Add Python source files

* And update .gitignore

.gitignore

@@ -1,3 +1,4 @@
+.DS_store
 # Byte-compiled / optimized / DLL files
 __pycache__/
 *.py[cod]

sources/calc.py

@@ -0,0 +1,5 @@
+'''
+A simple addition function.
+'''
+def add(a, b):
+    return a + b

sources/test_calc.py

@@ -0,0 +1,33 @@
+import calc
+import unittest
+
+class TestAdd(unittest.TestCase):
+    """
+    Test the add function from the calc library
+    """
+
+    def test_add_integers(self):
+        """
+        Test that the addition of two integers returns the correct total
+        """
+        result = calc.add(1, 2)
+        self.assertEqual(result, 3)
+
+    def test_add_floats(self):
+        """
+        Test that the addition of two floats returns the correct result
+        """
+        result = calc.add(10.5, 2)
+        self.assertEqual(result, 12.5)
+
+    def test_add_strings(self):
+        """
+        Test the addition of two strings returns the two string as one
+        concatenated string
+        """
+        result = calc.add('abc', 'def')
+        self.assertEqual(result, 'abcdef')
+
+
+if __name__ == '__main__':
+    unittest.main()