Deterministic scheduling via Hypothesis

newsfragments/73.feature.rst

@@ -0,0 +1,3 @@
+pytest-trio now makes the Trio scheduler deterministic while running
+inside a Hypothesis test.  Hopefully you won't see any change, but if
+you had scheduler-dependent bugs Hypothesis will be more effective now.

pytest_trio/plugin.py

@@ -22,6 +22,17 @@
     # Ordered dict (and **kwargs) not available with Python<3.6
     ORDERED_DICTS = False
 
+try:
+    import hypothesis.strategies as st
+except ImportError:  # pragma: no cover
+    pass
+else:
+    # On recent versions of Hypothesis, make the Trio scheduler deterministic
+    # even though it uses a module-scoped Random instance.  This works
+    # regardless of whether or not the random_module strategy is used.
+    if hasattr(st.random_module, 'register'):  # pragma: no branch
+        st.random_module.register(trio._core._run._r)
+
 
 def pytest_addoption(parser):
     parser.addini(
@@ -345,7 +356,8 @@ def pytest_runtest_call(item):
             item.obj.hypothesis.inner_test = _trio_test_runner_factory(
                 item, item.obj.hypothesis.inner_test
             )
-        elif getattr(item.obj, 'is_hypothesis_test', False):
+        elif getattr(item.obj, 'is_hypothesis_test',
+                     False):  # pragma: no cover
             pytest.fail(
                 'test function `%r` is using Hypothesis, but pytest-trio '
                 'only works with Hypothesis 3.64.0 or later.' % item