diff --git a/Lesson_4/02_Adding Users and Logins/models.py b/Lesson_4/02_Adding Users and Logins/models.py
index b665a04..e7f43d7 100644
--- a/Lesson_4/02_Adding Users and Logins/models.py	
+++ b/Lesson_4/02_Adding Users and Logins/models.py	
@@ -1,25 +1,57 @@
-from sqlalchemy import Column,Integer,String
+#!/usr/bin/env python
+
+"""Create a users database containing a User model."""
+
+from sqlalchemy import Column, Integer, String
 from sqlalchemy.ext.declarative import declarative_base
 from sqlalchemy.orm import relationship, sessionmaker
 from sqlalchemy import create_engine
+
+# Passlib is dedicated to password hashing
+# custom_app_context is an easy to use option
+# base on the SHA256 hashing algorithm
 from passlib.apps import custom_app_context as pwd_context
 
 Base = declarative_base()
+
+
 class User(Base):
+    """Define the User model for the database."""
+
     __tablename__ = 'user'
     id = Column(Integer, primary_key=True)
     username = Column(String(32), index=True)
+
+    # The hash of the user's password so passlib can verify a password by:
+    # 1. hashing it with the same function that was used during registration
+    # 2. compare the resulting hash against this one, stored in the database
+    # Thanks to this system, we never have to store a password in the database
     password_hash = Column(String(64))
 
     def hash_password(self, password):
+        """Store a hash of a plain user password string in the User table.
+
+        Called when:
+        - a new user is registering with the server
+        - a user changes their password
+
+        Argument:
+        password (string): plain password
+        """
         self.password_hash = pwd_context.encrypt(password)
 
     def verify_password(self, password):
+        """Return true if a password is correct, false if it's not.
+
+        Called when a user provides credentials that they need to be validated
+
+        Argument:
+        password (string): plain password
+        """
         return pwd_context.verify(password, self.password_hash)
 
 
 engine = create_engine('sqlite:///users.db')
- 
+
 
 Base.metadata.create_all(engine)
-