Merge pull request #4682 from arel/parameterize-conditional-raises-document-only

Document parametrizing conditional raises

doc/en/example/parametrize.rst

@@ -565,3 +565,50 @@ As the result:
 - The test ``test_eval[1+7-8]`` passed, but the name is autogenerated and confusing.
 - The test ``test_eval[basic_2+4]`` passed.
 - The test ``test_eval[basic_6*9]`` was expected to fail and did fail.
+
+.. _`parametrizing_conditional_raising`:
+
+Parametrizing conditional raising
+--------------------------------------------------------------------
+
+Use :func:`pytest.raises` with the
+:ref:`pytest.mark.parametrize ref` decorator to write parametrized tests
+in which some tests raise exceptions and others do not.
+
+It is helpful to define a no-op context manager ``does_not_raise`` to serve
+as a complement to ``raises``. For example::
+
+    from contextlib import contextmanager
+    import pytest
+
+    @contextmanager
+    def does_not_raise():
+        yield
+
+
+    @pytest.mark.parametrize('example_input,expectation', [
+        (3, does_not_raise()),
+        (2, does_not_raise()),
+        (1, does_not_raise()),
+        (0, pytest.raises(ZeroDivisionError)),
+    ])
+    def test_division(example_input, expectation):
+        """Test how much I know division."""
+        with expectation:
+            assert (6 / example_input) is not None
+
+In the example above, the first three test cases should run unexceptionally,
+while the fourth should raise ``ZeroDivisionError``.
+
+If you're only supporting Python 3.7+, you can simply use ``nullcontext``
+to define ``does_not_raise``::
+
+    from contextlib import nullcontext as does_not_raise
+
+Or, if you're supporting Python 3.3+ you can use::
+
+    from contextlib import ExitStack as does_not_raise
+
+Or, if desired, you can ``pip install contextlib2`` and use::
+
+    from contextlib2 import ExitStack as does_not_raise