Since we know the problem lies in digits(), let's set an explicit breakpoint in there using the pdb.set_trace() function mentioned earlier:

def digits(x):
    """Convert an integer into a list of digits.

    Args:
      x: The number whose digits we want.

    Returns: A list of the digits, in order, of ``x``.

    >>> digits(4586378)
    [4, 5, 8, 6, 3, 7, 8]
    """

    import pdb; pdb.set_trace()

    digs = []
    while x != 0:
        div, mod = divmod(x, 10)
        digs.append(mod)
        x = mod
    return digs

Remember that the set_trace() function will halt execution and enter the debugger.

So now we can just execute our script without specifying the PDB module:

% python palindrome.py
> /Users/sixty_north/examples/palindrome.py(8)digits()
-> digs = []
(Pdb)

And we see that we almost immediately go to a PDB prompt with execution halted at the beginning of our digits() function.

To verify that we know where we are, let's use where to see our call stack:

(Pdb) where
  /Users/sixty_north/examples/palindrome.py(35)<module>()
-> unittest.main()
  /Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/un
ittest/main.py(95)__init__()
-> self.runTests()
    
  /Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/uni
ttest/main.py(229)runTests()
-> self.result = testRunner.run(self.test)
  /Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/uni
ttest/runner.py(151)run()
-> test(result)
  /Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/uni
ttest/suite.py(70)__call__()
-> return self.run(*args, **kwds)
  
  /Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/uni
ttest/suite.py(108)run()
-> test(result)
  /Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/uni
ttest/suite.py(70)__call__()
-> return self.run(*args, **kwds)
  /Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/uni
ttest/suite.py(108)run()
-> test(result)
  /Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/uni
ttest/case.py(391)__call__()
-> return self.run(*args, **kwds)
  /Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/uni
ttest/case.py(327)run()
-> testMethod()
/Users/sixty_north/examples/palindrome.py(25)test_negative()
-> self.assertFalse(is_palindrome(1234))
/Users/sixty_north/examples/palindrome.py(17)is_palindrome()
-> digs = digits(x)
> /Users/sixty_north/examples/palindrome.py(8)digits()
-> digs = []

Remember that the most recent frames are at the end of this listing. After a lot of unittest functions, we see that we are indeed in the digits() function, and that it was called by is_palindrome(), just as we expected.