    from UserManager import UserManager
    from MiscUtils import NoDefault
    import os
    from glob import glob
    from User import User

    class UserManagerToFile(UserManager):
        """
        When using this user manager, make sure you invoke setUserDir() and that this directory is writeable by your application. It will contain 1 file per user with the user's serial number as the main filename and an extension of '.user'.

        The default user directory is the current working directory, but relying on the current directory is often a bad practice.
        """

        baseOfUserManagerToFile = UserManager

        ## Init ##

        def __init__(self, userClass=None):
            self.baseOfUserManagerToFile.__init__(self, userClass=None)
            try:
                from cPickle import load, dump
            except ImportError:
                from pickle import load, dump
            self.setEncoderDecoder(dump, load)
            self.setUserDir(os.getcwd())
            self.initNextSerialNum()

        def initNextSerialNum(self):
            if os.path.exists(self._userDir):
                serialNums = self.scanSerialNums()
                if serialNums:
                    self._nextSerialNum = max(serialNums)+1
                else:
                    self._nextSerialNum = 1
            else:
                self._nextSerialNum = 1

        ## WebKit integration ##

        def wasInstalled(self, owner):
            self.baseOfUserManagerToFile.wasInstalled(self, owner)
            self.setUserDir(owner.serverSidePath('Users'))
            self.initNextSerialNum()

        ## File storage specifics ##

        def userDir(self):
            return self._userDir

        def setUserDir(self, userDir):
            """ Sets the directory where user information is stored. You should strongly consider invoking initNextSerialNum() afterwards. """
            self._userDir = userDir

        def loadUser(self, serialNum, default=NoDefault):
            """
            Loads the user with the given serial number from disk.
            If there is no such user, a KeyError will be raised unless a default value was passed, in which case that value is returned.
            """
            filename = str(serialNum)+'.user'
            filename = os.path.join(self.userDir(), filename)
            if os.path.exists(filename):
                file = open(filename, 'r')
                user = self.decoder()(file)
                file.close()
                self._cachedUsers.append(user)
                self._cachedUsersBySerialNum[serialNum] = user
                return user
            else:
                if default is NoDefault:
                    raise KeyError, serialNum
                else:
                    return default

        def scanSerialNums(self):
            """
            Returns a list of all the serial numbers of users found on disk. Serial numbers are always integers.
            """
            chopIndex = -len('.user')
            nums = glob(os.path.join(self.userDir(), '*.user'))
            nums = [num[:chopIndex] for num in nums]
            nums = [os.path.basename(num) for num in nums]
            nums = [int(num) for num in nums]
            return nums

        ## UserManager customizations ##

        def setUserClass(self, userClass):
            """ Overridden to mix in UserMixIn to the class that is passed in. """
            from MiscUtils.MixIn import MixIn
            MixIn(userClass, UserMixIn)
            self.baseOfUserManagerToFile.setUserClass(self, userClass)

        ## UserManager concrete methods ##

        def nextSerialNum(self):
            result = self._nextSerialNum
            self._nextSerialNum += 1
            return result

        def addUser(self, user):
            assert isinstance(user, User)
            user._serialNum = self.nextSerialNum()
            self.baseOfUserManagerToFile.addUser(self, user)
            user.save()

        def userForSerialNum(self, serialNum, default=NoDefault):
            user = self._cachedUsersBySerialNum.get(serialNum, None)
            if user is not None:
                return user
            return self.loadUser(serialNum, default)

        def userForExternalId(self, externalId, default=NoDefault):
            for user in self._cachedUsers:
                if user.externalId()==externalId:
                    return user
            for user in self.users():
                if user.externalId()==externalId:
                    return user
            if default is NoDefault:
                raise KeyError, externalId
            else:
                return default

        def userForName(self, name, default=NoDefault):
            for user in self._cachedUsers:
                if user.name()==name:
                    return user
            for user in self.users():
                if user.name()==name:
                    return user
            if default is NoDefault:
                raise KeyError, name
            else:
                return default

        def users(self):
            return _UserList(self)

        def activeUsers(self):
            return _UserList(self, lambda user: user.isActive())

        def inactiveUsers(self):
            return _UserList(self, lambda user: not user.isActive())

        ## Encoder/decoder ##

        def encoder(self):
            return self._encoder

        def decoder(self):
            return self._decoder

        def setEncoderDecoder(self, encoder, decoder):
            self._encoder = encoder
            self._decoder = decoder

    class UserMixIn:

        def filename(self):
            return os.path.join(self.manager().userDir(), str(self.serialNum()))+'.user'

        def save(self):
            file = open(self.filename(), 'w')
            self.manager().encoder()(self, file)
            file.close()

    class _UserList:

        def __init__(self, mgr, filterFunc=None):
            self._mgr = mgr
            self._serialNums = mgr.scanSerialNums()
            self._count = len(self._serialNums)
            self._data = None
            if filterFunc:
                results = []
                for user in self:
                    if filterFunc(user):
                        results.append(user)
                self._count = len(results)
                self._data = results

        def __getitem__(self, index):
            if index>=self._count:
                raise IndexError
            if self._data:
                # We have the data directly. Just return it
                return self._data[index]
            else:
                # We have a list of the serial numbers.
                # Get the user from the manager via the cache or loading
                serialNum = self._serialNums[index]
                if self._mgr._cachedUsersBySerialNum.has_key(serialNum):
                    return self._mgr._cachedUsersBySerialNum[serialNum]
                else:
                    return self._mgr.loadUser(self._serialNums[index])

        def __len__(self):
            return self._count