning(pytester: Pytester) -> None:
+    pytester.makepyfile(
+        """
+        def test_with_default_param(param=42):
+            assert param == 42
+        """
+    )
+    result = pytester.runpytest()
+    result.stdout.fnmatch_lines([
+        "*PytestDefaultArgumentWarning: Test function 'test_with_default_param' has a default argument 'param=42'.*"
+    ])
+
+
+def test_no_warning_for_no_default_param(pytester: Pytester) -> None:
+    pytester.makepyfile(
+        """
+        def test_without_default_param(param):
+            assert param is None
+        """
+    )
+    result = pytester.runpytest()
+    assert "PytestDefaultArgumentWarning" not in result.stdout.str()
+
+
+def test_warning_for_multiple_default_params(pytester: Pytester) -> None:
+    pytester.makepyfile(
+        """
+        def test_with_multiple_defaults(param1=42, param2="default"):
+            assert param1 == 42
+            assert param2 == "default"
+        """
+    )
+    result = pytester.runpytest()
+    result.stdout.fnmatch_lines([
+        "*PytestDefaultArgumentWarning: Test function 'test_with_multiple_defaults' has a default argument 'param1=42'.*",
+        "*PytestDefaultArgumentWarning: Test function 'test_with_multiple_defaults' has a default argument 'param2=default'.*"
+    ])