The pytest testing tool makes it easy to write small tests, yet scales to support complex functional testing. It provides

auto-discovery of test modules and functions,
detailed info on failing assert statements (no need to remember self.assert* names)
modular fixtures for managing small or parametrized long-lived test resources.
multi-paradigm support: you can use pytest to run test suites based on unittest (or trial), nose
single-source compatibility from Python2.6 all the way up to Python3.4, PyPy-2.3, (jython-2.5 untested)
many external plugins.

A simple example for a test:

# content of test_module.py
def test_function():
    i = 4
    assert i == 3

which can be run with py.test test_module.py. See getting-started for more examples.

For much more info, including PDF docs, see

http://pytest.org

and report bugs at:

http://bitbucket.org/hpk42/pytest/issues/

and checkout or fork repo at:

http://bitbucket.org/hpk42/pytest/