Add `--no-loadscope-reorder` and `--loadscope-reorder` options (#1217)

Based on #1098.

Closes #1098

---------

Co-authored-by: Toan Vuong <toan.vuong@hyperscience.com>

changelog/1083.improvement.rst

@@ -0,0 +1,5 @@
+Add ``--no-loadscope-reorder`` and ``--loadscope-reorder`` option to control whether to automatically reorder tests in loadscope for tests where relative ordering matters. This only applies when using ``loadscope``. 
+
+For example, [test_file_1, test_file_2, ..., test_file_n] are given as input test files, if ``--no-loadscope-reorder`` is used, for either worker, the ``test_file_a`` will be executed before ``test_file_b`` only if ``a < b``.
+
+The default behavior is to reorder the tests to maximize the number of tests that can be executed in parallel.

src/xdist/plugin.py

@@ -127,6 +127,32 @@ def pytest_addoption(parser: pytest.Parser) -> None:
             "(default) no: Run tests inprocess, don't distribute."
         ),
     )
+    group.addoption(
+        "--loadscope-reorder",
+        dest="loadscopereorder",
+        action="store_true",
+        default=True,
+        help=(
+            "Pytest-xdist will default reorder tests by number of tests per scope "
+            "when used in conjunction with loadscope.\n"
+            "This option will enable loadscope reorder which will improve the "
+            "parallelism of the test suite.\n"
+            "However, the partial order of tests might not be retained.\n"
+        ),
+    )
+    group.addoption(
+        "--no-loadscope-reorder",
+        dest="loadscopereorder",
+        action="store_false",
+        help=(
+            "Pytest-xdist will default reorder tests by number of tests per scope "
+            "when used in conjunction with loadscope.\n"
+            "This option will disable loadscope reorder, "
+            "and the partial order of tests can be retained.\n"
+            "This is useful when pytest-xdist is used together with "
+            "other plugins that specify tests in a specific order."
+        ),
+    )
     group.addoption(
         "--tx",
         dest="tx",

src/xdist/scheduler/loadscope.py

@@ -371,11 +371,15 @@ class LoadScopeScheduling:
             work_unit = unsorted_workqueue.setdefault(scope, {})
             work_unit[nodeid] = False
 
-        # Insert tests scopes into work queue ordered by number of tests.
-        for scope, nodeids in sorted(
-            unsorted_workqueue.items(), key=lambda item: -len(item[1])
-        ):
-            self.workqueue[scope] = nodeids
+        if self.config.option.loadscopereorder:
+            # Insert tests scopes into work queue ordered by number of tests.
+            for scope, nodeids in sorted(
+                unsorted_workqueue.items(), key=lambda item: -len(item[1])
+            ):
+                self.workqueue[scope] = nodeids
+        else:
+            for scope, nodeids in unsorted_workqueue.items():
+                self.workqueue[scope] = nodeids
 
         # Avoid having more workers than work
         extra_nodes = len(self.nodes) - len(self.workqueue)

testing/acceptance_test.py

@@ -1254,6 +1254,24 @@ class TestLoadScope:
             "test_b.py::test", result.outlines
         ) == {"gw0": 20}
 
+    def test_workqueue_ordered_by_input(self, pytester: pytest.Pytester) -> None:
+        test_file = """
+            import pytest
+            @pytest.mark.parametrize('i', range({}))
+            def test(i):
+                pass
+        """
+        pytester.makepyfile(test_a=test_file.format(10), test_b=test_file.format(20))
+        result = pytester.runpytest(
+            "-n2", "--dist=loadscope", "--no-loadscope-reorder", "-v"
+        )
+        assert get_workers_and_test_count_by_prefix(
+            "test_a.py::test", result.outlines
+        ) == {"gw0": 10}
+        assert get_workers_and_test_count_by_prefix(
+            "test_b.py::test", result.outlines
+        ) == {"gw1": 20}
+
     def test_module_single_start(self, pytester: pytest.Pytester) -> None:
         """Fix test suite never finishing in case all workers start with a single test (#277)."""
         test_file1 = """