diff --git a/DIRECTORY.md b/DIRECTORY.md
index ac10afdc0..1dceb8879 100644
--- a/DIRECTORY.md
+++ b/DIRECTORY.md
@@ -85,10 +85,12 @@
   * [Harriscorner](https://github.com/TheAlgorithms/Python/blob/master/computer_vision/harriscorner.py)
 
 ## Conversions
+  * [Binary To Decimal](https://github.com/TheAlgorithms/Python/blob/master/conversions/binary_to_decimal.py)
   * [Decimal To Any](https://github.com/TheAlgorithms/Python/blob/master/conversions/decimal_to_any.py)
   * [Decimal To Binary](https://github.com/TheAlgorithms/Python/blob/master/conversions/decimal_to_binary.py)
   * [Decimal To Hexadecimal](https://github.com/TheAlgorithms/Python/blob/master/conversions/decimal_to_hexadecimal.py)
   * [Decimal To Octal](https://github.com/TheAlgorithms/Python/blob/master/conversions/decimal_to_octal.py)
+  * [Hexadecimal To Decimal](https://github.com/TheAlgorithms/Python/blob/master/conversions/hexadecimal_to_decimal.py)
   * [Prefix Conversions](https://github.com/TheAlgorithms/Python/blob/master/conversions/prefix_conversions.py)
   * [Roman To Integer](https://github.com/TheAlgorithms/Python/blob/master/conversions/roman_to_integer.py)
   * [Temperature Conversions](https://github.com/TheAlgorithms/Python/blob/master/conversions/temperature_conversions.py)
@@ -222,6 +224,8 @@
 ## File Transfer
   * [Receive File](https://github.com/TheAlgorithms/Python/blob/master/file_transfer/receive_file.py)
   * [Send File](https://github.com/TheAlgorithms/Python/blob/master/file_transfer/send_file.py)
+  * Tests
+    * [Test Send File](https://github.com/TheAlgorithms/Python/blob/master/file_transfer/tests/test_send_file.py)
 
 ## Fuzzy Logic
   * [Fuzzy Operations](https://github.com/TheAlgorithms/Python/blob/master/fuzzy_logic/fuzzy_operations.py)
@@ -725,6 +729,7 @@
   * [Binary Tree Traversals](https://github.com/TheAlgorithms/Python/blob/master/traversals/binary_tree_traversals.py)
 
 ## Web Programming
+  * [Covid Stats Via Xpath](https://github.com/TheAlgorithms/Python/blob/master/web_programming/covid_stats_via_xpath.py)
   * [Crawl Google Results](https://github.com/TheAlgorithms/Python/blob/master/web_programming/crawl_google_results.py)
   * [Current Stock Price](https://github.com/TheAlgorithms/Python/blob/master/web_programming/current_stock_price.py)
   * [Current Weather](https://github.com/TheAlgorithms/Python/blob/master/web_programming/current_weather.py)
@@ -735,5 +740,6 @@
   * [Fetch Jobs](https://github.com/TheAlgorithms/Python/blob/master/web_programming/fetch_jobs.py)
   * [Get Imdb Top 250 Movies Csv](https://github.com/TheAlgorithms/Python/blob/master/web_programming/get_imdb_top_250_movies_csv.py)
   * [Get Imdbtop](https://github.com/TheAlgorithms/Python/blob/master/web_programming/get_imdbtop.py)
+  * [Recaptcha Verification](https://github.com/TheAlgorithms/Python/blob/master/web_programming/recaptcha_verification.py)
   * [Slack Message](https://github.com/TheAlgorithms/Python/blob/master/web_programming/slack_message.py)
   * [World Covid19 Stats](https://github.com/TheAlgorithms/Python/blob/master/web_programming/world_covid19_stats.py)
diff --git a/project_euler/problem_04/sol1.py b/project_euler/problem_04/sol1.py
index 599345b5a..227b594d0 100644
--- a/project_euler/problem_04/sol1.py
+++ b/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())))