Rather than simply list all of the commonly useful PDB commands, we're going to instead debug a simple function. Our function – is_palindrome() – takes in an integer and determines if the digits of the integer are a palindrome or not. A palindrome is a sequence which is the same both forwards and backwards.

The first thing we'll do is create a new file, palindrome.py, with this code:

import unittest

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]
    """

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

def is_palindrome(x):
    """Determine if an integer is a palindrome.

    Args:
      x: The number to check for palindromicity.

    Returns: True if the digits of ``x`` are a palindrome,
      False otherwise.

    >>> is_palindrome(1234)
    False
    >>> is_palindrome(2468642)
    True
    """
    digs = digits(x)
    for f, r in zip(digs, reversed(digs)):
        if f != r:
            return False
    return True

class Tests(unittest.TestCase):
    """Tests for the ``is_palindrome()`` function."""
    def test_negative(self):
        "Check that it returns False correctly."
        self.assertFalse(is_palindrome(1234))

    def test_positive(self):
        "Check that it returns True correctly."
        self.assertTrue(is_palindrome(1234321))

    def test_single_digit(self):
        "Check that it works for single digit numbers."
        for i in range(10):
            self.assertTrue(is_palindrome(i))

if __name__ == '__main__':
    unittest.main()

As you can see, our code has three main parts:

• The first is the digits() function which converts an integer into a list of digits.
• The second is the is_palindrome() function which first calls digits() and then checks if the resulting list is a palindrome.
• The third part is a set of unit tests. We'll use these tests to drive the program.

As you might expect, this being a section on debugging, there's a bug in this code. We're going to first run the program and notice the bug, and then we'll see how to use PDB to find the bug.