Add recursive factorial method with tests

Author: khushipy

recursion/__init__.py

@@ -0,0 +1,28 @@
+"""
+Fibonacci
+https://en.wikipedia.org/wiki/Fibonacci_number
+"""
+def factorial(number: int) -> int:
+    """
+    Compute the factorial of a non-negative integer using recursion.
+
+    >>> factorial(5)
+    120
+    >>> factorial(0)
+    1
+    >>> factorial(1)
+    1
+    >>> factorial(3)
+    6
+    >>> factorial(10)
+    3628800
+    >>> factorial(-1)
+    Traceback (most recent call last):
+        ...
+    ValueError: Input must be a non-negative integer.
+    """
+    if number < 0:
+        raise ValueError("Input must be a non-negative integer.")
+    if number == 0:
+        return 1
+    return number * factorial(number - 1)

recursion/factorial.py

@@ -0,0 +1,18 @@
+import unittest
+from recursion.factorial import factorial
+
+
+class TestFactorial(unittest.TestCase):
+    def test_factorial_valid_inputs(self):
+        self.assertEqual(factorial(0), 1)
+        self.assertEqual(factorial(1), 1)
+        self.assertEqual(factorial(5), 120)
+        self.assertEqual(factorial(10), 3628800)
+
+    def test_factorial_invalid_input(self):
+        with self.assertRaises(ValueError):
+            factorial(-1)
+
+
+if __name__ == "__main__":
+    unittest.main()