Use a different random seed per test

CHANGELOG.rst

@@ -7,6 +7,13 @@ Unreleased
 
 * Support Python 3.14.
 
+* Use a different random seed per test, based on the test ID.
+
+  This change should mean that tests exercise more random data values in a given run, and that any randomly-generated identifiers have a lower chance of collision when stored in a shared resource like a database.
+
+  `PR #687 <https://github.com/pytest-dev/pytest-randomly/issues/687>`__.
+  Thanks to Bryce Drennan for the suggestion in `Issue #600 <https://github.com/pytest-dev/pytest-randomly/issues/600>`__ and initial implementation in `PR #617 <https://github.com/pytest-dev/pytest-randomly/pull/617>`__.
+
 3.16.0 (2024-10-25)
 -------------------
 

README.rst

@@ -36,32 +36,33 @@ All of these features are on by default but can be disabled with flags.
   modules, then at the level of test classes (if you have them), then at the
   order of functions. This also works with things like doctests.
 
-* Resets the global ``random.seed()`` at the start of every test case and test
-  to a fixed number - this defaults to ``time.time()`` from the start of your
-  test run, but you can pass in ``--randomly-seed`` to repeat a
-  randomness-induced failure.
-
-* If
-  `factory boy <https://factoryboy.readthedocs.io/en/latest/reference.html>`_
-  is installed, its random state is reset at the start of every test. This
-  allows for repeatable use of its random 'fuzzy' features.
-
-* If `faker <https://pypi.org/project/faker>`_ is installed, its random
-  state is reset at the start of every test. This is also for repeatable fuzzy
-  data in tests - factory boy uses faker for lots of data. This is also done
-  if you're using the ``faker`` pytest fixture, by defining the ``faker_seed``
-  fixture
-  (`docs <https://faker.readthedocs.io/en/master/pytest-fixtures.html#seeding-configuration>`__).
-
-* If
-  `Model Bakery <https://model-bakery.readthedocs.io/en/latest/>`_
-  is installed, its random state is reset at the start of every test. This
-  allows for repeatable use of its random fixture field values.
-
-* If `numpy <http://www.numpy.org/>`_ is installed, its legacy global random state in |numpy.random|__ is reset at the start of every test.
-
-  .. |numpy.random| replace:: ``numpy.random``
-  __ https://numpy.org/doc/stable/reference/random/index.html
+* Generates a base random seed or accepts one for reproduction with ``--randomly-seed``.
+  The base random seed is printed at the start of the test run, and can be passed in to repeat a failure caused by test ordering or random data.
+
+* At the start of the test run, and before each test setup, run, and teardown, it resets Python’s global random seed to a fixed value, using |random.seed()|__.
+  The fixed value is derived from the base random seed, the pytest test ID, and an offset for setup or teardown.
+  This ensures each test gets a different but repeatable random seed.
+
+  .. |random.seed()| replace:: ``random.seed()``
+  __ https://docs.python.org/3/library/random.html#random.seed
+
+* pytest-randomly also resets several libraries’ random states at the start of
+  every test, if they are installed:
+
+  * `factory boy <https://factoryboy.readthedocs.io/en/latest/reference.html>`__
+
+  * `Faker <https://pypi.org/project/faker>`__
+
+    The ``faker`` pytest fixture is also affected, as pytest-randomly defines |the faker_seed fixture|__.
+
+    .. |the faker_seed fixture| replace:: the ``faker_seed`` fixture
+    __ https://faker.readthedocs.io/en/master/pytest-fixtures.html#seeding-configuration
+
+  * `Model Bakery <https://model-bakery.readthedocs.io/en/latest/>`__
+
+  * `NumPy <https://www.numpy.org/>`_
+
+    Only its `legacy random state <https://numpy.org/doc/stable/reference/random/legacy.html>`__ is affected.
 
 * If additional random generators are used, they can be registered under the
   ``pytest_randomly.random_seeder``

src/pytest_randomly/__init__.py

@@ -4,12 +4,14 @@
 import hashlib
 import random
 import sys
+from functools import lru_cache
 from itertools import groupby
 from types import ModuleType
 from typing import Any, Callable, TypeVar
 
 from _pytest.config import Config
 from _pytest.config.argparsing import Parser
+from _pytest.fixtures import SubRequest
 from _pytest.nodes import Item
 from pytest import Collector, fixture, hookimpl
 
