Allow richer expressions in -n auto

[LivInTheLookingGlass]

It would be nice if we could put things like pytest -n auto-1 and have it correctly interpret that as "all CPUs except 1". This ensures that the processor is not hogged by tests, but also that it runs at a very fast clip.

Could be done something like:

cores = 1
if 'auto' in args.n:
    cores = cpu_count()
    if '-' in args.n:
        cores -= int(args.n.split('-')[1])
    elif args.n != 'auto':
        raise ValueError("Invalid option given!")
else:
    cores = int(args.n)

[RonnyPfannschmidt]

im -10 on this - not going to add a mini pseudo programming language into a option

[chrahunt]

This can be done directly in shell, no reason to implement it in xdist.

pytest -n $(($(lscpu | awk '/CPU\(s\)/ {print $2; exit}') - 1))

There's probably similar for Powershell.

[nicoddemus]

I agree, closing as "won't fix"

[graingert]

auto_detect_cpus is now a bit more complicated:

pytest-xdist/src/xdist/plugin.py

Lines 8 to 28 in 59caed8

	def auto_detect_cpus():
	try:
	from os import sched_getaffinity
	except ImportError:
	if os.environ.get("TRAVIS") == "true":
	# workaround https://bitbucket.org/pypy/pypy/issues/2375
	return 2
	try:
	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

pytest --numprocesses=$(python -c 'print(__import__("pytest.plugin").plugin.auto_detect_cpus() - 1)')

[stas00]

pytest --numprocesses=$(python -c 'print(__import__('pytest.plugin').plugin.auto_detect_cpus() - 1)')

that didn't work.

here is a fixed version

# count
python -c "print(__import__('xdist.plugin').plugin.auto_detect_cpus() - 1)"
# feed that to pytest
pytest --numprocesses=$(python -c "print(__import__('xdist.plugin').plugin.auto_detect_cpus() - 1)")