Do not chain the original mock error to wrapped assertion failures

CHANGELOG.rst

@@ -1,6 +1,11 @@
 Releases
 ========
 
+UNRELEASED
+----------
+
+* `#641 <https://github.com/pytest-dev/pytest-mock/pull/641>`_: Failures of mock assertions such as ``assert_called_with`` are again reported only once, instead of two or three times with the original error from the ``mock`` module chained before them.
+
 3.16.1
 ------
 

src/pytest_mock/plugin.py

@@ -565,7 +565,9 @@ def assert_wrapper(
                     msg += "\n\npytest introspection follows:\n" + introspection
         e = AssertionError(msg)
         e._mock_introspection_applied = True  # type:ignore[attr-defined]
-        raise e  # noqa:TRY201
+        # Suppress the context, otherwise pytest shows the original error
+        # (and the mock frames) chained before this one.
+        raise e from None
 
 
 def _unpack_expected_call(expect_call: Any) -> tuple[Any, Any] | None:
@@ -651,7 +653,9 @@ def assert_has_calls_wrapper(
                 msg += "\n\npytest introspection follows:\n" + introspection
         e = AssertionError(msg)
         e._mock_introspection_applied = True  # type:ignore[attr-defined]
-        raise e  # noqa:TRY201
+        # Suppress the context, otherwise pytest shows the original error
+        # (and the mock frames) chained before this one.
+        raise e from None
 
 
 def wrap_assert_not_called(*args: Any, **kwargs: Any) -> None:

tests/test_pytest_mock.py

@@ -1106,6 +1106,35 @@ def test_foo(mocker):
         result = testdir.runpytest()
         assert "pytest introspection follows:" not in result.stdout.str()
 
+    @pytest.mark.parametrize(
+        "assertion",
+        [
+            "stub.assert_called_with(2)",
+            # assert_called_once_with calls assert_called_with internally,
+            # so both wrappers are involved.
+            "stub.assert_called_once_with(2)",
+            "stub.assert_not_called()",
+            "stub.assert_has_calls([mocker.call(2)])",
+        ],
+    )
+    def test_no_chained_tracebacks(self, testdir: Any, assertion: str) -> None:
+        """
+        The wrapped assertions report the failure only once, without the
+        original exception from the mock module chained to it.
+        """
+        testdir.makepyfile(
+            f"""
+            def test_foo(mocker):
+                stub = mocker.stub()
+                stub(1)
+                {assertion}
+            """
+        )
+        result = testdir.runpytest()
+        result.assert_outcomes(failed=1)
+        assert "During handling of the above exception" not in result.stdout.str()
+        assert "mock.py" not in result.stdout.str()
+
 
 class TestAssertHasCalls:
     """``assert_has_calls``, including the shapes accepted as expected calls."""