@@ -196,17 +198,17 @@ def pytest_report_header(config: Config) -> str:
 
 def pytest_runtest_setup(item: Item) -> None:
     if item.config.getoption("randomly_reset_seed"):
-        _reseed(item.config, -1)
+        _reseed(item.config, int.from_bytes(_md5(item.nodeid), "big") - 1)
 
 
 def pytest_runtest_call(item: Item) -> None:
     if item.config.getoption("randomly_reset_seed"):
-        _reseed(item.config)
+        _reseed(item.config, int.from_bytes(_md5(item.nodeid), "big"))
 
 
 def pytest_runtest_teardown(item: Item) -> None:
     if item.config.getoption("randomly_reset_seed"):
-        _reseed(item.config, 1)
+        _reseed(item.config, int.from_bytes(_md5(item.nodeid), "big") + 1)
 
 
 @hookimpl(tryfirst=True)
@@ -279,6 +281,7 @@ def reduce_list_of_lists(lists: list[list[T]]) -> list[T]:
     return new_list
 
 
+@lru_cache
 def _md5(string: str) -> bytes:
     hasher = hashlib.md5(usedforsecurity=False)
     hasher.update(string.encode())
@@ -288,6 +291,10 @@ def _md5(string: str) -> bytes:
 if have_faker:  # pragma: no branch
 
     @fixture(autouse=True)
-    def faker_seed(pytestconfig: Config) -> int:
-        result: int = pytestconfig.getoption("randomly_seed")
+    def faker_seed(pytestconfig: Config, request: SubRequest) -> int:
+        print(type(request))
+        result: int = pytestconfig.getoption("randomly_seed") + int.from_bytes(
+            _md5(request.node.nodeid),
+            "big",
+        )
         return result

tests/test_pytest_randomly.py

@@ -58,7 +58,7 @@ def test_it_reports_a_header_when_set(simpletester):
     assert lines == ["Using --randomly-seed=10"]
 
 
-def test_it_reuses_the_same_random_seed_per_test(ourtester):
+def test_it_uses_different_random_seeds_per_test(ourtester):
     """
     Run a pair of tests that generate the a number and then assert they got
     what the other did.
@@ -67,18 +67,16 @@ def test_it_reuses_the_same_random_seed_per_test(ourtester):
         test_one="""
         import random
 
+
         def test_a():
-            test_a.num = random.random()
-            if hasattr(test_b, 'num'):
-                assert test_a.num == test_b.num
+            global num
+            num = random.random()
 
         def test_b():
-            test_b.num = random.random()
-            if hasattr(test_a, 'num'):
-                assert test_b.num == test_a.num
+            assert random.random() != num
         """
     )
-    out = ourtester.runpytest("--randomly-dont-reorganize")
+    out = ourtester.runpytest("--randomly-dont-reorganize", "--randomly-seed=1")
     out.assert_outcomes(passed=2, failed=0)
 
 
@@ -157,9 +155,8 @@ class A(TestCase):
 
             @classmethod
             def setUpClass(cls):
-                super(A, cls).setUpClass()
+                super().setUpClass()
                 cls.suc_num = random.random()
-                assert cls.suc_num == getattr(B, 'suc_num', cls.suc_num)
 
             def test_fake(self):
                 assert True
@@ -169,15 +166,15 @@ class B(TestCase):
 
             @classmethod
             def setUpClass(cls):
-                super(B, cls).setUpClass()
+                super().setUpClass()
                 cls.suc_num = random.random()
-                assert cls.suc_num == getattr(A, 'suc_num', cls.suc_num)
+                assert cls.suc_num != A.suc_num
 
             def test_fake(self):
                 assert True
         """
     )
-    out = ourtester.runpytest()
+    out = ourtester.runpytest("--randomly-seed=1")
     out.assert_outcomes(passed=2, failed=0)
 
 
@@ -195,9 +192,8 @@ def test_fake(self):
 
             @classmethod
             def tearDownClass(cls):
-                super(A, cls).tearDownClass()
+                super().tearDownClass()
                 cls.suc_num = random.random()
-                assert cls.suc_num == getattr(B, 'suc_num', cls.suc_num)
 
 
         class B(TestCase):
@@ -207,12 +203,12 @@ def test_fake(self):
 
             @classmethod
             def tearDownClass(cls):
-                super(B, cls).tearDownClass()
+                super().tearDownClass()
                 cls.suc_num = random.random()
-                assert cls.suc_num == getattr(A, 'suc_num', cls.suc_num)
+                assert cls.suc_num != A.suc_num
         """
     )
-    out = ourtester.runpytest()
+    out = ourtester.runpytest("--randomly-seed=1")
     out.assert_outcomes(passed=2, failed=0)
 
 
@@ -574,62 +570,20 @@ def test_one(myfixture):
     out.assert_outcomes(passed=1)
 
