Add recursive factorial method with tests

recursion/factorial.py

@@ -0,0 +1,30 @@
+"""
+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/tests/test_factorial.py

@@ -0,0 +1,15 @@
+import pytest
+
+from recursion.factorial import factorial
+
+
+def test_factorial_valid_inputs() -> None:
+    assert factorial(0) == 1
+    assert factorial(1) == 1
+    assert factorial(5) == 120
+    assert factorial(10) == 3628800
+
+
+def test_factorial_invalid_input() -> None:
+    with pytest.raises(ValueError):
+        factorial(-1)