feature: each test starts with a unique seed. --randomly-use-same-seed-per-test for old behavior

Author: brycedrennan

CHANGELOG.rst

@@ -2,6 +2,14 @@
 Changelog
 =========
 
+3.17.0
+-------------------
+
+* Change default seed to be different per test. Add the option ``--randomly-use-same-seed-per-test`` to enable the old behavior.
+
+  Resolves `Issue #600 <https://github.com/pytest-dev/pytest-randomly/issues/600>`__
+
+
 3.16.0 (2024-10-25)
 -------------------
 

README.rst

@@ -155,6 +155,9 @@ You can disable behaviours you don't like with the following flags:
 * ``--randomly-dont-reset-seed`` - turn off the reset of ``random.seed()`` at
   the start of every test
 * ``--randomly-dont-reorganize`` - turn off the shuffling of the order of tests
+* ``--randomly-dont-seed-per-test`` - turn off each test having a unique seed.
+  Each test will be seeded with the same seed.
+
 
 The plugin appears to Pytest with the name 'randomly'. To disable it
 altogether, you can use the ``-p`` argument, for example:

src/pytest_randomly/__init__.py

@@ -105,6 +105,13 @@ def pytest_addoption(parser: Parser) -> None:
         default=True,
         help="Stop pytest-randomly from randomly reorganizing the test order.",
     )
+    group._addoption(
+        "--randomly-use-same-seed-per-test",
+        action="store_false",
+        dest="same_seed_per_test",
+        default=False,
+        help="""Use a the same random seed for each test. Reproduces legacy behavior of pytest-randomly.""",
+    )
 
 
 def pytest_configure(config: Config) -> None:
@@ -203,9 +210,17 @@ def pytest_runtest_setup(item: Item) -> None:
         _reseed(item.config, -1)
 
 
+def seed_from_string(string: str) -> int:
+    return int(hashlib.md5(string.encode()).hexdigest(), 16)
+
+
 def pytest_runtest_call(item: Item) -> None:
     if item.config.getoption("randomly_reset_seed"):
-        _reseed(item.config)
+        if item.config.getoption("same_seed_per_test"):
+            test_offset = 0
+        else:
+            test_offset = seed_from_string(item.nodeid) + 100
+        _reseed(item.config, offset=test_offset)
 
 
 def pytest_runtest_teardown(item: Item) -> None:

tests/test_pytest_randomly.py

@@ -78,7 +78,33 @@ def test_b():
                 assert test_b.num == test_a.num
         """
     )
-    out = ourtester.runpytest("--randomly-dont-reorganize")
+    out = ourtester.runpytest(
+        "--randomly-dont-reorganize", "--randomly-use-same-seed-per-test"
+    )
+    out.assert_outcomes(passed=2, failed=0)
+
+
+def test_it_uses_different_random_seed_per_test(ourtester):
+    """
+    Run a pair of tests that generate a number and assert they produce
+    different numbers.
+    """
+    ourtester.makepyfile(
+        test_one="""
+        import random
+
+        def test_a():
+            test_a.num = random.random()
+            if hasattr(test_b, 'num'):
+                assert test_a.num != test_b.num
+
+        def test_b():
+            test_b.num = random.random()
+            if hasattr(test_a, 'num'):
+                assert test_b.num != test_a.num
+        """
+    )
+    out = ourtester.runpytest()
     out.assert_outcomes(passed=2, failed=0)
 
 
@@ -601,7 +627,7 @@ def test_two(myfixture):
             assert random.getstate() == state_at_seed_two
         """
     )
-    args = ["--randomly-seed=2"]
+    args = ["--randomly-seed=2", "--randomly-use-same-seed-per-test"]
 
     out = ourtester.runpytest(*args)
     out.assert_outcomes(passed=2)
@@ -633,7 +659,7 @@ def test_b():
         """
     )
 
-    out = ourtester.runpytest("--randomly-seed=1")
+    out = ourtester.runpytest("--randomly-seed=1", "--randomly-use-same-seed-per-test")
     out.assert_outcomes(passed=2)
 
 
@@ -645,10 +671,10 @@ def test_faker(ourtester):
         fake = Faker()
 
         def test_one():
-            assert fake.name() == 'Ryan Gallagher'
+            assert fake.name() == 'Justin Richard'
 
         def test_two():
-            assert fake.name() == 'Ryan Gallagher'
+            assert fake.name() == 'Tiffany Williams'
         """
     )
 
@@ -692,7 +718,7 @@ def test_b():
         """
     )
 
-    out = ourtester.runpytest("--randomly-seed=1")
+    out = ourtester.runpytest("--randomly-seed=1", "--randomly-use-same-seed-per-test")
     out.assert_outcomes(passed=2)
 
 
@@ -702,10 +728,10 @@ def test_numpy(ourtester):
         import numpy as np
 
         def test_one():
-            assert np.random.rand() == 0.417022004702574
+            assert np.random.rand() == 0.46479378116435255
 
         def test_two():
-            assert np.random.rand() == 0.417022004702574
+            assert np.random.rand() == 0.6413112443155088
         """
     )
 
@@ -765,7 +791,9 @@ def fake_entry_points(*, group):
     entry_points.append(_FakeEntryPoint("test_seeder", reseed))
 
     # Need to run in-process so that monkeypatching works
-    pytester.runpytest_inprocess("--randomly-seed=1")
+    pytester.runpytest_inprocess(
+        "--randomly-seed=1", "--randomly-use-same-seed-per-test"
+    )
     assert reseed.mock_calls == [
         mock.call(1),
         mock.call(1),
@@ -775,7 +803,9 @@ def fake_entry_points(*, group):
     ]
 
     reseed.mock_calls[:] = []
-    pytester.runpytest_inprocess("--randomly-seed=424242")
+    pytester.runpytest_inprocess(
+        "--randomly-seed=424242", "--randomly-use-same-seed-per-test"
+    )
     assert reseed.mock_calls == [
         mock.call(424242),
         mock.call(424242),