>>> import doctest
>>> doctest.testfile ('tests.txt', False)
**********************************************************************
File "tests.txt", line 55, in tests.txt
Failed example:
    snd = a1.left_to_right (snd, len(snd))
Exception raised:
    Traceback (most recent call last):
      File "C:\Python27\lib\doctest.py", line 1254, in __run
        compileflags, 1) in test.globs
      File "<doctest tests.txt[20]>", line 1, in <module>
        snd = a1.left_to_right (snd, len(snd))
      File "a1.py", line 51, in left_to_right
        i = sound.get_index(sample)
    NameError: global name 'fade_length' is not defined
**********************************************************************
File "tests.txt", line 56, in tests.txt
Failed example:
    snd == snd_left_to_right
Expected:
    True
Got:
    False
**********************************************************************
1 items had failures:
   2 of  22 in tests.txt
***Test Failed*** 2 failures.
TestResults(failed=2, attempted=22)