human
Solve the following python programming problem given the following code:
# %load test_compress.py
from nose.tools import assert_equal

class TestCompress(object):

    def test_compress(self, func):
        assert_equal(func(None), None)
        assert_equal(func(''), '')
        assert_equal(func('AABBCC'), 'AABBCC')
        assert_equal(func('AAABCCDDDD'), 'A3B1C2D4')
        print('Success: test_compress')

def main():
    test = TestCompress()
    test.test_compress(compress_string)

if __name__ == '__main__':
    main()
Given the above code, generate some markdown instructions for it.
assistant
#Here is the Python code you requested.

Unit Test
The following unit test is expected to fail until you solve the challenge.