    """The abstract UserManager class."""

    from MiscUtils import AbstractError, NoDefault
    from User import User

    class UserManager(object):
        """The base class for all user manager classes.

        A UserManager manages a set of users including authentication,
        indexing and persistence. Keep in mind that UserManager is abstract;
        you will always use one of the concrete subclasses (but please read
        the rest of this docstring):
          * UserManagerToFile
          * UserManagerToMiddleKit

        You can create a user through the manager (preferred):
            user = manager.createUser(name, password)

        Or directly through the user class:
            user = RoleUser(manager, name, password)
            manager.addUser(user)

        The manager tracks users by whether or not they are "active"
        (e.g., logged in) and indexes them by:
          * user serial number
          * external user id
          * user name

        These methods provide access to the users by these keys:
            def userForSerialNum(self, serialNum, default=NoDefault)
            def userForExternalId(self, extId, default=NoDefault)
            def userForName(self, name, default=NoDefault)

        UserManager provides convenient methods for iterating through the
        various users. Each method returns an object that can be used in a
        for loop and asked for its len():
            def users(self)
            def activeUsers(self)
            def inactiveUsers(self)

        You can authenticate a user by passing the user object and attempted
        password to login(). If the authentication is successful, then login()
        returns the User, otherwise it returns None:
            user = mgr.userForExternalId(externalId)
            if mgr.login(user, password):
                self.doSomething()

        As a convenience, you can authenticate by passing the serialNum,
        externalId or name of the user:
            def loginSerialNum(self, serialNum, password):
            def loginExternalId(self, externalId, password):
            def loginName(self, userName, password):

        The user will automatically log out after a period of inactivity
        (see below), or you can make it happen with:
            def logout(self, user):

        There are three user states that are important to the manager:
          * modified
          * cached
          * authenticated or "active"

        A modified user is one whose data has changed and eventually requires
        storage to a persistent location. A cached user is a user whose data
        resides in memory (regardless of the other states). An active user
        has been authenticated (e.g., their username and password were checked)
        and has not yet logged out or timed out.

        The manager keeps three timeouts, expressed in minutes, to:
          * save modified users after a period of time following the first
            unsaved modification
          * push users out of memory after a period of inactivity
          * deactive (e.g., log out) users after a period of inactivity

        The methods for managing these values deal with the timeouts as
        number-of-minutes. The default values and the methods are:
          * 20  modifiedUserTimeout()  setModifiedUserTimeout()
          * 20  cachedUserTimeout()    setCachedUserTimeout()
          * 20  activeUserTimeout()    setActiveUserTimeout()

        Subclasses of UserManager provide persistence such as to the file
        system or a MiddleKit store. Subclasses must implement all methods
        that raise AbstractError's. Subclasses typically override (while still
        invoking super) addUser().

        Subclasses should ensure "uniqueness" of users. For example, invoking
        any of the userForSomething() methods repeatedly should always return
        the same user instance for a given key. Without uniqueness, consistency
        issues could arise with users that are modified.

        Please read the method docstrings and other class documentation to
        fully understand UserKit.

        """

        ## Init ##

        def __init__(self, userClass=None):
            if userClass is None:
                self._userClass = None
            else:
                self.setUserClass(userClass)
            self._cachedUsers = []
            self._cachedUsersBySerialNum = {}
            self.setModifiedUserTimeout(20)
            self.setCachedUserTimeout(20)
            self.setActiveUserTimeout(20)
            self._numActive = 0

        def shutDown(self):
            """Perform any tasks necessary to shut down the user manager.

            Subclasses may override and must invoke super as their *last* step.

            """
            pass

        ## Settings ##

        def userClass(self):
            """Return the userClass, which is used by createUser.

            The default value is UserKit.User.User.

            """
            if self._userClass is None:
                self.setUserClass(User)
            return self._userClass

        def setUserClass(self, userClass):
            """Set the userClass, which cannot be None and must inherit from User.

            See also: userClass().

            """
            assert issubclass(userClass, User)
            self._userClass = userClass

        def modifiedUserTimeout(self):
            return self._modifiedUserTimeout

        def setModifiedUserTimeout(self, value):
            self._modifiedUserTimeout = value

        def cachedUserTimeout(self):
            return self._cachedUserTimeout

        def setCachedUserTimeout(self, value):
            self._cachedUserTimeout = value

        def activeUserTimeout(self):
            return self._activeUserTimeout

        def setActiveUserTimeout(self, value):
            self._activeUserTimeout = value

        ## Basic user access ##

        def createUser(self, name, password, userClass=None):
            """Return a newly created user that is added to the manager.

            If userClass is not specified, the manager's default user class
            is instantiated. This not imply that the user is logged in.
            This method invokes self.addUser().

            See also: userClass(), setUserClass()

            """
            if userClass is None:
                userClass = self.userClass()
            user = userClass(manager=self, name=name, password=password)
            self.addUser(user)
            return user

        def addUser(self, user):
            assert isinstance(user, User)
            self._cachedUsers.append(user)
            assert user.serialNum() not in self._cachedUsersBySerialNum
            self._cachedUsersBySerialNum[user.serialNum()] = user

        def userForSerialNum(self, serialNum, default=NoDefault):
            """Return the user with the given serialNum.

            The user record is pulled into memory if needed.

            """
            raise AbstractError(self.__class__)

        def userForExternalId(self, externalId, default=NoDefault):
            """Return the user with the given external id.

            The user record is pulled into memory if needed.

            """
            raise AbstractError(self.__class__)

        def userForName(self, name, default=NoDefault):
            """Return the user with the given name.

            The user record is pulled into memory if needed.

            """
            raise AbstractError(self.__class__)

        def users(self):
            """Return a list of all users (regardless of login status)."""
            raise AbstractError(self.__class__)

        def numActiveUsers(self):
            """Return the number of active users, e.g., users that are logged in."""
            return self._numActive

        def activeUsers(self):
            """Return a list of all active users."""
            raise AbstractError(self.__class__)

        def inactiveUsers(self):
            raise AbstractError(self.__class__)

        ## Logging in and out ##

        def login(self, user, password):
            """Return the user if the login is successful, otherwise return None."""
            assert isinstance(user, User)
            result = user.login(password, fromMgr=True)
            if result:
                self._numActive += 1
            return result

        def logout(self, user):
            assert isinstance(user, User)
            user.logout(fromMgr=True)
            self._numActive -= 1

        def loginSerialNum(self, serialNum, password):
            user = self.userForSerialNum(serialNum, None)
            if user:
                return self.login(user, password)
            else:
                return None

        def loginExternalId(self, externalId, password):
            user = self.userForExternalId(externalId, None)
            if user:
                return self.login(user, password)
            else:
                return None

        def loginName(self, userName, password):
            user = self.userForName(userName, None)
            if user:
                return self.login(user, password)
            else:
                return None

        ## Cached ##

        def clearCache(self):
            """Clear the cache of the manager.

            Use with extreme caution. If your program maintains a reference
            to a user object, but the manager loads in a new copy later on,
            then consistency problems could occur.

            The most popular use of this method is in the regression test suite.

            """
            self._cachedUsers = []
            self._cachedUsersBySerialNum = {}