New decorator @pytest_parametrize_plus able to handle the case where a fixture_ref(<fixture_name>) is present in the parameter values list. This decorator can be applied both on test functions and fixtures (if they are decorated with @pytest_fixture_plus). Fixes #40

Added appropriate test.

Major refactoring of the "union fixtures" mechanism.

 - The `NOT_USED` status is now correctly propagated between dependent fixtures. This should fix a few cases where user fixtures were setup/teardown while not used in the current test node.
 - Empty fixture unions are not permitted anymore.
 - The way unions are handled in parametrization was redesigned. The new design is based on a two-steps approach: first build the closure as a tree, and then apply parametrization based on this tree. This fixes several unintuitive behaviours.

Internal:

 - A new internal function `is_used_request(request)` is used everywhere to check if a request for a fixture is actually used or not. In which case user code is not called.
 - `UnionFixtureConfig` renamed `UnionFixtureAlternative` and the parametrization of a union fixture now contains the correct number of items. New associated helper `is_fixture_union_params`

Author: Sylvain MARIE

pytest_cases/__init__.py

@@ -1,7 +1,7 @@
 from pytest_cases.case_funcs import case_name, test_target, case_tags, cases_generator
 
 from pytest_cases.main_fixtures import cases_fixture, pytest_fixture_plus, param_fixtures, param_fixture, \
-    fixture_union, NOT_USED  # pytest_parametrize_plus
+    fixture_union, NOT_USED, pytest_parametrize_plus, fixture_ref
 
 from pytest_cases.main_params import cases_data, CaseDataGetter, unfold_expected_err, get_all_cases, THIS_MODULE, \
     get_pytest_parametrize_args
@@ -14,7 +14,7 @@
     'case_name',  'test_target', 'case_tags', 'cases_generator',
     # --main_fixtures
     'cases_fixture', 'pytest_fixture_plus', 'param_fixtures', 'param_fixture',  # 'pytest_parametrize_plus',
-    'fixture_union', 'NOT_USED',
+    'fixture_union', 'NOT_USED', 'pytest_parametrize_plus', 'fixture_ref',
     # --main params
     'cases_data', 'CaseDataGetter', 'THIS_MODULE', 'unfold_expected_err', 'get_all_cases',
     'get_pytest_parametrize_args',

pytest_cases/common.py

@@ -293,3 +293,10 @@ def transform_marks_into_decorators(marks):
     except Exception as e:
         warn("Caught exception while trying to mark case: [%s] %s" % (type(e), e))
     return marks_mod
+
+
+def get_pytest_nodeid(metafunc):
+    try:
+        return metafunc.definition.nodeid
+    except AttributeError:
+        return "unknown"