Warn when test functions return other than None (#9956)

Closes #7337

AUTHORS

@@ -63,6 +63,7 @@ Ceridwen
 Charles Cloud
 Charles Machalow
 Charnjit SiNGH (CCSJ)
+Cheuk Ting Ho
 Chris Lamb
 Chris NeJame
 Chris Rose

changelog/7337.improvement.rst

@@ -0,0 +1 @@
+A warning is now emitted if a test function returns something other than `None`. This prevents a common mistake among beginners that expect that returning a `bool` (for example `return foo(a, b) == result`) would cause a test to pass or fail, instead of using `assert`.

doc/en/deprecations.rst

@@ -252,6 +252,47 @@ or ``pytest.warns(Warning)``.
 
 See :ref:`warns use cases` for examples.
 
+
+Returning non-None value in test functions
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. deprecated:: 7.2
+
+A :class:`pytest.PytestReturnNotNoneWarning` is now emitted if a test function returns something other than `None`.
+
+This prevents a common mistake among beginners that expect that returning a `bool` would cause a test to pass or fail, for example:
+
+.. code-block:: python
+
+    @pytest.mark.parametrize(
+        ["a", "b", "result"],
+        [
+            [1, 2, 5],
+            [2, 3, 8],
+            [5, 3, 18],
+        ],
+    )
+    def test_foo(a, b, result):
+        return foo(a, b) == result
+
+Given that pytest ignores the return value, this might be surprising that it will never fail.
+
+The proper fix is to change the `return` to an `assert`:
+
+.. code-block:: python
+
+    @pytest.mark.parametrize(
+        ["a", "b", "result"],
+        [
+            [1, 2, 5],
+            [2, 3, 8],
+            [5, 3, 18],
+        ],
+    )
+    def test_foo(a, b, result):
+        assert foo(a, b) == result
+
+
 The ``--strict`` command-line option
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 

doc/en/reference/reference.rst

@@ -1130,6 +1130,9 @@ Custom warnings generated in some situations such as improper usage or deprecate
 .. autoclass:: pytest.PytestExperimentalApiWarning
    :show-inheritance:
 
+.. autoclass:: pytest.PytestReturnNotNoneWarning
+  :show-inheritance:
+
 .. autoclass:: pytest.PytestUnhandledCoroutineWarning
    :show-inheritance:
 

src/_pytest/python.py

@@ -77,10 +77,12 @@ from _pytest.pathlib import parts
 from _pytest.pathlib import visit
 from _pytest.scope import Scope
 from _pytest.warning_types import PytestCollectionWarning
+from _pytest.warning_types import PytestReturnNotNoneWarning
 from _pytest.warning_types import PytestUnhandledCoroutineWarning
 
 if TYPE_CHECKING:
     from typing_extensions import Literal
+
     from _pytest.scope import _ScopeName
 
 
@@ -192,6 +194,13 @@ def pytest_pyfunc_call(pyfuncitem: "Function") -> Optional[object]:
     result = testfunction(**testargs)
     if hasattr(result, "__await__") or hasattr(result, "__aiter__"):
         async_warn_and_skip(pyfuncitem.nodeid)
+    elif result is not None:
+        warnings.warn(
+            PytestReturnNotNoneWarning(
+                f"Expected None, but {pyfuncitem.nodeid} returned {result!r}, which will be an error in a "
+                "future version of pytest.  Did you mean to use `assert` instead of `return`?"
+            )
+        )
     return True
 
 

src/_pytest/warning_types.py

@@ -55,6 +55,13 @@ class PytestRemovedIn8Warning(PytestDeprecationWarning):
     __module__ = "pytest"
 
 
+@final
+class PytestReturnNotNoneWarning(PytestDeprecationWarning):
+    """Warning emitted when a test function is returning value other than None."""
+
+    __module__ = "pytest"
+
+
 @final
 class PytestExperimentalApiWarning(PytestWarning, FutureWarning):
     """Warning category used to denote experiments in pytest.

src/pytest/__init__.py

@@ -69,6 +69,7 @@ from _pytest.warning_types import PytestConfigWarning
 from _pytest.warning_types import PytestDeprecationWarning
 from _pytest.warning_types import PytestExperimentalApiWarning
 from _pytest.warning_types import PytestRemovedIn8Warning
+from _pytest.warning_types import PytestReturnNotNoneWarning
 from _pytest.warning_types import PytestUnhandledCoroutineWarning
 from _pytest.warning_types import PytestUnhandledThreadExceptionWarning
 from _pytest.warning_types import PytestUnknownMarkWarning
@@ -127,6 +128,7 @@ __all__ = [
     "PytestDeprecationWarning",
     "PytestExperimentalApiWarning",
     "PytestRemovedIn8Warning",
+    "PytestReturnNotNoneWarning",
     "Pytester",
     "PytestPluginManager",
     "PytestUnhandledCoroutineWarning",

testing/acceptance_test.py

@@ -1292,3 +1292,14 @@ def test_no_brokenpipeerror_message(pytester: Pytester) -> None:
 
     # Cleanup.
     popen.stderr.close()
+
+
+def test_function_return_non_none_warning(testdir) -> None:
+    testdir.makepyfile(
+        """
+        def test_stuff():
+            return "something"
+    """
+    )
+    res = testdir.runpytest()
+    res.stdout.fnmatch_lines(["*Did you mean to use `assert` instead of `return`?*"])