Changes from code review

README.rst

@@ -57,10 +57,11 @@ Install the plugin with::
 
     pip install pytest-xdist
 
-or use the package in develop/in-place mode with
-a checkout of the `pytest-xdist repository`_ ::
 
-    pip install --editable .
+To use ``psutil`` for detection of the number of CPUs available, install the ``psutil`` extra::
+
+    pip install pytest-xdist[psutil]
+
 
 .. _parallelization:
 

changelog/585.feature.rst

@@ -1 +1 @@
-New ``pytest_xdist_auto_num_workers`` hook can be implemented by plugins or ``conftets.py`` to control the number of workers when ``--numprocesses=auto`` is given in the command-line.
+New ``pytest_xdist_auto_num_workers`` hook can be implemented by plugins or ``conftest.py`` files to control the number of workers when ``--numprocesses=auto`` is given in the command-line.

changelog/585.trivial.rst

@@ -1,7 +1,3 @@
-``psutil`` is no longer a dependency, as it has proven to make ``pytest-xdist`` installation in certain platforms and containers problematic.
+``psutil`` has proven to make ``pytest-xdist`` installation in certain platforms and containers problematic, so to use it for automatic number of CPUs detection users need to install the ``psutil`` extra::
 
-To those interested to continue to use ``psutil`` to detect the number of CPUs, the new ``pytest_xdist_auto_num_workers`` hook can be used in the root ``conftest.py`` file of the project::
-
-    def pytest_xdist_auto_num_workers(config):
-        import psutil
-        return psutil.cpu_count(logical=False)
+    pip install pytest-xdist[psutil]

setup.py

@@ -18,7 +18,7 @@
     platforms=["linux", "osx", "win32"],
     packages=find_packages(where="src"),
     package_dir={"": "src"},
-    extras_require={"testing": ["filelock"]},
+    extras_require={"testing": ["filelock"], "psutil": ["psutil>=3.0"]},
     entry_points={
         "pytest11": ["xdist = xdist.plugin", "xdist.looponfail = xdist.looponfail"]
     },

src/xdist/plugin.py

@@ -6,8 +6,20 @@
 
 
 def pytest_xdist_auto_num_workers():
+    try:
+        import psutil
+    except ImportError:
+        pass
+    else:
+        count = psutil.cpu_count(logical=False) or psutil.cpu_count()
+        if count:
+            return count
     try:
         from os import sched_getaffinity
+
+        def cpu_count():
+            return len(sched_getaffinity(0))
+
     except ImportError:
         if os.environ.get("TRAVIS") == "true":
             # workaround https://bitbucket.org/pypy/pypy/issues/2375
@@ -16,11 +28,6 @@ def pytest_xdist_auto_num_workers():
             from os import cpu_count
         except ImportError:
             from multiprocessing import cpu_count
-    else:
-
-        def cpu_count():
-            return len(sched_getaffinity(0))
-
     try:
         n = cpu_count()
     except NotImplementedError:

testing/test_plugin.py

@@ -1,7 +1,11 @@
+from contextlib import suppress
+
 import py
 import execnet
 from xdist.workermanage import NodeManager
 
+import pytest
+
 
 def test_dist_incompatibility_messages(testdir):
     result = testdir.runpytest("--pdb", "--looponfail")
@@ -38,6 +42,11 @@ def test_auto_detect_cpus(testdir, monkeypatch):
     import os
     from xdist.plugin import pytest_cmdline_main as check_options
 
+    with suppress(ImportError):
+        import psutil
+
+        monkeypatch.setattr(psutil, "cpu_count", lambda logical=True: None)
+
     if hasattr(os, "sched_getaffinity"):
         monkeypatch.setattr(os, "sched_getaffinity", lambda _pid: set(range(99)))
     elif hasattr(os, "cpu_count"):
@@ -67,6 +76,18 @@ def test_auto_detect_cpus(testdir, monkeypatch):
     assert config.getoption("numprocesses") == 2
 
 
+def test_auto_detect_cpus_psutil(testdir, monkeypatch):
+    from xdist.plugin import pytest_cmdline_main as check_options
+
+    psutil = pytest.importorskip("psutil")
+
+    monkeypatch.setattr(psutil, "cpu_count", lambda logical=True: 42)
+
+    config = testdir.parseconfigure("-nauto")
+    check_options(config)
+    assert config.getoption("numprocesses") == 42
+
+
 def test_hook_auto_num_workers(testdir, monkeypatch):
     from xdist.plugin import pytest_cmdline_main as check_options
 

tox.ini

@@ -3,18 +3,26 @@ envlist=
   linting
   py{35,36,37,38,39}-pytestlatest
   py38-pytestmaster
+  py38-psutil
 
 [testenv]
-passenv = USER USERNAME
 extras = testing
 deps =
   pytestlatest: pytest
   pytestmaster: git+https://github.com/pytest-dev/pytest.git@master
 commands=
   pytest {posargs}
 
+[testenv:py38-psutil]
+extras =
+  testing
+  psutil
+deps = pytest
+commands =
+  pytest {posargs:-k psutil}
+
 [testenv:linting]
-skipsdist = True
+skip_install = True
 usedevelop = True
 deps =
   pre-commit
@@ -28,9 +36,9 @@ skipsdist = True
 usedevelop = True
 passenv = *
 deps =
-    towncrier
+  towncrier
 commands =
-    towncrier --version {posargs} --yes
+  towncrier --version {posargs} --yes
 
 [pytest]
 addopts = -ra