for PR

solutions/challenge_36/factorial.py

@@ -0,0 +1,26 @@
+"""this function allows to calculate the factorial of a non-negative integer n using recursive function"""
+
+
+def factorial(n):
+    """
+    Calculate the factorial of a non-negative integer n using recursion.
+
+    Args:
+        n (int): Non-negative integer whose factorial is to be calculated.
+
+    Returns:
+        int: Factorial of the input number n.
+    """
+    if n < 0:
+        raise ValueError("Factorial is not defined for negative numbers.")
+    if n == 0:
+        return 1
+    return n * factorial(n - 1)
+
+
+if __name__ == "__main__":
+    try:
+        num = int(input("Enter a non-negative integer: "))
+        print(f"Factorial of {num} is {factorial(num)}")
+    except ValueError as e:
+        print(f"Error: {e}")

solutions/tests/challenge_36/test_factorial.py

@@ -0,0 +1,24 @@
+"""test the program factorial"""
+
+import unittest
+from solutions.challenge_36.factorial import factorial
+
+
+class TestFactorial(unittest.TestCase):
+    """test the program for positive numbers, negative numbers and zero"""
+
+    def test_factorial_positive(self):
+        """test the program for positive numbers"""
+        self.assertEqual(factorial(5), 120)
+        self.assertEqual(factorial(4), 24)
+        self.assertEqual(factorial(3), 6)
+        self.assertEqual(factorial(1), 1)
+
+    def test_factorial_zero(self):
+        """test the program for zero"""
+        self.assertEqual(factorial(0), 1)
+
+    def test_factorial_negative(self):
+        """test the program for negative numbers"""
+        with self.assertRaises(ValueError):
+            factorial(-1)