example

conftest.py

import pytest


pytest_plugins = 'pytester'


@pytest.fixture
def testdir(testdir):
    testdir.foo = 1

test.out

% py.test --tb=short
============================= test session starts =============================
platform linux2 -- Python 2.7.3 -- pytest-2.3.5.dev16
plugins: boxed, xdist, codecheckers, couchdbkit, cov
collected 1 items 

test_foo.py F

================================== FAILURES ===================================
____________________________________ test _____________________________________
/home/ronny/foo/test_foo.py:4: in test
>       assert testdir.foo
E       AttributeError: TmpTestdir instance has no attribute 'foo'
========================== 1 failed in 0.07 seconds ===========================

test_foo.py

def test(testdir):
    assert testdir.foo

however it does work if -p pytester is passed