Fix doctest power recursion

maths/power_using_recursion.py

@@ -15,18 +15,35 @@
 
 def power(base: int, exponent: int) -> float:
     """
-    power(3, 4)
+    >>> power(3, 4)
     81
     >>> power(2, 0)
     1
     >>> all(power(base, exponent) == pow(base, exponent)
     ...     for base in range(-10, 10) for exponent in range(10))
     True
+    >>> power('a', 1)
+    'a'
+    >>> power('a', 2)
+    Traceback (most recent call last):
+        ...
+    TypeError: can't multiply sequence by non-int of type 'str'
+    >>> power('a', 'b')
+    Traceback (most recent call last):
+        ...
+    TypeError: unsupported operand type(s) for -: 'str' and 'int'
+    >>> power(2, -1)
+    Traceback (most recent call last):
+        ...
+    RecursionError: maximum recursion depth exceeded
     """
     return base * power(base, (exponent - 1)) if exponent else 1
 
 
 if __name__ == "__main__":
+    from doctests import testmod
+
+    testmod()
     print("Raise base to the power of exponent using recursion...")
     base = int(input("Enter the base: ").strip())
     exponent = int(input("Enter the exponent: ").strip())