A sesssion-fixture which can look at all collected tests
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A session-scoped fixture effectively has access to all 
collected test items.  Here is an example of a fixture
function which walks all collected tests and looks
if their test class defines a ``callme`` method and
calls it::

    # content of conftest.py

    import pytest

    @pytest.fixture(scope="session", autouse=True)
    def callattr_ahead_of_alltests(request):
        print "callattr_ahead_of_alltests called"
        seen = set([None])
        session = request.node
        for item in session.items:
            cls = item.getparent(pytest.Class)
            if cls not in seen:
                if hasattr(cls.obj, "callme"):
                   cls.obj.callme()
                seen.add(cls)

test classes may now define a ``callme`` method which
will be called ahead of running any tests::

    # content of test_module.py

    class TestHello:
        @classmethod
        def callme(cls):
            print "callme called!"

        def test_method1(self):
            print "test_method1 called"
        
        def test_method2(self):
            print "test_method1 called"

    class TestOther:
        @classmethod
        def callme(cls):
            print "callme other called"
        def test_other(self):
            print "test other"

    # works with unittest as well ...
    import unittest
    
    class SomeTest(unittest.TestCase):
        @classmethod
        def callme(self):
            print "SomeTest callme called"

        def test_unit1(self):
            print "test_unit1 method called"

If you run this without output capturing::

    $ py.test -q -s test_module.py 
    ....
    callattr_ahead_of_alltests called
    callme called!
    callme other called
    SomeTest callme called
    test_method1 called
    test_method1 called
    test other
    test_unit1 method called