Add psutil extra and introduce pytest_xdist_auto_num_workers hook

This makes using psutil optional and opens up the possibility of customization
through the pytest_xdist_auto_num_workers hook, making things like #477 possible.

Fix #585

.appveyor.yml

@@ -5,6 +5,7 @@ environment:
   - TOXENV: "py37-pytestlatest"
   - TOXENV: "py38-pytestlatest"
   - TOXENV: "py38-pytestmaster"
+  - TOXENV: "py38-psutil"
 
 install:
   - C:\Python38\python -m pip install -U pip setuptools virtualenv

.travis.yml

@@ -44,6 +44,8 @@ jobs:
       env: TOXENV=py39-pytestlatest
     - python: "3.8"
       env: TOXENV=py38-pytestmaster
+    - python: "3.8"
+      env: TOXENV=py38-psutil
 
     - stage: deploy
       python: '3.8'

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

@@ -0,0 +1 @@
+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

@@ -0,0 +1,3 @@
+``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::
+
+    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/newhooks.py

@@ -55,3 +55,13 @@ def pytest_xdist_node_collection_finished(node, ids):
 @pytest.mark.firstresult
 def pytest_xdist_make_scheduler(config, log):
     """ return a node scheduler implementation """
+
+
+@pytest.mark.firstresult
+def pytest_xdist_auto_num_workers(config):
+    """
+    Return the number of workers to spawn when ``--numprocesses=auto`` is given in the
+    command-line.
+
+    .. versionadded:: 2.1
+    """

src/xdist/plugin.py

@@ -5,9 +5,21 @@
 import pytest
 
 
-def auto_detect_cpus():
+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
@@ -16,25 +28,16 @@ def auto_detect_cpus():
             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:
         return 1
     return n if n else 1
 
 
-class AutoInt(int):
-    """Mark value as auto-detected."""
-
-
 def parse_numprocesses(s):
     if s == "auto":
-        return AutoInt(auto_detect_cpus())
+        return "auto"
     elif s is not None:
         return int(s)
 
@@ -187,12 +190,13 @@ def pytest_configure(config):
 @pytest.mark.tryfirst
 def pytest_cmdline_main(config):
     usepdb = config.getoption("usepdb", False)  # a core option
-    if isinstance(config.option.numprocesses, AutoInt):
+    if config.option.numprocesses == "auto":
         if usepdb:
             config.option.numprocesses = 0
             config.option.dist = "no"
         else:
-            config.option.numprocesses = int(config.option.numprocesses)
+            auto_num_cpus = config.hook.pytest_xdist_auto_num_workers(config=config)
+            config.option.numprocesses = auto_num_cpus
 
     if config.option.numprocesses:
         if config.option.dist == "no":

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"):
@@ -51,6 +60,7 @@ def test_auto_detect_cpus(testdir, monkeypatch):
     assert config.getoption("numprocesses") == 2
 
     config = testdir.parseconfigure("-nauto")
+    check_options(config)
     assert config.getoption("numprocesses") == 99
 
     config = testdir.parseconfigure("-nauto", "--pdb")
@@ -62,9 +72,36 @@ def test_auto_detect_cpus(testdir, monkeypatch):
     monkeypatch.delattr(os, "sched_getaffinity", raising=False)
     monkeypatch.setenv("TRAVIS", "true")
     config = testdir.parseconfigure("-nauto")
+    check_options(config)
     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
+
+    testdir.makeconftest(
+        """
+        def pytest_xdist_auto_num_workers():
+            return 42
+    """
+    )
+    config = testdir.parseconfigure("-nauto")
+    check_options(config)
+    assert config.getoption("numprocesses") == 42
+
+
 def test_boxed_with_collect_only(testdir):
     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