update factorial (#1535)

* update factorial

* update factorial

maths/factorial_python.py

@@ -1,21 +1,34 @@
 def factorial(input_number: int) -> int:
     """
-        Non-recursive algorithm of finding factorial of the
-        input number.
-        >>> factorial(1)
-        1
-        >>> factorial(6)
-        720
-        >>> factorial(0)
-        1
+    Calculate the factorial of specified number
+
+    >>> factorial(1)
+    1
+    >>> factorial(6)
+    720
+    >>> factorial(0)
+    1
+    >>> factorial(-1)
+    Traceback (most recent call last):
+        ...
+    ValueError: factorial() not defined for negative values
+    >>> factorial(0.1)
+    Traceback (most recent call last):
+        ...
+    ValueError: factorial() only accepts integral values
     """
 
     if input_number < 0:
-        raise ValueError("Input input_number should be non-negative")
-    elif input_number == 0:
-        return 1
-    else:
-        result = 1
-        for i in range(input_number):
-            result = result * (i + 1)
+        raise ValueError("factorial() not defined for negative values")
+    if not isinstance(input_number, int):
+        raise ValueError("factorial() only accepts integral values")
+    result = 1
+    for i in range(1, input_number):
+        result = result * (i + 1)
     return result
+
+
+if __name__ == '__main__':
+    import doctest
+
+    doctest.testmod()