disallow yield in non-yield-fixtures for now. This is an incompataibility but we want to prepare for possibly merging fixture and yield_fixture some day.

CHANGELOG

@@ -14,12 +14,18 @@ known incompatibilities:
 - the pytest_plugin_unregister hook wasn't ever properly called
   and there is no known implementation of the hook - so it got removed.
 
+- pytest.fixture-decorated functions cannot be generators (i.e. use yield) anymore.  
+  This change might be reversed in 2.4.1 if it causes unforeseen real-life issues.
+  However, you can always write and return an inner function/generator
+  and change the fixture consumer to iterate over the returned generator. 
+  This change was done in lieu of the new ``pytest.yield_fixture`` decorator, see below.
+
 new features:
 
-- experimentally introduce a new pytest.yield_fixture decorator which
-  has exactly the same parameters as pytest.fixture but expects
+- experimentally introduce a new ``pytest.yield_fixture`` decorator which
+  accepts exactly the same parameters as pytest.fixture but mandates
   a ``yield`` statement instead of a ``return statement`` from fixture functions.
-  This allows direct integration with with-context managers
+  This allows direct integration with "with-style" context managers
   in fixture functions and generally avoids registering of finalization callbacks
   in favour of treating the "after-yield" as teardown code.
   Thanks Andreas Pelme, Vladimir Keleshev, Floris Bruynooghe, Ronny Pfannschmidt

_pytest/python.py

@@ -1705,6 +1705,11 @@ def call_fixture_func(fixturefunc, request, kwargs, yieldctx):
                     "yield_fixture function has more than one 'yield'")
         request.addfinalizer(teardown)
     else:
+        if is_generator(fixturefunc):
+            fail_fixturefunc(fixturefunc,
+                msg="pytest.fixture functions cannot use ``yield``. "
+                    "Instead write and return an inner function/generator "
+                    "and let the consumer call and iterate over it.")
         res = fixturefunc(**kwargs)
     return res
 

testing/python/fixture.py

@@ -2087,3 +2087,18 @@ class TestContextManagerFixtureFuncs:
             *def arg1*
         """)
 
+    def test_yield_not_allowed_in_non_yield(self, testdir):
+        testdir.makepyfile("""
+            import pytest
+            @pytest.fixture(scope="module")
+            def arg1():
+                yield 1
+            def test_1(arg1):
+                pass
+        """)
+        result = testdir.runpytest("-s")
+        result.stdout.fnmatch_lines("""
+            *fixture*cannot use*yield*
+            *def arg1*
+        """)
+