 
-def test_fixtures_dont_interfere_with_tests_getting_same_random_state(ourtester):
-    ourtester.makepyfile(
-        test_one="""
-        import random
-
-        import pytest
-
-
-        random.seed(2)
-        state_at_seed_two = random.getstate()
-
-
-        @pytest.fixture(scope='module')
-        def myfixture():
-            return random.random()
-
-
-        @pytest.mark.one()
-        def test_one(myfixture):
-            assert random.getstate() == state_at_seed_two
-
-
-        @pytest.mark.two()
-        def test_two(myfixture):
-            assert random.getstate() == state_at_seed_two
-        """
-    )
-    args = ["--randomly-seed=2"]
-
-    out = ourtester.runpytest(*args)
-    out.assert_outcomes(passed=2)
-
-    out = ourtester.runpytest("-m", "one", *args)
-    out.assert_outcomes(passed=1)
-    out = ourtester.runpytest("-m", "two", *args)
-    out.assert_outcomes(passed=1)
-
-
 def test_factory_boy(ourtester):
     """
-    Rather than set up factories etc., just check the random generator it uses
-    is set between two tests to output the same number.
+    Check that the random generator factory boy uses is different between two tests.
     """
     ourtester.makepyfile(
         test_one="""
         from factory.random import randgen
 
         def test_a():
-            test_a.num = randgen.random()
-            if hasattr(test_b, 'num'):
-                assert test_a.num == test_b.num
+            assert randgen.random() == 0.9988532989147809
+
 
         def test_b():
-            test_b.num = randgen.random()
-            if hasattr(test_a, 'num'):
-                assert test_b.num == test_a.num
+            assert randgen.random() == 0.18032546798434612
         """
     )
 
@@ -645,10 +599,10 @@ def test_faker(ourtester):
         fake = Faker()
 
         def test_one():
-            assert fake.name() == 'Ryan Gallagher'
+            assert fake.name() == 'Mrs. Lisa Ryan'
 
         def test_two():
-            assert fake.name() == 'Ryan Gallagher'
+            assert fake.name() == 'Kaitlyn Mitchell'
         """
     )
 
@@ -660,10 +614,10 @@ def test_faker_fixture(ourtester):
     ourtester.makepyfile(
         test_one="""
         def test_one(faker):
-            assert faker.name() == 'Ryan Gallagher'
+            assert faker.name() == 'Mrs. Lisa Ryan'
 
         def test_two(faker):
-            assert faker.name() == 'Ryan Gallagher'
+            assert faker.name() == 'Kaitlyn Mitchell'
         """
     )
 
@@ -673,22 +627,17 @@ def test_two(faker):
 
 def test_model_bakery(ourtester):
     """
-    Rather than set up models, just check the random generator it uses is set
-    between two tests to output the same number.
+    Check the Model Bakery random generator is reset between tests.
     """
     ourtester.makepyfile(
         test_one="""
-        from model_bakery.random_gen import baker_random
+        from model_bakery.random_gen import gen_slug
 
         def test_a():
-            test_a.num = baker_random.random()
-            if hasattr(test_b, 'num'):
-                assert test_a.num == test_b.num
+            assert gen_slug(10) == 'XjpU5br7ej'
 
         def test_b():
-            test_b.num = baker_random.random()
-            if hasattr(test_a, 'num'):
-                assert test_b.num == test_a.num
+            assert gen_slug(10) == 'xJHS-PD_WT'
         """
     )
 
@@ -702,10 +651,10 @@ def test_numpy(ourtester):
         import numpy as np
 
         def test_one():
-            assert np.random.rand() == 0.417022004702574
+            assert np.random.rand() == 0.36687834264514585
 
         def test_two():
-            assert np.random.rand() == 0.417022004702574
+            assert np.random.rand() == 0.7050715833365834
         """
     )
 
@@ -769,19 +718,19 @@ def fake_entry_points(*, group):
     assert reseed.mock_calls == [
         mock.call(1),
         mock.call(1),
-        mock.call(0),
-        mock.call(1),
-        mock.call(2),
+        mock.call(116362448262735926321257785636175308268),
+        mock.call(116362448262735926321257785636175308269),
+        mock.call(116362448262735926321257785636175308270),
     ]
 
     reseed.mock_calls[:] = []
     pytester.runpytest_inprocess("--randomly-seed=424242")
     assert reseed.mock_calls == [
         mock.call(424242),
         mock.call(424242),
-        mock.call(424241),
-        mock.call(424242),
-        mock.call(424243),
+        mock.call(116362448262735926321257785636175732509),
+        mock.call(116362448262735926321257785636175732510),
+        mock.call(116362448262735926321257785636175732511),
     ]