Added ability to run some tests serially, not in subprocesses.

Pipfile

@@ -6,6 +6,7 @@ name = "pypi"
 [packages]
 pytest = ">=3.0.0"
 tblib = "*"
+six = "*"
 
 [dev-packages]
 pylint = "*"

README.md

@@ -53,6 +53,19 @@ pytest --tests-per-worker auto
 pytest --workers 2 --tests-per-worker auto
 ```
 
+## Marking some tests to run in a single process
+
+Sometimes you might have one or more tests which should not be run in parallel.
+In this case, you might mark them with `no_parallel` marker:
+
+```python
+@pytest.mark.no_parallel
+def test_sequential(driver):
+    do_something()
+```
+
+Such tests will be runned in the main process in a serialized manner.
+
 ## Notice
 
 Beginning with Python 3.8, forking behavior is forced on macOS at the expense of safety.

pytest_parallel/__init__.py

@@ -93,8 +93,6 @@ def run(self):
                 run_test(self.session, item, None)
             except BaseException:
                 import pickle
-                import sys
-
                 self.errors.put((self.name, pickle.dumps(sys.exc_info())))
             finally:
                 try:
@@ -110,6 +108,10 @@ def pytest_configure(config):
     if not config.option.collectonly and (workers or tests_per_worker):
         config.pluginmanager.register(ParallelRunner(config), 'parallelrunner')
 
+    config.addinivalue_line(
+        "markers", "no_parallel: mark test to run in a single process"
+    )
+
 
 class ThreadLocalEnviron(os._Environ):
     def __init__(self, env):
@@ -271,8 +273,15 @@ def pytest_runtestloop(self, session):
         # This way, report generators like JUnitXML will work as expected.
         self.responses_queue = queue_cls()
 
-        for i in range(len(session.items)):
-            queue.put(i)
+        # Current process is not a worker.
+        # This flag will be changed after the worker's fork.
+        self._config.parallel_worker = False
+
+        for idx, item in enumerate(session.items):
+            if has_no_parallel_marker(item):
+                run_test(session, item, None)
+            else:
+                queue.put(idx)
 
         # Now we need to put stopping sentinels, so that worker
         # processes will know, there is time to finish the work.
@@ -292,10 +301,6 @@ def wait_for_responses_processor():
 
         processes = []
 
-        # Current process is not a worker.
-        # This flag will be changed after the worker's fork.
-        self._config.parallel_worker = False
-
         args = (self._config, queue, session, tests_per_worker, errors)
         for _ in range(self.workers):
             process = Process(target=process_with_threads, args=args)
@@ -363,3 +368,8 @@ def process_responses(self, queue):
                     queue.task_done()
                 except ConnectionRefusedError:
                     pass
+
+
+def has_no_parallel_marker(item):
+    markers = list(item.iter_markers(name='no_parallel'))
+    return len(markers) > 0

tests/test_no_parallel.py

@@ -0,0 +1,27 @@
+import pytest
+import re
+
+
+def test_no_parallel_marker(testdir):
+    testdir.makepyfile("""
+        import os
+        import pytest
+
+        # Remember the PID of a process
+        # which loaded the module.
+        ppid = os.getpid()
+
+        def test_parallel():
+            # This test should be executed in a subprocess
+            assert ppid != os.getpid()
+            assert ppid == os.getppid()
+
+        @pytest.mark.no_parallel
+        def test_no_parallel():
+            # And this one is in the main process
+            assert ppid == os.getpid()
+
+    """)
+    result = testdir.runpytest('--workers=2')
+    result.assert_outcomes(passed=2)
+    assert result.ret == 0