document fixture override techniques

--HG--
branch : parametrized-fixture-override

doc/en/fixture.txt

@@ -780,4 +780,182 @@ to a :ref:`conftest.py <conftest.py>` file or even separately installable
 fixtures functions starts at test classes, then test modules, then
 ``conftest.py`` files and finally builtin and third party plugins.
 
+Overriding fixtures on various levels
+-------------------------------------
 
+In relatively large test suite, you most likely need to ``override`` a ``global`` or ``root`` fixture with a ``locally``
+defined one, keeping the test code readable and maintainable.
+
+Override a fixture on a folder (conftest) level
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Given the tests file structure is:
+
+::
+
+    tests/
+        __init__.py
+
+        conftest.py
+            # content of tests/conftest.py
+            import pytest
+
+            @pytest.fixture
+            def username():
+                return 'username'
+
+        test_something.py
+            # content of tests/test_something.py
+            def test_username(username):
+                assert username == 'username'
+
+        subfolder/
+            __init__.py
+
+            conftest.py
+                # content of tests/subfolder/conftest.py
+                import pytest
+
+                @pytest.fixture
+                def username(username):
+                    return 'overridden-' + username
+
+            test_something.py
+                # content of tests/subfolder/test_something.py
+                def test_username(username):
+                    assert username == 'overridden-username'
+
+As you can see, a fixture with the same name can be overridden for certain test folder level.
+Note that the ``base`` or ``super`` fixture can be accessed from the ``overriding``
+fixture easily - used in the example above.
+
+Override a fixture on a test module level
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Given the tests file structure is:
+
+::
+
+    tests/
+        __init__.py
+
+        conftest.py
+            # content of tests/conftest.py
+            @pytest.fixture
+            def username():
+                return 'username'
+
+        test_something.py
+            # content of tests/test_something.py
+            import pytest
+
+            @pytest.fixture
+            def username(username):
+                return 'overridden-' + username
+
+            def test_username(username):
+                assert username == 'overridden-username'
+
+        test_something_else.py
+            # content of tests/test_something_else.py
+            import pytest
+
+            @pytest.fixture
+            def username(username):
+                return 'overridden-else-' + username
+
+            def test_username(username):
+                assert username == 'overridden-else-username'
+
+In the example above, a fixture with the same name can be overridden for certain test module.
+
+
+Override a fixture with direct test parametrization
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Given the tests file structure is:
+
+::
+
+    tests/
+        __init__.py
+
+        conftest.py
+            # content of tests/conftest.py
+            import pytest
+
+            @pytest.fixture
+            def username():
+                return 'username'
+
+            @pytest.fixture
+            def other_username(username):
+                return 'other-' + username
+
+        test_something.py
+            # content of tests/test_something.py
+            import pytest
+
+            @pytest.mark.parametrize('username', ['directly-overridden-username'])
+            def test_username(username):
+                assert username == 'directly-overridden-username'
+
+            @pytest.mark.parametrize('username', ['directly-overridden-username-other'])
+            def test_username_other(other_username):
+                assert username == 'other-directly-overridden-username-other'
+
+In the example above, a fixture value is overridden by the test parameter value. Note that the value of the fixture
+can be overridden this way even if the test doesn't use it directly (doesn't mention it in the function prototype).
+
+
+Override a parametrized fixture with non-parametrized one and vice versa
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Given the tests file structure is:
+
+::
+
+    tests/
+        __init__.py
+
+        conftest.py
+            # content of tests/conftest.py
+            import pytest
+
+            @pytest.fixture(params=['one', 'two', 'three'])
+            def parametrized_username(request):
+                return request.param
+
+            @pytest.fixture
+            def non_parametrized_username(request):
+                return 'username'
+
+        test_something.py
+            # content of tests/test_something.py
+            import pytest
+
+            @pytest.fixture
+            def parametrized_username():
+                return 'overridden-username'
+
+            @pytest.fixture(params=['one', 'two', 'three'])
+            def non_parametrized_username(request):
+                return request.param
+
+            def test_username(parametrized_username):
+                assert parametrized_username == 'overridden-username'
+
+            def test_parametrized_username(non_parametrized_username):
+                assert non_parametrized_username in ['one', 'two', 'three']
+
+        test_something_else.py
+            # content of tests/test_something_else.py
+            def test_username(parametrized_username):
+                assert parametrized_username in ['one', 'two', 'three']
+
+            def test_username(non_parametrized_username):
+                assert non_parametrized_username == 'username'
+
+In the example above, a parametrized fixture is overridden with a non-parametrized version, and
+a non-parametrized fixture is overridden with a parametrized version for certain test module.
+The same applies for the test folder level obviously.