def default(session):
    """Default unit test session.

    This is intended to be run **without** an interpreter set, so
    that the current ``python`` (on the ``PATH``) or the version of
    Python corresponding to the ``nox`` binary the ``PATH`` can
    run the tests.
    """
    # Install all test dependencies, then install local packages in-place.
    session.install('mock', 'pytest', 'pytest-cov')
    for local_dep in LOCAL_DEPS:
        session.install('-e', local_dep)
    session.install('-e', '.')

    # Run py.test against the unit tests.
    session.run(
        'py.test',
        '--quiet',
        '--cov=google.cloud.storage',
        '--cov=tests.unit',
        '--cov-append',
        '--cov-config=.coveragerc',
        '--cov-report=',
        '--cov-fail-under=97',
        'tests/unit',
        *session.posargs
    )