Add docstring fir helper function

login_provider/helper.py

 import re
 
+
 def is_safe_url(url):
+    """Checks if a url is safe
+
+    Check if a url is safe to be used in redirects. This function is used whenever the user
+    passes a redirect url to the application. In case of the login process the user passes
+    a url to the application via a HTTP-GET variable, namely `next`. Once the user successfully
+    authenticated, the url is used by the server to redirect to the page the user initially
+    requested. The url validation prevents attacks where an attacker creates links that
+    redirect users to malicious urls once they are loged in.
+    example: http://login-provider/login?next=malicious\.org/
+
+    Args:
+        url: Url that needs to be validated
+
+    Returns:
+        True if the url is trusted. False if not.
+    """
     safe_urls = [
         "^[/]*$",                             # Home page
         "^/\?login_challenge=[a-z|A-Z|0-9]+$" # Login challenge with alphanumeric code