queue.py

#!/usr/bin/python

# Udacity's Python Engineering Interview Question
# Implement the push/pop operation of a queue only using
# stack operations

import unittest

class Q:
    def __init__(self, elements = []):
        self.stack1 = elements
        self.stack2 = []

    def push(self, element):
        self.stack1.append(element)

    def pop(self):
        if not self.stack2:
            while self.stack1:
                self.stack2.append(self.stack1.pop())
        return self.stack2.pop()

class QTest(unittest.TestCase):

    def test_empty_constructor(self):
        q = Q()
        self.assertRaises(IndexError, q.pop)

    def test_constructor(self):
        q = Q(['a','b','c','d'])
        self.assertEqual('a', q.pop())
        self.assertEqual('b', q.pop())
        self.assertEqual('c', q.pop())
        self.assertEqual('d', q.pop())

    def test_multiple_push_pop(self):
        q= Q()
        q.push(1)
        q.push(2)
        q.push(3)
        self.assertEqual(1, q.pop())
        self.assertEqual(2, q.pop())
        q.push(4)
        self.assertEqual(3, q.pop())
        self.assertEqual(4, q.pop())
        q.push(5)
        self.assertEqual(5, q.pop())
        self.assertRaises(IndexError, q.pop)


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