From 687b2a8f25d72495c3db6f2d76eb03e6c4f658f4 Mon Sep 17 00:00:00 2001
From: tmeissner <programming@goodcleanfun.de>
Date: Thu, 11 Sep 2014 23:42:30 +0200
Subject: [PATCH] added case study of chapter 4: auth module

---
 python_3_oop/chapter04/auth.py | 138 +++++++++++++++++++++++++++++++++
 1 file changed, 138 insertions(+)
 create mode 100644 python_3_oop/chapter04/auth.py

diff --git a/python_3_oop/chapter04/auth.py b/python_3_oop/chapter04/auth.py
new file mode 100644
index 0000000..98b1021
--- /dev/null
+++ b/python_3_oop/chapter04/auth.py
@@ -0,0 +1,138 @@
+import hashlib
+
+
+class User:
+
+    def __init__(self, username, password):
+        '''Create a new user object. The password will be encrypted before
+        storing'''
+        self.username = username
+        self.password = self._encrypt_pw(password)
+        self.is_logged_in = False
+
+    def _encrypt_pw(self, password):
+        '''Encrypt the password with the username and return the sha digest.'''
+        hash_string = self.username + password
+        hash_string = hash_string.encode("utf8")
+        return hashlib.sha256(hash_string).hexdigest()
+
+    def check_password(self, password):
+        '''Return True if the password is valid for this user,
+        False otherwise'''
+        encrypted = self._encrypt_pw(password)
+        return encrypted == self.password
+
+
+class Authenticator:
+
+    def __init__(self):
+        '''Construct an authenticator to manage users logging in and out.
+        The User objects are stored in a simple dictionary'''
+        self.users = {}
+
+    def add_user(self, username, password):
+        '''Add a User object with given username & password to the users
+        dictionary after checking if given username & password are valid'''
+        if username in self.users:
+            raise UsernameAlreadyExists(username)
+        if len(password) < 6:
+            raise PasswordTooShort(username)
+        self.users[username] = User(username, password)
+
+    def login(self, username, password):
+        try:
+            user = self.users[username]
+        except KeyError:
+            raise InvalidUsername(username)
+        if not user.check_password(password):
+            raise InvalidPassword(username, user)
+        user.is_logged_in = True
+        return True
+
+    def is_logged_in(self, username):
+        if username in self.users:
+            return self.users[username].is_logged_in
+        return False
+
+
+authenticator = Authenticator()
+
+
+class Authorizor():
+
+    def __init__(self, authenticator):
+        self.authenticator = authenticator
+        self.permissions = {}
+
+    def add_permission(self, perm_name):
+        '''Create a new permission that users can be added to'''
+        try:
+            perm_set = self.permissions[perm_name]
+        except KeyError:
+            self.permissions[perm_name] = set()
+        else:
+            raise PermissionError("Permission exists")
+
+    def permit_user(self, perm_name, username):
+        '''Grant the given permission to the given user'''
+        try:
+            perm_set = self.permissions[perm_name]
+        except KeyError:
+            raise PermissionError("Permission does not exist")
+        else:
+            if username not in self.authenticator.users:
+                raise InvalidUsername(username)
+            perm_set.add(username)
+
+    def check_permission(self, perm_name, username):
+        if not self.authenticator.is_logged_in(username):
+            raise NotLoggedInError(username)
+        try:
+            perm_set = self.permissions[perm_name]
+        except KeyError:
+            raise PermissionError("Permission does not exist")
+        if username not in perm_set:
+            raise NotPermittedError(username)
+        else:
+            return True
+
+
+authorizor = Authorizor(authenticator)
+
+
+# Exception classes
+
+class AuthException(Exception):
+
+    def __init__(self, username, user = None):
+        super().__init__(username, user)
+        self.username = username
+        self.user = user
+
+
+class UsernameAlreadyExists(AuthException):
+    pass
+
+
+class PasswordTooShort(AuthException):
+    pass
+
+
+class InvalidUsername(AuthException):
+    pass
+
+
+class InvalidPassword(AuthException):
+    pass
+
+
+class NotLoggedInError(AuthException):
+    pass
+
+
+class NotPermittedError(AuthException):
+    pass
+
+
+class PermissionError(Exception):
+    pass