Updated problem_04 in project_euler (#2427)

* Updated problem_04 in project_euler

* fixup! Format Python code with psf/black push

* That number is larger than our acceptable range.

* updating DIRECTORY.md

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
Co-authored-by: Christian Clauss <cclauss@me.com>

project_euler/problem_04/sol1.py

@@ -18,25 +18,34 @@ def solution(n):
     29992
     >>> solution(40000)
     39893
+    >>> solution(10000)
+    Traceback (most recent call last):
+    ...
+    ValueError: That number is larger than our acceptable range.
     """
     # fetches the next number
-    for number in range(n - 1, 10000, -1):
+    for number in range(n - 1, 9999, -1):
 
         # converts number into string.
-        strNumber = str(number)
+        str_number = str(number)
 
-        # checks whether 'strNumber' is a palindrome.
-        if strNumber == strNumber[::-1]:
+        # checks whether 'str_number' is a palindrome.
+        if str_number == str_number[::-1]:
 
             divisor = 999
 
             # if 'number' is a product of two 3-digit numbers
             # then number is the answer otherwise fetch next number.
             while divisor != 99:
-                if (number % divisor == 0) and (len(str(int(number / divisor))) == 3):
+                if (number % divisor == 0) and (len(str(number // divisor)) == 3.0):
                     return number
                 divisor -= 1
+    raise ValueError("That number is larger than our acceptable range.")
 
 
 if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()
+
     print(solution(int(input().strip())))