Added documentation about @pytest_parametrize_plus and fixture_ref. Added a corresponding test.

Author: Sylvain MARIE

docs/index.md

@@ -6,7 +6,7 @@
 
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-!!! success "New `@pytest_fixture_plus` decorator is there, [check it out](#d-case-fixtures) !"
+!!! success "New `fixture_union` and `@pytest_parametrize_plus` are there, [check them out](#fixture_union) !"
 
 Did you ever thought that most of your test functions were actually *the same test code*, but with *different data inputs* and expected results/exceptions ?
 
@@ -275,6 +275,50 @@ This feature has been tested in very complex cases (several union fixtures, fixt
 !!! note "fixture unions vs. cases" 
     If you're familiar with `pytest-cases` already, you might note `@cases_data` is not so different than a fixture union: we do a union of all case functions. If one day union fixtures are directly supported by `pytest`, we will probably refactor this lib to align all the concepts.
 
+### `@pytest_parametrize_plus`
+
+`@pytest_parametrize_plus` is a replacement for `@pytest.mark.parametrize` that allows you to include references to fixtures in the parameter values. Simply use `fixture_ref(<fixture>)` in the parameter values, where `<fixture>` can be the fixture name or fixture function.
+
+For example:
+
+```python
+import pytest
+from pytest_cases import pytest_parametrize_plus, pytest_fixture_plus, fixture_ref
+
+@pytest.fixture
+def world_str():
+    return 'world'
+
+@pytest_fixture_plus
+@pytest_parametrize_plus('who', [fixture_ref(world_str), 
+                                 'you'])
+def greetings(who):
+    return 'hello ' + who
+
+@pytest_parametrize_plus('main_msg', ['nothing', 
+                                      fixture_ref(world_str), 
+                                      fixture_ref(greetings)])
+@pytest.mark.parametrize('ending', ['?', '!'])
+def test_prints(main_msg, ending):
+    print(main_msg + ending)
+```
+
+yields the following
+
+```bash
+> pytest -s -v
+nothing?
+nothing!
+world?
+world!
+hello world?
+hello world!
+hello you?
+hello you!
+```
+
+Note: for this to be performed, the parameters are replaced with a union fixture. Therefore the relative priority order with standard 'mark' parameters will get impacted. You may solve this by replacing your mark parameters with `param_fixture`s (see [above](#param_fixtures).)
+
 ## Main features / benefits
 
  * **Separation of concerns**: test code on one hand, test cases data on the other hand. This is particularly relevant for data science projects where a lot of test datasets are used on the same block of test code.

pytest_cases/tests/fixtures/test_fixture_in_parametrize_basic.py

@@ -0,0 +1,30 @@
+import pytest
+from pytest_cases import pytest_parametrize_plus, pytest_fixture_plus, fixture_ref
+
+
+@pytest.fixture
+def world_str():
+    return 'world'
+
+
+@pytest_fixture_plus
+@pytest_parametrize_plus('who', [fixture_ref(world_str), 'you'])
+def greetings(who):
+    return 'hello ' + who
+
+
+@pytest_parametrize_plus('main_msg', ['nothing', fixture_ref(world_str), fixture_ref(greetings)])
+@pytest.mark.parametrize('ending', ['?', '!'])
+def test_prints(main_msg, ending):
+    print(main_msg + ending)
+
+
+def test_synthesis(module_results_dct):
+    assert list(module_results_dct) == ['test_prints[test_prints_param__main_msg__0-nothing-?]',
+                                        'test_prints[test_prints_param__main_msg__0-nothing-!]',
+                                        'test_prints[world_str-?]',
+                                        'test_prints[world_str-!]',
+                                        'test_prints[greetings-world_str-?]',
+                                        'test_prints[greetings-world_str-!]',
+                                        'test_prints[greetings-greetings_param__who__1-you-?]',
+                                        'test_prints[greetings-greetings_param__who__1-you-!]']