Added a test and improved documentation to explain why @fixture should be used instead of @pytest.fixture. Fixed #125

Author: Sylvain MARIE

docs/api_reference.md

@@ -124,6 +124,8 @@ Transforms a list of cases (obtained from [`get_all_cases`](#get_all_cases)) int
 
 Identical to `@pytest.fixture` decorator, except that 
 
+ - when used in a fixture union (either explicit `fixture_union` or indirect through `@parametrize`+`fixture_ref` or `@parametrize_with_cases`), it will not be setup/teardown unnecessarily in tests that do not require it.
+
  - it supports multi-parametrization with `@pytest.mark.parametrize` as requested in [pytest#3960](https://github.com/pytest-dev/pytest/issues/3960). As a consequence it does not support the `params` and `ids` arguments anymore.
 
  - it supports a new argument `unpack_into` where you can provide names for fixtures where to unpack this fixture into.

docs/index.md

@@ -356,6 +356,8 @@ test_generators.py::test_foo[simple_generator-who=there] PASSED [100%]
 
 Cases can use fixtures the same way as [test functions do](https://docs.pytest.org/en/stable/fixture.html#fixtures-as-function-arguments): simply add the fixture names as arguments in their signature and make sure the fixture exists either in the same module, or in a [`conftest.py`](https://docs.pytest.org/en/stable/fixture.html?highlight=conftest.py#conftest-py-sharing-fixture-functions) file in one of the parent packages. See [`pytest` documentation on sharing fixtures](https://docs.pytest.org/en/stable/fixture.html?highlight=conftest.py#conftest-py-sharing-fixture-functions).
 
+!!! warning "Use `@fixture` instead of `@pytest.fixture`"
+    If a fixture is used by *some* of your cases only, then you *should* use the `@fixture` decorator from pytest-cases instead of the standard `@pytest.fixture`. Otherwise you fixture will be setup/teardown for all cases even those not requiring it. See [`@fixture` doc](./api_reference.md#fixture).
 
 ```python
 from pytest_cases import parametrize_with_cases, fixture, parametrize

pytest_cases/fixture_core2.py

@@ -277,6 +277,9 @@ def fixture_plus(scope="function",  # type: str
 
     Identical to `@pytest.fixture` decorator, except that
 
+     - when used in a fixture union (either explicit `fixture_union` or indirect through `@parametrize`+`fixture_ref`
+       or `@parametrize_with_cases`), it will not be setup/teardown unnecessarily in tests that do not require it.
+
      - it supports multi-parametrization with `@pytest.mark.parametrize` as requested in
        https://github.com/pytest-dev/pytest/issues/3960. As a consequence it does not support the `params` and `ids`
        arguments anymore.

pytest_cases/tests/cases/issues/test_issue_125.py

@@ -0,0 +1,40 @@
+from pytest_cases import parametrize_with_cases, fixture
+
+
+@fixture
+def one():
+    return 1
+
+
+time_to_call_two = False
+
+
+@fixture
+def two():
+    global time_to_call_two
+    assert time_to_call_two, "Should not be called."
+    time_to_call_two = False
+
+
+def case_one(one):
+    return one
+
+
+def case_two(two):
+    return two
+
+
+def case_three(one):
+    return one
+
+
+test_id = 0
+
+
+@parametrize_with_cases("case", cases=".")
+def test(case):
+    global time_to_call_two, test_id
+    if test_id == 0:
+        # next test will need the fixture two
+        time_to_call_two = True
+    pass