emit warning when pytest.warns receives unknown keyword arguments

changelog/4724.deprecation.rst

@@ -0,0 +1,3 @@
+``pytest.warns()`` now emits a warning when it receives unknown keyword arguments.
+
+This will be changed into an error in the future.

src/_pytest/deprecated.py

@@ -14,6 +14,7 @@
 
 from _pytest.warning_types import PytestDeprecationWarning
 from _pytest.warning_types import RemovedInPytest4Warning
+from _pytest.warning_types import UnformattedWarning
 
 
 YIELD_TESTS = "yield tests were removed in pytest 4.0 - {name} will be ignored"
@@ -87,3 +88,9 @@
     "pytest_logwarning is deprecated, no longer being called, and will be removed soon\n"
     "please use pytest_warning_captured instead"
 )
+
+PYTEST_WARNS_UNKNOWN_KWARGS = UnformattedWarning(
+    PytestDeprecationWarning,
+    "pytest.warns() got unexpected keyword arguments: {args!r}.\n"
+    "This will be an error in future versions.",
+)

src/_pytest/recwarn.py

@@ -11,6 +11,7 @@
 import six
 
 import _pytest._code
+from _pytest.deprecated import PYTEST_WARNS_UNKNOWN_KWARGS
 from _pytest.deprecated import WARNS_EXEC
 from _pytest.fixtures import yield_fixture
 from _pytest.outcomes import fail
@@ -84,10 +85,12 @@ def warns(expected_warning, *args, **kwargs):
 
     """
     __tracebackhide__ = True
-    match_expr = None
     if not args:
-        if "match" in kwargs:
-            match_expr = kwargs.pop("match")
+        match_expr = kwargs.pop("match", None)
+        if kwargs:
+            warnings.warn(
+                PYTEST_WARNS_UNKNOWN_KWARGS.format(args=sorted(kwargs)), stacklevel=2
+            )
         return WarningsChecker(expected_warning, match_expr=match_expr)
     elif isinstance(args[0], str):
         warnings.warn(WARNS_EXEC, stacklevel=2)
@@ -97,12 +100,12 @@ def warns(expected_warning, *args, **kwargs):
         loc = frame.f_locals.copy()
         loc.update(kwargs)
 
-        with WarningsChecker(expected_warning, match_expr=match_expr):
+        with WarningsChecker(expected_warning):
             code = _pytest._code.Source(code).compile()
             six.exec_(code, frame.f_globals, loc)
     else:
         func = args[0]
-        with WarningsChecker(expected_warning, match_expr=match_expr):
+        with WarningsChecker(expected_warning):
             return func(*args[1:], **kwargs)
 
 

testing/deprecated_test.py

@@ -6,6 +6,7 @@
 import sys
 
 import pytest
+from _pytest.warning_types import PytestDeprecationWarning
 from _pytest.warnings import SHOW_PYTEST_WARNINGS_ARG
 
 pytestmark = pytest.mark.pytester_example_path("deprecated")
@@ -238,3 +239,11 @@ def test_python_deprecation(testdir):
         )
     else:
         assert msg not in result.stdout.str()
+
+
+def test_pytest_warns_unknown_kwargs():
+    with pytest.warns(
+        PytestDeprecationWarning,
+        match=r"pytest.warns\(\) got unexpected keyword arguments: \['foo'\]",
+    ):
+        pytest.warns(UserWarning, foo="hello")