Does pytest support "default" markers?

Let's tackle the full problem. I think you can put a conftest.py file along with your tests and it will take care to skip all non-matching tests (non-marked tests will always match and thus never get skipped). Here i am using sys.platform but i am sure you have a different way to compute your platform value.

# content of conftest.py
#
import sys
import pytest

ALL = set("osx linux2 win32".split())

def pytest_runtest_setup(item):
    if isinstance(item, item.Function):
        plat = sys.platform
        if not hasattr(item.obj, plat):
            if ALL.intersection(set(item.obj.__dict__)):
                pytest.skip("cannot run on platform %s" %(plat))

With this you can mark your tests like this::

# content of test_plat.py

import pytest

@pytest.mark.osx
def test_if_apple_is_evil():
    pass

@pytest.mark.linux2
def test_if_linux_works():
    pass

@pytest.mark.win32
def test_if_win32_crashes():
    pass

def test_runs_everywhere_yay():
    pass

and if you run with::

$ py.test -rs

then you can run it and will see at least two test skipped and always at least one test executed::

then you will see two test skipped and two executed tests as expected::

$ py.test -rs # this option reports skip reasons
=========================== test session starts ============================
platform linux2 -- Python 2.7.3 -- pytest-2.2.5.dev1
collecting ... collected 4 items

test_plat.py s.s.
========================= short test summary info ==========================
SKIP [2] /home/hpk/tmp/doc-exec-222/conftest.py:12: cannot run on platform linux2

=================== 2 passed, 2 skipped in 0.01 seconds ====================

Note that if you specify a platform via the marker-command line option like this::

$ py.test -m linux2
=========================== test session starts ============================
platform linux2 -- Python 2.7.3 -- pytest-2.2.5.dev1
collecting ... collected 4 items

test_plat.py .

=================== 3 tests deselected by "-m 'linux2'" ====================
================== 1 passed, 3 deselected in 0.01 seconds ==================

then the unmarked-tests will not be run. It is thus a way to restrict the run to the specific tests.

Late to the party, but I just solved a similar problem by adding a default marker to all unmarked tests.

As a direct answer to the Question: you can have unmarked tests always run, and include marked test only as specified via the -m option, by adding the following to the conftest.py

def pytest_collection_modifyitems(items, config):
    # add `always_run` marker to all unmarked items
    for item in items:
        if not any(item.iter_markers()):
            item.add_marker("always_run")
    # Ensure the `always_run` marker is always selected for
    markexpr = config.getoption("markexpr", 'False')
    config.option.markexpr = f"always_run or ({markexpr})"