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

Document parametrizing conditional raises

Author: asottile

AUTHORS

@@ -27,6 +27,7 @@ Anthony Shaw
 Anthony Sottile
 Anton Lodder
 Antony Lee
+Arel Cordero
 Armin Rigo
 Aron Coyle
 Aron Curzon

changelog/4324.doc.rst

@@ -0,0 +1 @@
+Document how to use ``raises`` and ``does_not_raise`` to write parametrized tests with 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

src/_pytest/python_api.py

@@ -621,6 +621,14 @@ def raises(expected_exception, *args, **kwargs):
            ...
            >>> assert exc_info.type is ValueError
 
+    **Using with** ``pytest.mark.parametrize``
+
+    When using :ref:`pytest.mark.parametrize ref`
+    it is possible to parametrize tests such that
+    some runs raise an exception and others do not.
+
+    See :ref:`parametrizing_conditional_raising` for an example.
+
     **Legacy form**
 
     It is possible to specify a callable by passing a to-be-called lambda::

testing/python/raises.py

@@ -94,6 +94,54 @@ def test_raise_wrong_exception_passes_by():
         result = testdir.runpytest()
         result.stdout.fnmatch_lines(["*3 passed*"])
 
+    def test_does_not_raise(self, testdir):
+        testdir.makepyfile(
+            """
+            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
+        """
+        )
+        result = testdir.runpytest()
+        result.stdout.fnmatch_lines(["*4 passed*"])
+
+    def test_does_not_raise_does_raise(self, testdir):
+        testdir.makepyfile(
+            """
+            from contextlib import contextmanager
+            import pytest
+
+            @contextmanager
+            def does_not_raise():
+                yield
+
+            @pytest.mark.parametrize('example_input,expectation', [
+                (0, does_not_raise()),
+                (1, pytest.raises(ZeroDivisionError)),
+            ])
+            def test_division(example_input, expectation):
+                '''Test how much I know division.'''
+                with expectation:
+                    assert (6 / example_input) is not None
+        """
+        )
+        result = testdir.runpytest()
+        result.stdout.fnmatch_lines(["*2 failed*"])
+
     def test_noclass(self):
         with pytest.raises(TypeError):
             pytest.raises("wrong", lambda: None)