One can currently write:

@pytest.xfail("sys.platform == 'darwin'")

But not

@pytest.xfail("platform.platform() == 'Darwin'")

If one tries that, an error is raised:

Error evaluating 'xfail' expression
    platform.platform() == "Darwin"
NameError: name 'platform' is not defined

It seems reasonable to me the platform module should be exposed in the namespace for these conditional methods for the same reason sys is.

I searched but could not find an existing issue or even a definition of what these conditional expressions are guaranteed to support, so I hope this is not a duplicate report.