How to disable pytest xdist only when pytest is called with filters?

When I run all pytests I do want to fully benefit from spreading the load on all m cores (xdist) but when I run a subset of them is almost always for development/debugging purposes, case in which I do want to avoid the downsides of xdist as:

slower start time
unable to use pdb debugging

To examplify: calling pytest -k foo should effectively run pytest -n0 -k foo. Keep in mind that calling pytest without arguments should not affect the -n value.

Solution

You can dynamically add the option, for example:

conftest.py

import sys

def pytest_cmdline_preparse(args):
    if "xdist" in sys.modules and "-k" in args:
        for i, arg in enumerate(args):
            # remove -n # option
            if arg == "-n":
                del args[i]
                del args[i]
                break
            # remove -n# option
            if re.match(r"-n\d+", arg):
                del args[i]
                break

        args[:] = ["-n0"] + args

This removes an existing "-n" option (both of the form -n # and -n# - you may not need both, depending on how your command line looks), and adds the "-n0" option instead.

Update:
pytest_cmdline_preparse is deprecated in current (6.x) pytest versions, pytest_load_initial_conftests can be used instead in the same manner.