New exercise: python octal

EXERCISES.txt

@@ -4,6 +4,7 @@ word-count
 anagram
 beer-song
 nucleotide-count
+octal
 point-mutations
 phone-number
 grade-school

octal/example.py

@@ -0,0 +1,13 @@
+class Octal(object):
+    def __init__(self, octal_string):
+        self.digits = self.__validate(octal_string)
+
+    def __validate(self, s):
+        for char in s:
+            if not '0' <= char < '8':
+                raise ValueError("Invalid octal digit: " + char)
+        return s
+
+    def to_decimal(self):
+        return sum(int(digit) * 8 ** i
+                   for (i, digit) in enumerate(reversed(self.digits)))

octal/octal_test.py

@@ -0,0 +1,45 @@
+try:
+    from octal import Octal
+except ImportError:
+    raise SystemExit('Could not find octal.py. Does it exist?')
+
+import unittest
+
+
+class OctalTest(unittest.TestCase):
+    def test_octal_1_is_decimal_1(self):
+        self.assertEqual(1, Octal("1").to_decimal())
+
+    def test_octal_10_is_decimal_8(self):
+        self.assertEqual(8, Octal("10").to_decimal())
+
+    def test_octal_17_is_decimal_15(self):
+        self.assertEqual(15, Octal("17").to_decimal())
+
+    def test_octal_130_is_decimal_88(self):
+        self.assertEqual(88, Octal("130").to_decimal())
+
+    def test_octal_2047_is_decimal_1063(self):
+        self.assertEqual(1063, Octal("2047").to_decimal())
+
+    def test_octal_1234567_is_decimal_342391(self):
+        self.assertEqual(342391, Octal("1234567").to_decimal())
+
+    def test_8_is_seen_as_invalid(self):
+        self.assertRaisesRegexp(ValueError, "^Invalid octal digit: 8$",
+                                Octal, "8")
+
+    def test_invalid_octal_is_recognized(self):
+        self.assertRaisesRegexp(ValueError, "^Invalid octal digit: c$",
+                                Octal, "carrot")
+
+    def test_6789_is_seen_as_invalid(self):
+        self.assertRaisesRegexp(ValueError, "^Invalid octal digit: 8$",
+                                Octal, "6789")
+
+    def test_valid_octal_formatted_string_011_is_decimal_9(self):
+        self.assertEqual(9, Octal("011").to_decimal())
+
+
+if __name__ == '__main__':
+    unittest.main()